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+Project Gutenberg's Optical Projection, by Lewis Wright and Russell S. Wright
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Optical Projection
+       Part 1: Projection of Lantern Slides
+
+Author: Lewis Wright
+        Russell S. Wright
+
+Release Date: October 31, 2010 [EBook #33899]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK OPTICAL PROJECTION ***
+
+
+
+
+Produced by Chris Curnow, Keith Edkins and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+OPTICAL PROJECTION
+
+A TREATISE ON THE USE OF THE LANTERN IN
+EXHIBITION AND SCIENTIFIC DEMONSTRATION
+
+BY
+
+LEWIS WRIGHT
+
+AUTHOR OF 'LIGHT: A COURSE OF EXPERIMENTAL OPTICS'
+
+5TH EDITION
+
+RE-WRITTEN AND BROUGHT UP-TO-DATE BY
+
+RUSSELL S. WRIGHT, M.I.E.E.
+
+IN TWO PARTS
+
+PART I
+
+_THE PROJECTION OF LANTERN SLIDES_
+
+
+
+_WITH ILLUSTRATIONS_
+
+
+
+LONGMANS, GREEN, AND CO.
+39 PATERNOSTER ROW, LONDON, E.C. FOURTH AVENUE & 30TH STREET, NEW YORK
+BOMBAY, CALCUTTA, AND MADRAS
+
+1920
+
+(_All rights reserved_)
+
+       *       *       *       *       *
+
+
+{v}
+
+PREFACE TO THE FIFTH EDITION
+
+The first edition of this work was written by my father, the late Mr. Lewis
+Wright, and was published in 1890.
+
+The reception that it received testified to the fact that it met a
+long-felt want, and successive editions were published in 1895, 1901, and
+1906.
+
+My father, unfortunately, met his death in a railway accident in 1905, and
+the corrections and additions to the last edition, which had been to a
+certain extent prepared by him, were completed and written by myself, and
+the work as published then was again reprinted in 1911.
+
+As the original text is now thirty years old, it has seemed better entirely
+to re-write the whole book rather than make fresh revisions, the more so as
+the last ten years have seen great advances in the science of Lantern
+Projection, and especially in the developments of Acetylene and Electric
+Lighting.
+
+It has also seemed best at the present juncture to issue the book in two
+parts, the first dealing with the Projection of Lantern Slides only, and
+the second with the Demonstration of Opaque and Microscopic Objects,
+Scientific Phenomena and accessory apparatus, including Cinematograph
+Projection.
+
+It must of necessity be many months before this second volume can be
+produced, for the simple reason that Optical {vi} Instrument Makers have as
+yet hardly had time to turn round after the war and produce their new
+models, and therefore any such book written now could do little more than
+describe apparatus that was on the market prior to 1914.
+
+The present work, therefore, deals solely with the exhibition of Lantern
+Slides in the Optical Lantern, and as such I trust will be found of value
+to Schoolmasters, Social Workers, Lecturers, and, in fact, to all who use
+the lantern as a means of illustration.
+
+  RUSSELL S. WRIGHT.
+  _January 1920._
+
+       *       *       *       *       *
+
+
+{vii}
+
+CONTENTS
+
+  CHAPTER                                                     PAGE
+
+  I. INTRODUCTORY                                                1
+
+  II. THE ILLUMINANT                                             3
+
+  III. PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS   6
+
+  IV. THE ACETYLENE LIGHT                                       11
+
+  V. LIMELIGHT AND THE ACETYLENE BLAST                          16
+
+  VI. THE ELECTRIC LIGHT                                        39
+
+  VII. THE OPTICAL SYSTEM OF A LANTERN                          57
+
+  VIII. THE BODY OF THE LANTERN                                 70
+
+  IX. LANTERN BOXES, STANDS, READING LAMPS, ETC.                76
+
+  X. SCREENS AND SCREEN STANDS                                  79
+
+  XI. THE PRACTICAL MANIPULATION OF A LANTERN                   82
+
+{viii}
+
+ILLUSTRATIONS
+
+  FIG.                                                     PAGE
+
+   1. Oil Lamp                                                6
+   2. Inverted  Incandescent Lamp                             8
+   3. Methylated Spirit Burner                                9
+   4. Luna Lamp                                              10
+   5. The Moss Generator                                     12
+   6. The A.L. or 'Popular' Model                            14
+   7. Acetylene Jet                                          15
+   8. Oxygen Cylinder in hemp cover                          17
+   9. Double Lever Key                                       18
+  10. Fine Adjustment Valve                                  19
+  11. Construction of Beard's Regulator                      20
+  12. Beard's Regulator                                      21
+  13. Regulator and Gauge                                    22
+  14. Gas-bags                                               24
+  15. 'Blow-through' Nozzles                                 25
+  16. 'Blow-through' Jet                                     25
+  17. Mixed Jet                                              27
+  18. Mixed Jet, Gwyer pattern                               27
+  19. Mixing Chamber of Jet                                  28
+  20. 'Injector' Jet                                         30
+  21. 'Gridiron' Saturator                                   32
+  22. 'Pendant' Saturator                                    33
+  23. Fallot Air Blast                                       37
+  24. Fallot Air Blast, and Cylinder                         37
+  25. Lime-tongs                                             39
+  26. Universal Hand-fed Arc Lamp                            45
+  27.                                                        46
+  28. Resistance                                             49
+  29. 'Scissors' Arc Lamp                                    51
+  30. 'Right-angled' Arc Lamp                                52
+  31. 'Westminster' Arc Lamp                                 53
+  32. Arc Lamp with Induction Ring                           56
+  33. The Optical System of a Lantern            _facing p._ 57
+  33A. Optical System of Lantern                             57
+  34. Optical System without Condenser                       59
+  35. Action of Condenser                                    59
+  36. Forms of Condensers                                    60
+  37. Double Sliding Carrier                                 62
+  38. Beard's Dissolving Carrier                             63
+  39. Focussing Action of Lens                               64
+  40. Achromatic Lens                                        65
+  41. Petzval Combination                                    66
+  42. Hughes' Short-Range Lantern                            71
+  43. Long-Range Lantern                                     72
+  44. Connections for a Bi-unial Lantern                     73
+  45. Beard's Circulating Water Tank                         75
+  46. Quadruple Lantern Stand                                78
+  47. Reading Lamp                                           79
+  48. Roller Screen                                          80
+  49. Portable Screen Stand                                  81
+  50. Adjustment of the Light                                84
+
+       *       *       *       *       *
+
+
+{1}
+
+OPTICAL PROJECTION
+
+A TREATISE ON THE USE OF THE OPTICAL LANTERN
+
+CHAPTER I
+
+INTRODUCTORY
+
+Lantern Projection, as commonly understood, may be broadly subdivided into
+two branches: (A) The Projection of Lantern Slides, and (B) The Projection
+of Scientific Phenomena, Opaque Objects, Microscopic Specimens, &c.,
+usually referred to broadly under the heading of 'Scientific
+Demonstration.'
+
+To these two classes may perhaps now be added a third, viz. The Projection
+of So-called Living Pictures, or, in other words, the Cinematograph. In the
+earlier editions of this work both A and B were dealt with in the same
+volume, but, as there are thousands who require to use a lantern for the
+demonstration of lantern slides only, and who have no interest or concern
+with Science Projection, it has seemed to the writer that the work might,
+with advantage, be divided into two portions, Vol. I. dealing with slides
+only, and Vol. II. with the various adaptations of the science lantern.
+This present book therefore only deals with the exhibition of lantern
+slides, and as such it will, I trust, be found to be of real assistance to
+the ordinary user of the optical lantern, including clergymen,
+schoolmasters, army and cadet officers, and others {2} who require advice
+and instruction in the purchase or use of a lantern.
+
+The essential parts of a lantern are: (_a_) A _slide-holder_ or _carrier_
+to hold the slide; (_b_) a _lens_ to 'focus' it on the screen; (_c_) a
+_condenser_ to converge the light upon slide and lens; (_d_) a source of
+light or _radiant_ to provide the necessary illumination; and (_e_) a
+_body_ or framework to hold the whole together. All possible variations in
+choice of a suitable lantern relate to one or another of the above parts,
+and will be treated of in turn; but, fortunately, we have this
+all-important simplification that every ordinary English lantern slide is
+the same _standard size_, viz. 3¼ inches square. Some Continental and
+American slides differ in one dimension from the above, but not enough to
+cause any serious difficulty, and the convenient English standard is being
+gradually adopted throughout the world.
+
+The varieties of slide-holders or carriers are therefore comparatively few
+and are chiefly concerned with the question of rapidly and easily changing
+the slides. The choice of a focussing lens or objective is mainly a matter
+of the size of picture required, and the most convenient distance from the
+screen for the lantern to be placed. Variations in condensers, which are
+comparatively small, are usually only a matter of conforming these with the
+size or type of objective to be used, and should be left to the
+manufacturer's judgment. The question of a suitable radiant is partly a
+matter of the amount of illumination required, and partly that of the
+practical possibilities; for example, if electric current is available some
+form of electric light is usually the most convenient, as well as the least
+expensive, but where this is not the case, paraffin-oil, methylated spirit,
+incandescent gas, acetylene, limelight, &c., are alternatives which all
+have their uses and must be considered on their own merits.
+
+Sometimes, as for example in the case of a travelling lecturer, a lantern
+is required fitted with a range of lenses for {3} halls of different size,
+and also with a variety of illuminants, and this in most lanterns can be
+easily provided for.
+
+The body is usually a matter of taste and price only, and may range from a
+simple but efficient shell of Russian iron to an elaborate mahogany
+instrument with a brass front, screw tilting arrangements and other
+adornments; but of late years there has been a wholesome reaction against
+unnecessary finish, and a simple metal body of some description is now
+chiefly the order of the day. In the foregoing remarks the various parts of
+a lantern have been mentioned in what I should consider the correct order,
+starting from the slide and slide-holder, and so to speak building up the
+rest of the instrument round these items; but I now propose somewhat to
+vary the procedure and for convenience deal in detail first with the
+Radiant, or _Illuminant_.
+
+       *       *       *       *       *
+
+
+CHAPTER II
+
+THE ILLUMINANT
+
+The first necessity for lantern projection is a strong light, and this can
+be obtained from a variety of sources, the principal means in common use
+being approximately in order of excellence as follows: paraffin-oil,
+incandescent spirit, incandescent gas, acetylene, acetylene air blast,
+oxyhydrogen (limelight), oxyether, and electric light in its various forms.
+The ideal characteristics to be sought for are (1) great intrinsic
+brilliancy; (2) minimum _size_ of luminous spot; (3) freedom from flicker;
+(4) freedom from smell; (5) absence of any preponderating colour; (6)
+cheapness; and (7) convenience. There is no question whatever as to which
+of the available sources of light most perfectly combines all the above if
+it is available, viz. the electric arc. If a current supply is in {4} the
+building, this form of lighting easily excels all others, except possibly
+in the matter of flicker, and even in this respect there is very little
+fault to be found with it.
+
+From all other points of view it is wellnigh perfect, inasmuch as it
+provides an extremely concentrated and intensely luminous spot, of almost
+perfect whiteness (if anything slightly bluish), no smell, comparatively
+little heat, convenient and inexpensive. So great is the advantage of the
+electric arc that attempts have been made to use it from accumulators in
+places where a current supply is not available, but this cannot be
+seriously recommended, except in special cases. Where an electric supply
+is, however, available there can be no real choice, whether the lantern is
+required for use in a large hall or a small class-room. The advantages of
+using the arc are so great that no other method need be seriously
+considered.
+
+The one real objection that I have heard urged against it is due, curiously
+enough, to its very perfection, and that is, that it lends itself to such
+exceedingly sharp definition that any slight imperfection in the slide is
+too faithfully reproduced on the screen, for which reason it is sometimes
+recommended that the operator shall work with the objective the least
+fraction out of focus; but this is a matter for individual taste and
+judgment.
+
+If, however, there is no possibility of using the electric current, one of
+the other sources of illumination must perforce be adopted, and for a
+_large_ hall this can only be limelight in one of its many forms, viz.
+oxyhydrogen, oxyether, oxyacetylene, &c. As regards results on the screen,
+this light compares well even against the electric arc, but it involves the
+expense and trouble of compressed gas cylinders, or the infinitely worse
+recourse to the now obsolete method of filling gas-bags.
+
+Limelight is therefore now but little used in this country, as the majority
+of large halls are equipped with the electric {5} current, and for smaller
+buildings it is deemed unnecessary and too expensive.
+
+ACETYLENE is undoubtedly the illuminant most in favour next to electric
+light, as the light is brilliant enough to illuminate a picture 12 feet in
+diameter at a distance up to, say, 30 feet from the screen, and this
+suffices in a large majority of cases, and acetylene is comparatively
+cheap, and reasonably simple to work.
+
+INCANDESCENT-GAS is often employed for small class-rooms and is fairly
+effective for a picture not exceeding 9 or 10 feet in diameter, and the
+same can be said of the same type of burner heated by methylated spirit.
+
+PARAFFIN-OIL is the poorest of all present-day forms of lantern
+illuminants. The flame is large, impairing the definition, yellow in
+colour, uneven in illumination, liable to smoke and smell, and barely equal
+to incandescent gas in illuminating power.
+
+It is therefore going gradually out of use in this country, but in
+out-of-the-way places, especially abroad, it is sometimes the only
+practicable light, and is therefore still employed from the best of all
+reasons, necessity.
+
+It is not the intention of the author to give precise working instruction
+for all and every variety of the above illuminants as manufactured by
+different firms. For such the reader must be referred to the directions
+usually issued by the makers themselves, but a general description of the
+various types offered for choice will not be out of place, and it will be
+more convenient to begin with the poorest, viz. paraffin-oil, and finish
+with the most perfect, the electric arc.
+
+       *       *       *       *       *
+
+
+{6}
+
+CHAPTER III
+
+PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS
+
+[Illustration: FIG. 1.--Oil Lamp.]
+
+There are several varieties of oil lamps on the market, but in practically
+every case they take the same general form, a metal reservoir sliding in
+grooves in the lantern body and holding approximately a pint of oil with
+(usually) four wicks _nearly_ parallel, but slightly converging from rear
+to front, these enclosed in a flame chamber of Russian iron, with _loose_
+well-annealed ends of sheet glass and an adjustable reflector at the back,
+or sometimes the reflector itself forms the rear end of the flame chamber.
+The chimney must be tall and is now usually made adjustable, though I have
+never been able to trace any real advantage from this complication {7}
+(Fig. 1). The whole secret of obtaining the best results from these lamps
+may be summed up--_good oil and perfect cleanliness_; and it is wonderful
+what can be done when these points are properly attended to.
+
+Care should be taken in trimming the wicks to see that no charred parts
+fall down between the wick holders, but it makes little difference whether
+the trimming is done with scissors or by rubbing with the finger. Special
+lamp scissors are sold by all makers with a large flat on one side to catch
+the portions cut off.
+
+These lamps should be well rubbed over the last thing before use, as
+paraffin-oil is apt to 'creep,' and the operator does not want to be told
+that his apparatus is suggestive of a fried fish shop. In working with
+these lamps it is difficult to avoid a dark streak down the centre of the
+sheet, representing the space between the two centre wicks; to a certain
+extent this can be obviated by adjusting the reflector, and in any case is
+not very obvious when the slide is in place. Lamps constructed with either
+three or five wicks are better in this respect, but the former are usually
+considered to be too poor in illuminating power, and the latter are apt to
+crack the sheet-glass ends by excessive heat.
+
+INCANDESCENT GAS.--Incandescent gas burners do not need much description,
+as they are practically similar to those in general use for house lighting.
+They may be either of the erect or inverted forms, the latter being
+preferable owing to the light being more concentrated, and a reflector is
+provided to increase the illumination (Fig. 2).
+
+These reflectors should be _spherical_ and so adjusted that the radiant is
+in the centre of curvature, thus ensuring that the light from the reflector
+passes again through the original source. If this point is not attended to,
+we shall be dealing with essentially two sources of light instead of one,
+to the detriment of the definition.
+
+The same remark applies to every lantern illuminant {8} which is
+supplemented by a reflector, and it is extraordinary how often it is
+neglected by the manufacturer. Of course the opacity of the illuminant
+destroys much of the efficiency of the reflector, and hence in the case of
+incandescent gas mantles there is not much real gain in making use of them,
+but with these comparatively weak illuminants every fraction tells, and the
+reflector does not add much to the cost.
+
+[Illustration: FIG. 2.--Inverted Incandescent Lamp.]
+
+In light the inverted gas burner is very little superior to oil, but it is
+whiter, slightly more concentrated, and freer from smell, and therefore to
+be regarded as preferable if a supply of gas is available.
+
+METHYLATED SPIRIT BURNERS.--Incandescent mantles heated by methylated
+spirit are also largely used, and provide a light decidedly superior to gas
+and nearly equal to acetylene. Some arrangement must be made for
+volatilising the spirit and driving the vapour out under pressure, and the
+most usual contrivance is somewhat as illustrated in Fig. 3.
+
+In this apparatus the spirit is contained in a metal reservoir at the rear
+and air pressure is provided by a pair of rubber balls and valves after the
+manner of a medical spray. Sufficient {9} pressure having been obtained,
+the liquid spirit is forced into a vaporising chamber immediately behind
+the mantle, and a kind of miniature pitchfork, with its prongs wrapped in
+asbestos wool, is soaked in spirit, and pushed over the brass fitting of
+the burner in such a way that when lighted the flame heats the chamber and
+volatilises the spirit. The burner can now be lit, and although the fork
+burns out in the course of a minute or so, the heat from the mantle itself
+is thereafter sufficient to vaporise the spirit as rapidly as required.
+This lamp works exceedingly well in practice, but has one drawback, viz.
+that it is possible to obtain too much pressure and squirt _liquid_ spirit
+through the burner, when it naturally catches fire and may even run on to
+the floor.
+
+[Illustration: FIG. 3.--Methylated Spirit Burner.]
+
+An accident of this sort is rare and usually harmless even if it does
+occur, but an audience is easily frightened, and hence this burner should
+only be used by _an operator with experience_. An altogether better
+arrangement is that made by Messrs. Hughes of Kingsland and known as the
+'Luna' Lamp (Fig. 4).
+
+In this burner there is no pump and no volatilising chamber; {10} the
+spirit is contained as before in a metal reservoir and a separate burner
+underneath is used to keep this sufficiently hot to both vaporise the
+spirit and provide the necessary pressure. The heat can be regulated by
+means of an adjustable sheath to the burner, and a simple safety valve
+provides against an excess of vapour.
+
+I do not say that an accident of the sort previously referred to is
+impossible even with this burner, but I have never heard of it happening,
+and the lamp is certainly the best apparatus of its kind that I am
+acquainted with.
+
+[Illustration: FIG. 4.--Luna Lamp.]
+
+INCANDESCENT ELECTRIC LAMPS.--Incandescent electric lamps of the ordinary
+metal or carbon filament type are also frequently used in small
+class-rooms, and should be mentioned here, as they provide approximately
+the same illumination as a gas mantle, or in some cases rather better. It
+will, however, be more convenient to deal with the question of electric
+lighting as a whole in the chapter devoted to it.
+
+It will suffice here to say that lamps are made for the purpose with a
+special filament arranged to provide a concentrated light, the ordinary
+type being almost useless in this respect, and that small battery lamps,
+worked by a suitable accumulator, can also be used, but except under very
+special circumstances are hardly worth the trouble of keeping the batteries
+charged.
+
+       *       *       *       *       *
+
+
+{11}
+
+CHAPTER IV
+
+ACETYLENE
+
+There is no doubt that at present acetylene holds second place to electric
+light in popularity for optical lantern work. The light is good; not, it is
+true, _so_ good as limelight or the electric arc, but still sufficient for
+a picture up to 12 feet in diameter at a working distance from the screen
+of not more than 30 feet, and this suffices for the large majority of
+halls.
+
+It has great advantages over limelight in convenience and cheapness,
+although on both these points it must yield place to the electric arc,
+always providing that current is available, and therefore it is chiefly
+used in country districts and in gas-lit halls in large towns.
+
+Acetylene gas is formed, as is well known, by the action of water upon
+carbide of calcium, and the generators constructed for lantern work are
+essentially the same in construction as for other purposes.
+
+The alterations introduced are chiefly directed towards obtaining a light
+as _steady_ as possible from a comparatively small generator, and,
+secondly, towards the entire elimination of smell, which obviously is far
+more serious in a lecture hall than, for instance, on a motor car. The
+generators in most common use may be divided into two classes, i.e. those
+on the gasometer principle in which the carbide is gradually lowered into
+the water, and those in which the water is allowed slowly to gain access to
+the carbide. A good example of the former is perhaps that made by Messrs.
+Moss of Birmingham, though there are several others equally good, and clear
+and explicit directions for working should be supplied by the makers. The
+Moss Generator (Fig. 5) consists of a tall iron vessel A fitted with a gas
+tap at bottom, this communicating {12} with a vertical iron tube within the
+vessel. Into this container fits the inner bell or container B, divided
+internally into two concentric portions entirely separated from each other,
+but connected by the pipe P P and the tap T.
+
+A guide inside the bell encircles the iron tube in the outer tank and
+prevents rotation. Into the inner portion fits again the carbide-container
+(shown separately on the left), which is locked when in place by giving it
+a half turn, when a hook inside the bell engages with the lower edge of the
+carbide container and prevents it from falling.
+
+The carbide container is fitted with a series of shelves, and the contents
+of a 2 lb. tin of carbide should be roughly divided among them; there is no
+need to make any accurate division. The carbide used should be that known
+as ½ inch mesh, and should be _pure_. That described as 'chemically'
+treated is apt to give trouble by over-generation in these gasometers and
+should be scrupulously avoided.
+
+[Illustration: FIG. 5.--The Moss Generator.]
+
+The carbide having been placed in the receptacles, these should be closed
+by means of the loose flap and the whole pushed into the bell and secured.
+
+Water should be poured into the outer vessel up to a mark on the iron tube,
+and the bell placed in position. The lower tap being then turned on and the
+upper one closed, air from the outer portion of the bell can gradually
+escape by means of the iron tube and lower tap, and the bell gradually
+sinks by its own weight until it is on the bottom, but still {13} no water
+can reach the carbide, the air imprisoned in the inner portion of the bell
+effectually excluding it.
+
+The lower tap should now be connected by means of india-rubber or flexible
+metallic tubing to the burner in the lantern (of which more anon), and the
+upper tap on the generator turned on, the tap or taps on the burner being
+likewise opened. The air from the inner portion of the bell can now escape
+by the pipe P P into the outer part, and thence through the iron tube, and
+out through tubing and jet, and as it does so water will rise in the
+interior and attack the carbide.
+
+In a few moments the burner can be lit; but the gas, being generated far in
+excess of requirements, and filling both the inner and outer portions of
+the bell faster than it can escape, lifts the latter until the carbide is
+entirely out of the water, when in a few minutes generation ceases.
+
+If the jet is left burning the bell will gradually sink again as the gas is
+used up, and should thereafter maintain an automatic balance without
+attention.
+
+It can be turned off at any moment by simply closing the taps at the jet
+or, better, the lower tap at the generator, when the bell rises
+sufficiently to take the carbide out of the water; but if it is required to
+leave the generator unlit for a considerable time, it is better to turn off
+the tap on the top first. This causes the inner portion of the bell to fill
+with gas which cannot escape, and as that in the outer part burns out, the
+bell sinks to the bottom and remains there, the gas itself imprisoned in
+the inner chamber excluding the water from the carbide. The exact
+arrangement varies in different patterns of generator, but the above may be
+taken as roughly indicating the action, and further information may always
+be obtained from the maker or dealer.
+
+_Emptying_ should always be done out of doors, as the odour of acetylene
+gas is most objectionable, and for the same reason rubber tubes, &c.,
+should be securely tied on, so that the slightest escape may be avoided.
+{14}
+
+If the exhibition has been a short one it will often be found that the
+upper cells have not been affected by the water, in which case they may be
+put back in the tin and used again, but it is not generally advisable to
+put in less than the full charge to begin with as the weight of the carbide
+plays a definite part in securing the smooth action of the apparatus. The
+sludge should be thrown away (it forms a good manure for the garden) and
+the entire generator thoroughly dried, otherwise rust will quickly appear.
+
+[Illustration: FIG. 6.--The A.L. or 'Popular' Model.]
+
+Theoretically one of these generators may be filled and left standing
+indefinitely, but in practice it is not advisable, as the damp in the
+atmosphere is apt to produce a very slow generation of gas, sufficient
+often to cause a decided smell.
+
+Of generators which act by admitting water to the carbide perhaps the best
+known is the A.L. or 'Popular' Model (Fig. 6), this being, in fact, a
+pattern designed for motor-car head-lights, but which answers well for
+lantern work.
+
+Its exact operation need hardly be described here in full detail. It will
+suffice to say that the water gains access to the carbide by 'creeping' up
+between two concentric copper cones, and in the event of over-generation
+the pressure of the gas automatically checks the flow.
+
+This generator is smaller than the gasometer pattern, and hence can be
+recommended for portability; but in my experience the light is not quite so
+steady, and the control rather less delicate, thereby causing on occasions
+a perceptible smell, especially if left standing for a considerable time.
+
+There are other types of generators, such as the 'Water {15} dropping'
+variety, in which the water drips on to the carbide, and the reverse, in
+which fine granulated carbide drops a little at a time into water; but
+these types are not very frequently met with and need hardly be described.
+
+It should never be forgotten that acetylene is an explosive gas and should
+be treated as such. Searching for a leak with a lighted match, though
+perhaps permissible when the operator knows his business, may be a
+dangerous proceeding when the contrary is the case.
+
+[Illustration: FIG. 7.--Acetylene Jet.]
+
+Acetylene burners are generally of the 'Batswing' type, and are as a rule
+four in number, mounted in a row with a reflector behind, each burner being
+separately controlled by its own tap (Fig. 7). An acetylene flame is very
+smoky, and care must be taken that the burners are not turned too high. A
+nipple cleaner, consisting of a fine wire in a short handle, can usually be
+obtained from any dealer, and is very handy.
+
+Acetylene gas can also be used for lantern illumination in quite another
+way, viz. by a blast from a blowpipe, in combination with either air or
+oxygen, on to a special 'Pastille' provided for the purpose, or an ordinary
+limelight jet can be used. These methods entail the use of acetylene _under
+pressure_, and are so analogous to limelight that I shall for convenience
+deal with them in the chapter devoted to that illuminant.
+
+       *       *       *       *       *
+
+
+{16}
+
+CHAPTER V
+
+LIMELIGHT AND THE ACETYLENE BLAST
+
+The illumination possible with this light is almost unlimited, and for
+really large halls it is, as remarked before, the _only_ substitute for the
+electric arc. It consists essentially of a blowpipe flame, composed of
+oxyhydrogen, oxyether, oxyspirit, oxy-acetylene, &c., or acetylene air
+blast, heating to incandescence a block of lime, or other refractory
+material, and the essential feature is that one at least of these gases
+must be under _pressure_. Thirty years ago this was usually achieved by
+storing the gas in rubber bags, and obtaining the requisite pressure by
+means of heavy weights; but except in a very few outlying districts this
+method has now been completely superseded by the use of compressed gas
+cylinders. The earlier editions of this work contained very full directions
+for manufacturing gas for storage in bags, but it is so exceptional now to
+find an operator who uses this method that it seems hardly necessary to
+devote much space to it, and the same may be said of automatic oxygen
+'generators.' The present work will therefore deal chiefly with compressed
+gas cylinders.
+
+Most elaborate precautions are now enforced by the Board of Trade to ensure
+the absolute safety of these, and any doubt existing from occasional
+accidents of years ago may be promptly dismissed. Humanly speaking, an
+accident nowadays _cannot_ happen, except by such wilful negligence on the
+part of the maker or filler as would almost render the culprit subject to
+criminal proceedings.
+
+Compressed gas cylinders are painted a distinctive colour, oxygen for
+example being black and coal gas or hydrogen red; the screw connections to
+the pumps, and all nozzle {17} and regulator fittings, are made with a
+totally different screw and therefore cannot be interchanged; the cylinders
+themselves are bound by law to be reannealed and retested under hydraulic
+pressure at regular intervals; the steel itself has to be of a guaranteed
+quality; and, in fact, every possible risk is guarded against.
+
+The most usual sizes of cylinders supplied for lantern exhibitions are
+those containing 6, 12, 20, or 40 cubic feet, and are usually sent out in
+wooden or hemp cases.
+
+[Illustration: FIG. 8.--Oxygen Cylinder in hemp cover.]
+
+Fig. 8 shows a 12-foot cylinder in its hemp case, the approximate size
+without case being 22 in. by 4 in. This size cylinder will supply an
+average limelight jet for just over two hours. The extra powerful jets as
+used for cinematograph work or for illuminating a very large screen take a
+good deal more, but for the usual apparatus as supplied for ordinary
+lantern purposes this is a pretty safe figure.
+
+A 12-foot cylinder is therefore the favourite size for a lantern exhibition
+lasting from an hour to one and a half hours, as it leaves a fair margin
+for gas used in adjusting the instrument, &c., and a 20-foot cylinder will
+usually suffice for _two_ such exhibitions.
+
+The price of gas per cubic foot varies with the size of the cylinder, being
+less for large cylinders than for small ones, and the cost of transit is
+also less in proportion--hence it is frequently an economy to hire a large
+cylinder and retain it for several exhibitions. On the other hand most
+suppliers charge a small rent if a cylinder is retained beyond a definite
+{18} time, so this is a question to be decided by each user on its own
+merits.
+
+Alternatively, of course, cylinders can be _purchased_, and the question of
+rent does not then come in; also gas is supplied a little cheaper in a
+customer's own cylinder than if sent on hire. If purchase is decided on it
+is frequently an economy to buy _two_, or two of each gas, if coal gas
+cylinders are required as well.
+
+[Illustration: FIG. 9.--Double Lever Key.]
+
+The whole contents of the cylinders can then be used up without waste, as
+if a cylinder should become exhausted during the course of a lecture, it is
+only a matter of a minute or two to change over to the spare one, whereas
+the compressors are required by law to empty out every cylinder returned to
+them for refilling, and any remaining gas is thereby wasted.
+
+It is extremely tantalising, to say the least of it, to find the pressure
+gauge indicating that there is, say, 8 feet of gas remaining in a cylinder,
+and to be compelled to waste this or else risk running short for the next
+exhibition, and duplicate cylinders are the only way of avoiding the loss.
+
+The cylinders are filled to a pressure of 120 atmospheres, or 1800 lb. per
+square inch, and are closed by strong screw nozzles. The keys for opening
+or closing these are of three types, viz. the 'T' pattern, 'Spanner'
+pattern, and that known as the 'Double Lever' type. This latter is so made
+that in closing the valve it shuts up to half its length and {19} opens out
+to double the leverage when being used to _open_ the cylinder (Fig. 9). The
+idea is to avoid the possibility, which has been known to occur, of the
+cylinder valve being screwed down by a powerful wrist and defying the
+efforts of the despairing lanternist to open it.
+
+[Illustration: FIG. 10.--Fine Adjustment Valve.]
+
+Cylinder nozzles are unfortunately not yet standardised, but those most
+frequently met with in this country are those adopted by the British Oxygen
+Company, both oxygen and coal gas cylinders being fitted with corresponding
+_internal_ screws 7/8 inch diameter, those for oxygen being _right-handed_,
+and those for coal gas _left-handed_, and in each case terminated at the
+bottom by a hollow metal cone.
+
+As such an internal screw cannot obviously be connected to a piece of
+rubber tubing, some type of screw connector must be employed, and this may
+take one of three forms: (1) A simple connecting nozzle, (2) a fine
+adjustment valve, or (3) a regulator. The first is seldom used in practice
+for lantern work, for the reason that the direct pressure of a full
+cylinder (120 atmospheres) cannot be checked or controlled by a tap on the
+jet, as the intervening rubber tubing would either burst or blow off, and
+must therefore be regulated at the cylinder nozzle itself, and gradually
+readjusted as the pressure diminishes.
+
+To achieve this regulation with the ordinary cylinder key is difficult,
+though possible to a careful operator, but for a slight extra expense a
+combined nozzle and _fine adjustment valve_ (Fig. 10) can be obtained, and
+regulation with this is {20} infinitely easier. The best plan of all,
+however, is to use an automatic regulator, which not only reduces the
+pressure so as to permit of the required adjustments being made at the
+jet-taps, but also maintains a practically steady supply as the cylinder
+empties, thereby obviating continual readjustments. Regulators are now so
+inexpensive that they have come into almost universal use, and are
+generally reckoned an indispensable part of a limelight lantern equipment.
+The form of regulator in most common use is that usually known as
+'Beard's,' having been originally designed and patented by Messrs. R. Beard
+& Sons, though as the patent has now expired it is open to any firm to make
+the same article if they desire.
+
+[Illustration: FIG. 11.--Construction of Beard's Regulator.]
+
+The construction of Beard's Regulator is shown in Fig. 11. The gas enters
+from below into a rubber bag, C, from which it can emerge through the
+nozzle.
+
+Any accumulation of gas raises the bellows against the pressure of a spiral
+spring pressing it down, and this brings into action an arrangement of
+so-called 'Lazy Levers,' which in turn presses down a small conical valve
+and closes the supply from the cylinder, this valve re-opening immediately
+the pressure diminishes.
+
+The outward form of this regulator is shown in Fig. 12, {21} which
+incidentally also illustrates the usual form of connection to the cylinder,
+referred to later on.
+
+In Beard's Regulator the pressure at which the gas can be delivered is
+determined by the strength of the spiral spring, and can only be altered by
+changing this spring.
+
+[Illustration: FIG. 12.--Beard's Regulator.]
+
+In practice Beard's Regulators are supplied set to a low pressure for
+ordinary mixed or 'blow-through' jets and for a higher pressure (14-16
+inches) for 'injector' jets. At this latter pressure the rubber tubing used
+must be fairly thick and strong and well tied on, and even so the taps of
+the jet should not be turned entirely off unless the gas at the cylinder is
+likewise turned off immediately afterwards. The British Oxygen Company make
+a regulator which can be set to any desired pressure, but it is not quite
+so delicate in its action as Beard's, and Messrs. Clarkson also make a
+pattern regulator which is widely used and well spoken of. The attachment
+of any of these fittings to the cylinder is a somewhat peculiar one, as
+will be seen on reference to Fig. 10 or Fig. 12. The regulator or nozzle
+ends at its lower extremity in a screw and cone, the latter being intended
+to make a gas-tight connection with the internal cone on the cylinder, and
+over this screws a loose wing piece with another outer screw, this latter
+fitting the thread in the cylinder.
+
+In making the connection care must be taken that the wing piece is not
+screwed too far down the inner screw, or the cone will not reach down and
+make a tight fit on its {22} seating; in its correct position the wing
+piece when clamped down should leave a turn or two of its thread exposed,
+in order to ensure that the cone does bed properly.
+
+[Illustration: FIG. 13.--Regulator and Gauge.]
+
+Care should be taken that the nozzle of the cylinder is free from dust
+before attaching any of these fittings: the best plan is first to blow into
+it, and finally wipe it round with the finger. Most professional operators
+_hammer_ the wing piece home with a spanner or other convenient implement a
+barbarous method and really unnecessary if the cones are in good condition,
+but, nevertheless, almost always adopted in practice.
+
+PRESSURE GAUGES.--These are useful in determining the amount of gas
+remaining in a cylinder and are of a very usual type; they may either be
+screwed on to the cylinder before commencing to work and taken off again to
+screw on the regulator, or they can be supplied fitted to the regulator
+itself, in which case they can be observed during the course of the
+exhibition (Fig. 13). As the same gauge may be used for cylinders of
+different sizes (though _never_ for those containing {23} different gases),
+they simply register in atmospheres, and knowing that a full cylinder shows
+a pressure of 120 atmospheres, the requisite calculation must be made to
+determine how many cubic feet are unused.
+
+In the case of oxygen cylinders an approximate idea of the amount of gas
+remaining can be got by _weighing_ it carefully when known to be either
+absolutely full or absolutely empty, and re-weighing it when information is
+required. Oxygen weighs approximately 1.4 oz. per cubic foot, and this is
+easily detected by an average scale. Coal gas is too light to be gauged in
+this way.
+
+GAS-BAGS AND GENERATORS.--It has already been remarked that there are two
+alternative methods of obtaining gas under pressure for limelight purposes,
+viz. gas-bags and generators (the latter for oxygen alone: there is no good
+hydrogen generator that I know of). In both these cases the oxygen is
+generated by heating a mixture of chlorate of potash and manganese black
+oxide. In the case of gas-bags the gas is prepared beforehand and passed
+through suitable purifiers into a rubber gas-bag. With a generator the
+oxygen is evolved during the exhibition itself; but this method has never
+come into very general use.
+
+Coal gas or hydrogen is very seldom home generated; a gas-bag can, if
+necessary, be filled a few miles away and brought full to the place of
+exhibition, or filled on the spot if gas is laid on; or, failing this,
+acetylene or ether, or even methylated spirit may be utilised instead.
+
+The bags in use are placed between double pressure boards (if _both_ gases
+are required under pressure) and weights sufficiently heavy placed on the
+top (Fig. 14), or with a 'blow-through' jet only the oxygen need be stored
+in a bag and the coal gas used from the supply main.
+
+Cylinders have, however, so universally superseded these appliances, that
+space is hardly warranted in fully describing them, especially as any
+operator wishing to adopt {24} the process can obtain full directions from
+any responsible dealer.
+
+LIMELIGHT JETS.--These are of three general types, viz. the 'Blow-through,'
+the 'Mixed,' and the 'Injector.'
+
+Of these the 'Blow-through' is now very little made, having been largely
+superseded by the 'Injector' pattern, but, as there are hundreds in common
+use in this country, they cannot yet be regarded as a thing of the past.
+
+[Illustration: FIG. 14.--Gas-bags.]
+
+The exact design of this jet varies considerably, but all are alike in
+this, that a jet of coal gas is burned at the orifice of a more or less
+open nozzle, and a stream of oxygen _blown_ _through_ it on to a cylinder
+of lime which it thereby renders incandescent. Fig. 15 represents the
+various designs chiefly adopted for this jet, that marked A being perhaps
+the most usual, though C is also frequently met with.
+
+In light-giving power there is not much to choose between the various
+types; probably D on the whole is the best in this respect, but so much
+depends upon the exact position of the two nozzles, and the _smoothness_ or
+otherwise of that {25} provided for the oxygen blast, that exact
+comparisons are difficult.
+
+[Illustration: FIG. 15.--'Blow-through' Nozzles.]
+
+'Blow-through' jets are the weakest form of limelight as used at the
+present day, and may be taken roughly as some 50 per cent. better than
+acetylene, or in other words, sufficient to illuminate a 12-foot picture at
+a distance of some 40 to 50 feet; but their advantage is, or was, that they
+only required one gas (oxygen) under pressure, the coal gas supply being
+obtained from the ordinary house main.
+
+[Illustration: FIG. 16.--'Blow-through' Jet.]
+
+This advantage is now shared by the more recently introduced 'Injector'
+jets, which give a far better light, and have therefore rendered the
+'Blow-through' type nearly extinct.
+
+The general construction of a 'Blow-through' jet is shown in Fig. 16, and
+it will be seen that a short vertical spindle is {26} provided to carry the
+lime cylinder, and that this can be rotated from the back by means of
+bevelled gear wheels, which at the same time screw the spindle up and down.
+A lime cylinder of the usual pattern being placed on this spindle can be
+rotated from time to time to expose a fresh surface, as that in use
+gradually becomes 'pitted' by the blast, while the screw provides
+sufficient vertical movement to ensure that a complete rotation does not
+bring round the same position again.
+
+Some arrangement is also generally provided by which the distance between
+the lime spindle and the jet can be adjusted. The exact position of this
+does not matter within a reasonable margin, but limes vary in size, and
+'Pastilles,' and other substitutes for limes, which will be referred to
+later, vary still more, at any rate as regards this adjustment. The average
+distance which gives the best result is usually about half an inch, and
+once set need not be altered with that particular jet unless a lime of
+different size is employed; minor variations due to limes being drilled
+slightly out of centre, &c., do not seriously matter.
+
+There is no accepted rule for colouring jet-taps in accordance with the
+cylinders, and although jets are sometimes met with painted in this way,
+_i.e._ red for coal gas and black for oxygen, it is more usual to find coal
+gas taps _black_ and oxygen _bright_, or sometimes both black or both
+bright. Care must therefore be taken that the right cylinder is connected
+to the right tap on the jet, but there should be no difficulty in telling
+which is which, and fortunately any mistake, even if it be made, is quite
+harmless.
+
+THE MIXED-GAS OR DOUBLE-PRESSURE JET.--This jet is fundamentally different
+from the 'blow-through' form, inasmuch as the two gases are combined in one
+mixing chamber before combustion, and burn in their correct proportions at
+one nipple.
+
+It is usually stated that this jet necessitates both gases being under
+equal or approximately equal pressure, but this {27} is not literally
+accurate, and I have given many a lantern exhibition with one of these
+jets, using coal gas from the ordinary supply, and oxygen from a cylinder.
+To use a mixed jet in this way needs care, as a very slight excess of
+oxygen puts the light out with a 'pop' which, although not dangerous, is
+disconcerting, while the light obtained under these conditions is very
+little better than with a 'blow-through' jet, and far inferior to the
+'Injector' jets to be described next.
+
+[Illustration: FIG. 17.--Mixed Jet.]
+
+[Illustration: FIG. 18.--Mixed Jet, Gwyer pattern.]
+
+The mixed-gas jet is intended then to be used with both gases under
+pressure, and is the _only_ jet to be seriously {28} considered in cases
+where a really powerful light is required. The power of this jet is indeed
+almost unlimited, and those made with large bores, such for example as used
+for cinematograph work, provide a light amounting often to some two or
+three thousand candles, and consume an enormous amount of gas; but the
+ordinary pattern, with a nipple of one-twentieth to one-sixteenth of an
+inch bore, and using some 5 feet of each gas per hour, or perhaps slightly
+more for the coal gas, will suffice for all ordinary work.
+
+[Illustration: FIG. 19.--Mixing Chamber of Jet.]
+
+The mixed-gas jet, like the 'blow-through,' is made in many forms, but
+these may be roughly divided into two main types, viz. those with small
+mixing chambers immediately below the nipple (Fig. 17), and those with
+larger chambers in the horizontal part of the jet as in the 'Gwyer' pattern
+(Fig. 18).
+
+The construction of the mixing chamber itself varies also, but that
+advocated by my father, the original author of this work, is generally
+followed, the chamber being packed with alternate discs pierced as in Fig.
+19, which ensures a thorough mixture of the gases. A layer or two of gauze
+is often introduced as well by way of further improvement. The distance
+between the lime and nipple is much less than with the 'blow-through' jet,
+and the adjustment has to be more exactly made. About 1/8 inch is
+approximately correct for a jet of moderate power, and rather more for a
+bigger bore; also care must be taken to turn the lime frequently, as the
+latter 'pits' pretty quickly with these jets, and if it is neglected the
+jet may spurt back out of the hole, which is gradually formed, and crack
+the condenser. {29}
+
+There is still an erroneous opinion extant that these jets are dangerous,
+and if the operator is working with the now obsolete gas-bags it is
+certainly a fact that an accident in careless hands is _possible_; but with
+cylinders there is, so far as I know, no possibility even of an accident
+under ordinary conditions.
+
+It is true that if too much oxygen is turned on the jet may suddenly go out
+with a loud snap or pop, and this is in reality a miniature explosion in
+the mixing chamber; but it can in any case hardly be serious enough to
+matter, though I have found after such a snap that the gauze packing,
+inside the chamber above referred to, has been pierced right through, and,
+when first lit afterwards, the jet has for a few minutes burnt with a
+characteristic green flame, denoting the presence in the gas of fine copper
+or brass particles.
+
+To obtain a good light with these jets, and in fact with _all_ jets, great
+care must be taken that the nipple is absolutely smooth, otherwise the
+flame is bound to hiss. The simplest plan is to slightly roughen a suitable
+sized needle with emery paper and to burnish the inside of the nipple from
+time to time with this. Especially if there has been one of the 'snaps'
+referred to is it desirable to see that the inside of the nipple is
+thoroughly smooth and polished.
+
+MANIPULATION OF THE MIXED-GAS JET.--On this point there is not much to be
+said. A good hard stone lime must be used--'soft' limes are useless for
+this jet--and the coal gas flame should be lit first, and the lime
+thoroughly heated with this before the oxygen is slowly turned on. As the
+oxygen increases the flame will gradually disappear and the light increase,
+until it is at a maximum for that particular amount of coal gas. This
+latter can then be turned on a little more, and more oxygen passed to
+balance it until the jet begins to 'roar,' when we are getting the maximum
+light for that particular sized nipple. When the two gases are, however, in
+the proper proportion to give the best light, there will always be a slight
+excess of coal gas flame visible playing about the lime. {30}
+
+THE INJECTOR JET.--This is essentially a mixed jet, and in outward
+appearance differs but little from one of the ordinary type (Fig. 20), but
+is so constructed that the pressure of oxygen 'sucks' coal gas into the
+mixing chamber, and so obviates all necessity for the latter being under
+pressure.
+
+With this jet there is little or no danger of the jet 'snapping' out
+through a surplus of oxygen, as the greater the flow of this gas, the
+greater the suction on the coal gas side.
+
+The light is not quite equal to a good mixed jet, but very nearly so, and
+therefore this jet is deservedly gaining in favour every day.
+
+[Illustration: FIG. 20.--'Injector' Jet.]
+
+One point must be noted: the oxygen itself must be under greater pressure
+than with the ordinary mixed jet if the best light is to be obtained, and
+therefore a special regulator must be used, or one of ordinary type
+modified (which can easily be done by the maker), and rubber connections
+must be securely tied both on to jet and regulator, as the pressure
+required to work this jet to advantage, while not enough to burst a rubber
+tube, is enough to blow it off an easy fitting connection.
+
+THE OXYETHER LIGHT.--This is practically similar to the oxyhydrogen, except
+that ether vapour is used in place of the hydrogen or coal gas. The method
+adopted consists essentially of passing a stream of oxygen through a vessel
+packed with some porous material (such as cotton wool or cotton gauze)
+which is saturated with ether. The oxygen {31} becomes saturated with ether
+vapour, and the mixture is then used in place of the coal gas supply in a
+double-pressure jet, an additional supply of free oxygen being still
+required through the ordinary oxygen tap.
+
+The arrangement is cheap, as it dispenses with the necessity for a coal gas
+cylinder, and effective, as the light is little, if at all inferior to the
+oxyhydrogen, but differs from the latter in this, that with careless
+handling an accident is _possible_.
+
+In competent hands there is no danger, and I have used ether saturators
+myself scores of times without one single contretemps; but it should _not_
+be entrusted to any chance amateur.
+
+The use of the ether light has a curious history. In the earlier days
+before the proper construction of ether saturators was understood, and
+gas-bags were still in vogue, it was largely condemned on the score of
+danger. Modern improvements in apparatus rendered it perfectly safe against
+anything but gross carelessness or bungling, and the London County Council
+and other similar bodies immediately supplied it broadcast to elementary
+schools (in disregard of warnings offered by myself and others), where it
+was often entrusted to incompetent operators or even senior boys. So far as
+I know no serious accident ever resulted, a pretty conclusive proof that
+the light is really safe, but in time the London County Council realised
+that the universal adoption of this illuminant was not advisable, and I
+believe _now_ prohibit it altogether in halls licensed by them for
+entertainments.
+
+In time, no doubt, they will learn to adopt a sane policy between the two
+extremes, but at present the official attitude in many localities has
+placed ether saturators out of the running, and before purchasing one the
+would-be operator should ascertain that he will be allowed to use it.
+
+Ether saturators as made at the present day may be divided into two
+principal patterns, viz. those in which saturator and jet are combined in
+one piece of apparatus {32} which fits bodily into the lantern, and
+saturators which are used outside and connected by means of tubing to any
+ordinary oxyhydrogen double-pressure jet.
+
+Both forms have their advantages and disadvantages; the first pattern tends
+to become too warm from its position in the lantern and generates ether
+vapour too quickly, while the second has the fault of becoming too cold
+(owing to evaporation of the ether) and therefore not vaporising quickly
+_enough_.
+
+[Illustration: FIG. 21.--'Gridiron' Saturator.]
+
+Writing at the present date, when manufacturers are slowly beginning to
+resume their normal occupations after the stress of war work, it is
+impossible to say exactly what models will or will not be made, but I will
+mention one typical example of each pattern as made in pre-war days.
+
+The first of these is the 'Gridiron' (Fig. 21), adopted largely by the
+London County Council in the days I have referred to, and certainly one of
+the best designed saturators ever put on the market.
+
+In the 'Gridiron' saturator there are three taps: two at the {33} rear and
+one in front, between the saturator and the mixing chamber. Between the
+rear taps is the inlet for the oxygen, which divides into two channels,
+that on the left passing upwards through the U tube shown in the
+illustration (the corresponding tube on the right is merely a dummy), and
+thence through the saturator and out through the horizontal tube and tap
+into the mixing chamber, whence the saturated stream of oxygen finally
+passes to the nipple, and the combination burns with a whitish flame
+closely resembling that produced by coal gas.
+
+[Illustration: FIG. 22.--'Pendant' Saturator.]
+
+The other channel for the oxygen is to the right, down the vertical tube
+shown there (the lower vertical tube on the left is also a dummy),
+underneath the saturator, and finally coming up into the mixing chamber
+from below, transforming the white flame into an intensely hot blowpipe
+exactly as it does with a coal gas jet. The front tap controls the supply
+of saturated ether to the mixing chamber, and whereas at first a good
+stream of oxygen is needed to pick up enough ether, by degrees as the
+instrument warms in the lantern, the oxygen passing through the saturator
+can be cut off entirely, and even then the front tap must be gradually
+closed down to prevent the hot ether coming off too fast.
+
+There is a disagreeable feeling of 'sitting on the safety-valve' in doing
+this, but in reality the pressure is never likely to become great enough to
+cause danger.
+
+Of saturators for use outside the lantern the best-known is probably the
+'Pendant' (Fig. 22). With this instrument the oxygen supply is connected to
+the inlet marked A; B goes {34} direct to the oxygen tap of any ordinary
+mixed-gas jet; while C, from whence issues the saturated stream, is
+connected to the coal gas tap of the jet. Whichever pattern is used, the
+essential thing is to keep a good supply of oxygen well saturated. If the
+lime becomes incandescent without any free oxygen, or it is found that this
+requires gradually turning off, it indicates that the saturation is
+becoming defective, and to continue is to risk the jet snapping out. In the
+case of an outside saturator such as the 'Pendant,' this may even blow off
+the connecting tubes with a loud report, though no worse accident is likely
+to happen, and for this reason an outside saturator should be placed _as
+close_ to the jet as possible, so that the rubber tube may be kept short,
+and incidentally this keeps the saturator warm and accelerates
+vaporisation.
+
+As ether vapour usually contains a certain amount of moisture which does
+not vaporise to any great extent, this gradually accumulates and the
+capacity of the instrument becomes reduced. It is therefore usually
+necessary to return a saturator to the makers every now and again for
+repacking.
+
+The only real danger with a modern saturator is not in using but in
+_filling_. This should be done if possible in the open air, and at any rate
+never near a light. Ordinary sulphuric ether of specific gravity 720-730 is
+usually considered the best, and a quarter of a pint will keep an ordinary
+small-bore jet going for nearly two hours.
+
+More precise directions are usually sent out by the makers, and as the
+various patterns of saturator in use are pretty numerous, it would be
+useless here to attempt more detailed instructions for working.
+
+OXY-ACETYLENE JETS.--Any good mixed gas jet may be used with acetylene
+instead of coal gas, provided that it is under pressure more or less
+corresponding to that from an oxygen cylinder, and at the present day there
+is no difficulty in obtaining this, in civilised countries at all events,
+by {35} means of compressed or, to speak more correctly, 'dissolved'
+acetylene cylinders, referred to later on.
+
+With an 'Injector' jet there is no need for the acetylene gas to be under
+pressure at all, and a simple generator such as described on page 12 will
+answer perfectly, though in practice very seldom used. With such a
+generator the pressure is so low that in many cases the jet will not even
+burn until _some_ oxygen is turned on; but this introduces no real
+difficulty, as with a good 'Injector' a snap is practically impossible,
+provided the generator is large enough to evolve sufficient acetylene. It
+is far better in every way, however, to use the acetylene from a cylinder,
+just as with coal gas. Only in this case the cylinder is completely filled
+with a porous material, and this again filled with liquid acetone or other
+suitable fluid, in which the acetylene is dissolved as rapidly as it is
+pumped into the cylinder.
+
+To compress acetylene in the ordinary way is neither safe nor practicable;
+but these 'dissolved' cylinders are now used extensively for both
+oxy-acetylene welding and motor car lighting, and may be entirely relied
+upon.
+
+The D.A. (Dissolved Acetylene) Company were the pioneers in this country of
+the industry, and their methods of business are peculiar and ingenious. The
+user is requested in the first place to purchase a cylinder, and he then
+becomes the owner of _a_ cylinder, but not of one _particular_ cylinder. A
+list is supplied to him of various depots in the country where the
+Company's cylinders are stored, and when empty he can, on payment of a
+fixed sum, exchange his empty cylinder for a full one, which then becomes
+_his_ cylinder _pro tem_.
+
+This saves the delay and expense of returning a cylinder to London, and
+incidentally clears the customer of any question of deterioration, this
+being obviously covered by degrees with each individual exchange. The
+system was first introduced in connection with the lighting of cars and
+only applies to the standard size for this purpose, viz. 20 cubic feet
+capacity, {36} but as this is, on the whole, the most convenient size for
+lantern work also, the limitation is not a disadvantage. The arrangement is
+also in vogue to a less extent with cylinders of 6 feet capacity (a size
+sometimes used for motor _cycles_), but the depots of exchange are at
+present far fewer for this size.
+
+The oxy-acetylene blast is much _hotter_ than the ordinary oxyhydrogen, and
+therefore produces a more intense light. I have therefore used it with
+success on occasions when even the ordinary limelight would fail, and the
+choice has lain between an oxyhydrogen jet of enormous bore (and, of
+course, corresponding consumption of gas), and the oxy-acetylene.
+
+For this very reason great care must be taken only to use the hardest
+limes, and even then to use the lime-turning movement frequently, or the
+lime will pit or crack and a broken condenser follow.
+
+THE FALLOT ACETYLENE LIGHT.--This light consists of a jet of acetylene
+under pressure, without oxygen, but producing its own _air blast_ from the
+atmosphere by suction, much as the 'Injector' jet does, but the reverse way
+round.
+
+The light is better than with an ordinary acetylene jet, though not quite
+so good as with a 'blow-through' jet; but as it only requires a cylinder of
+dissolved acetylene, or even a 'Pressure' generator, it is fast coming into
+favour.
+
+The peculiarity of the Fallot apparatus is that, instead of providing a
+direct beam of light in the direction of the screen, it projects the beam
+_backwards_ on to a concave mirror, and it is the reflected light from this
+that is used (Fig. 23).
+
+Instead of a lime is used a spherical 'Pastille' of peculiar composition,
+and before use each pastille must be burnt off exactly like an incandescent
+gas mantle, after which it is extremely fragile and difficult to handle.
+
+To use this illuminant one lens of the condenser must be removed, the
+curvature of the mirror taking its place, and it will be seen at once that
+the pastille itself will get in its own {37} light and throw a shadow,
+which actually happens, but it is hardly perceptible unless specially
+looked for.
+
+[Illustration: FIG. 23.--Fallot Air Blast.]
+
+A complete Fallot Air Blast Outfit, with cylinder, fine adjustment valve,
+pressure gauge and burner, with two spare pastilles, is shown in Fig. 24,
+but if preferred a regulator, such as previously described for oxygen, can
+be used instead of the fine adjustment valve.
+
+[Illustration: FIG. 24.--Fallot Air Blast, and Cylinder.]
+
+{38}
+
+FALLOT OXY-ACETYLENE BLAST.--This is similar to the foregoing, utilising
+oxygen from a cylinder instead of air, and the light is equal to a powerful
+limelight, and may be considered as an efficient substitute, though for
+_long range_ work the shadow before alluded to becomes more noticeable (for
+optical reasons which need not be here discussed). The Fallot Company also
+make a special 'Pressure Generator' which can be used instead of a D.A.
+Cylinder; but my experience of this so far is that, although perfectly
+safe, the blast from it is a little unsteady as compared with a cylinder.
+
+LIMES AND ACCESSORIES.--Limes for Optical Lantern work are usually supplied
+in the form of cylinders, the 'ordinary' size being 7/8 inch in diameter
+and about 1½ inches long, with a hole drilled longitudinally to take the
+lime pin. Extra large limes up to 2 inches in diameter are supplied for
+more powerful jets.
+
+So-called 'soft' limes used to be recommended for 'blow-through' jets as
+giving a better light than 'hard' limes, but the advantage, if any, is very
+little, and these limes are now very seldom heard of, possibly because
+'blow-through' jets themselves are becoming less and less used, and 'soft'
+limes will not stand the heat of a mixed or 'Injector' jet for long.
+
+'Hard' limes are turned out of the hardest stone lime, and must be kept in
+sealed tins until used, as they rapidly disintegrate when exposed to the
+air. There are one or two quarries known to provide the best lime for
+lantern purposes, and the various good brands on the market practically
+have the same origin as regards raw material, though called by different
+trade names; and the 'Hardazion' (hard as iron) limes, placed on the market
+some years ago by a well-known wholesale firm, to be countered shortly
+after by the 'Hardastil' (harder still) brand, are, I take it, legitimate
+though amusing instances of phonetic advertisement.
+
+Even the best of limes is liable to crack under the heat {39} of a powerful
+jet, and so a pair of lime-tongs should always be provided, and there is
+nothing better than the simple form shown in Fig. 25, and which is, or
+should be, sold by all dealers.
+
+[Illustration: FIG. 25.--Lime-tongs.]
+
+SUBSTITUTES FOR LIMES.--A good substitute for lime, that will give the same
+light, stand heat equally as well, and _not_ deteriorate if exposed to the
+atmosphere, has long been sought for, and some of the more recently
+discovered refractory materials are more or less satisfactory. 'Mabor'
+limes, for example, belong to this class, and so do some of the
+'pastilles,' which before the war came chiefly from France and to a less
+extent from Germany.
+
+       *       *       *       *       *
+
+
+CHAPTER VI
+
+THE ELECTRIC LIGHT
+
+The electric current provides _the_ light for an optical lantern, though it
+may take various forms, such as the incandescent glow-lamp in some shape or
+other, the comparatively new Ediswan 'Pointolite' lamp, the enclosed arc,
+and the open arc. This little book is not a treatise on electricity, but a
+few elementary notes may not be out of place, and may be of assistance to
+the non-technical lanternist.
+
+The first point then to be considered in adopting the electric light for
+the purpose of lantern projection is the character of the supply, and the
+information required may be summed up thus: (1) _E.M.F._, _voltage_, or
+_tension_ (these three expressions having exactly the same meaning); (2)
+_ampèrage_ or amount of current available; (3) whether current is (_a_)
+_continuous_, _constant_, or _direct_ (again three words meaning {40} the
+same thing), or (_b_) _alternating_. The E.M.F. or tension corresponds to
+_pressure_, to use the mechanical analogy of a water pipe, and the
+_ampèrage_ to volume, and the voltage of the supply currents in this
+country are usually between 100 and 250 volts. Private lighting sets are
+frequently as low as 50, and current derived from accumulators may be
+anything from a few volts and upwards. _Power_ currents, such as commonly
+employed for tramways, &c., are usually about 500 volts, but the use of
+these currents for lighting purposes, though practicable, is not to be
+advocated.
+
+Ampères and volts are convertible terms in a sense; that is to say, a
+current of 10 ampères at 100 volts requires the same horse-power to
+generate it as one of 5 ampères at 200 volts, or 20 ampères at 50 volts,
+but they are by no means convertible as regards their _efficient_ use for
+our purpose. The ampères used multiplied by the number of volts give the
+total power consumed in _watts_, and 1000 watts used for one hour represent
+1 _unit_ as charged for on our dreaded lighting bills. The current
+available from a public supply may be said to be unlimited so far as our
+purpose is concerned, and the amount actually used depends only on the
+total electrical resistance of our circuit, and this is measured in _ohms_,
+the three factors, viz. volts, ampères, and resistance, being connected by
+the well-known and simple equation C = E / R, C representing the current in
+ampères, E the tension or E.M.F. (electro-motive force) in volts, and R the
+resistance in ohms. The total current we _can_ use, however, is limited by
+the size of the cable laid on in the building, and this is automatically
+safeguarded (or should be) by the _fuses_, which consist, as is generally
+known, of thin wires or strips of tin or lead fixed on a fuse board in an
+easily accessible place, and which melt directly the current exceeds a safe
+amount in ampères. Whatever method of lighting we use therefore, _enough_
+resistance must always be kept in the circuit to ensure {41} that no more
+current can pass than has been provided for, and in the case of an arc lamp
+this usually means a _resistance_ or rheostat being retained in the circuit
+in addition to the arc itself, through which the current is passed and
+absolutely wasted, though fortunately the waste in money is negligible, and
+for reasons to be discussed later such a resistance is necessary with an
+optical lantern arc lamp in any case.
+
+In the case of a glow-lamp, the entire resistance is provided by the
+filament of the lamp itself, and that is why an ordinary metal or carbon
+filament lamp, for say 200 volts, has to be manufactured with an extremely
+long and slender, and therefore fragile, filament, while with an ordinary
+pocket-torch, which is usually supplied with current from a dry battery of
+some 3 or 4 volts only, the filament can be short and thick.
+
+Speaking generally, glow-lamps on a low voltage current can be made more
+efficient than on a high one, and are also longer lived for very obvious
+reasons; but, on the other hand, the transmission of current over long
+distances is cheaper the higher the tension, as for a given number of watts
+the ampèrage is less, and therefore smaller cables can be employed. On the
+whole, then, currents of 200 to 250 volts have during recent years become
+more common than 100, in spite of greater difficulties in making the lamps;
+but occasionally one finds a hall where two or more lamps are wired in
+_series_, two 100-volt lamps for example being wired together in series on
+a 200-volt circuit. If we are using current for our lantern from an
+ordinary lamp socket, this is a possibility that must be borne in mind.
+
+The same considerations, viz. the economy of transmitting power at high
+tension and of _using_ it at a lower one, have been mainly responsible for
+the rapidly increasing number of alternating current circuits now met with,
+especially in sparsely populated districts. An alternating current main is
+one in which the current reverses its direction, usually in this country
+50, but sometimes 60, 80, 90, or even 100 times {42} per second (there
+being unfortunately in Great Britain no standard 'Periodicity' or number of
+cycles per second), and for technical reasons which need not be entered
+upon here, with these alternating currents the tension and ampèrage can be
+mutually converted by means of _transformers_, so that current can be
+transmitted at so high a tension, for instance, as 10,000 volts, and used
+at a voltage of 50 or 100 or whatever is required, the ampèrage available
+being increased in inverse ratio as the tension is decreased. The same
+ready power of transformation unfortunately does not apply to the
+continuous current, or alternating currents would probably never have been
+heard of, but as it is they are very common. For glow-lamps it is
+immaterial which current is available, but for arc lamps the continuous is
+much to be preferred, though both can be utilised.
+
+With these initial remarks, I will now take in order of illumination the
+various methods of utilising the electric current for optical lantern work.
+
+THE ELECTRIC GLOW-LAMP.--The ordinary metal filament lamp is not very
+suitable for lantern work, the light not being sufficiently concentrated,
+but from what has already been said, it will be evident that this method of
+lighting is more suitable where currents of low voltage are available.
+
+An extremely good and intense light can be obtained from a comparatively
+small battery of accumulators, which can easily be carried in the hand, and
+a short and thick metal filament lamp, similar to those supplied with a
+powerful electric torch; and this arrangement is actually used to some
+extent by travelling lecturers, but the mess and trouble of keeping the
+accumulators in order have prevented the method being generally adopted.
+
+When _alternating_ current is available such a lamp will work well with a
+transformer to step down the voltage to the required degree, and the
+arrangement is simple, cheap, and efficient, and produces a light at least
+equal to that from {43} acetylene. In comparatively small halls, where the
+current is alternating, this is undoubtedly the best method of working, as
+it is simpler than the arc and amply brilliant enough for all practical
+purposes.
+
+With the continuous current the problem is not so simple, as transformation
+of voltage is not an easy matter, and a glow-lamp on; say, a 200-volt
+circuit involves a long and fragile filament, which it is difficult to
+arrange in a small space.
+
+Many years ago the Ediswan Company produced a 'Focus' lamp for the purpose,
+with the filament arranged in the form of a square grid, and this lamp gave
+a light of about 100 candles, and was fairly successful for a small room.
+More recently the Osram Company introduced a similar lamp with a metal
+filament arranged somewhat in the form of a cone, and this lamp also
+sufficed for a small class-room. It was, I believe, made in Germany and was
+practically unobtainable during the war. I understand the Osram Company are
+at present arranging to manufacture it in this country, but up to the time
+of writing it has not made its appearance.
+
+None of these lamps worked direct on a public lighting circuit can be
+regarded as really satisfactory, as it has been found impossible so far to
+get a _concentrated_ light; the 100-volt lamps have, of course, been
+superior to those made for 200 or 250, but they are all for lantern
+purposes far behind low voltage lamps, which are really good when a
+suitable current can be obtained.
+
+THE POINTOLITE LAMP.--This lamp produced by the Ediswan Company is in
+reality a miniature arc with tungsten electrodes in a highly exhausted
+vacuum bulb. To attempt a technical description would be beyond the scope
+of this book; it will suffice to say that the action depends upon the same
+principle as the various wireless vacuum valves or the Coolidge X-ray tube.
+
+This lamp requires a peculiar starting device which is supplied with it,
+and gives a good, intense, and concentrated {44} light, not equal to the
+ordinary arc or to limelight, but comparing well with any other form of
+illuminant. It can only be used with the continuous current.
+
+THE NERNST LAMP.--This lamp at the present moment is practically
+non-existent in this country, having been made exclusively in Germany. Also
+as recent improvements in metal filament lamps have rendered it almost
+obsolete for ordinary lighting purposes, it is, I think, very doubtful
+whether it is still manufactured even in that country, and hence I do not
+propose to waste space in an extensive description.
+
+It will suffice to say that the lamp consists of one or more straight rods
+or filaments of a refractory material, which are semi-conducting to the
+electric current when hot, but non-conducting when cold. To commence with
+the filament must be heated, and in the lamps as supplied for lantern work
+this is usually done by means of a spirit lamp, which can be removed
+immediately the current begins to pass, as the filament is thereafter
+maintained at a white heat automatically.
+
+A three-filament Nernst lamp gives as much as 1000 candles, but it is
+extremely hot, and the light rather diffuse. The filaments are also very
+fragile, so on the whole the lamp was never very much in favour; but on the
+other hand it consumed very little current, and could be worked from any
+ordinary house lighting main, points which led to its adoption in certain
+cases.
+
+THE ELECTRIC ARC.--We now come to _the_ light for optical lantern work, the
+brightest, the most concentrated, the cheapest, the easiest to work, in
+fact, the illuminant which combines all the virtues and but few drawbacks,
+but of course requires one indispensable condition, viz. electric current
+laid on. This current may be of any voltage from 70-250, or even higher; it
+may be continuous or alternating, though the former is to be preferred; and
+it requires a cable for _at least_ 5 ampères, and for a large hall 10 or 12
+ampères.
+
+The simplest form of arc lamp for lantern purposes is the {45} hand-fed
+type as illustrated in Fig. 26. The essential feature is the pair of carbon
+rods, the remainder of the apparatus consisting of mechanical adjustments
+to 'feed' these as they burn away, and to accurately maintain them in their
+proper positions and in the optical centre of the lantern. Just because the
+electric arc provides so small and concentrated a light, it is of extreme
+importance that the centring should be exact; and hence mechanical
+movements are usually provided for this which are unnecessary with other
+illuminants.
+
+[Illustration: FIG. 26.--Hand-fed Arc Lamp.]
+
+The whole question of optical adjustments has, however, been left over for
+a future chapter, as it more or less applies to whatever illuminant is
+used.
+
+The illustration shows a lamp arranged for continuous current, the upper
+carbon, which must be connected to the _positive_ wire, being larger than
+the lower (the negative), and very slightly behind it. The light from a
+continuous current arc lamp comes chiefly from this upper or positive
+carbon, {46} which 'craters' as it is used, and this arrangement has the
+effect of radiating the light in the direction required (Fig. 27).
+
+The positive carbon is usually of the 'cored' type, that is provided with a
+core of softer carbon, as this assists the 'cratering' action, while the
+negative is generally used 'solid,' that is homogeneous right through.
+
+The arc has to be 'struck' in the first place by touching the carbons
+together for a moment by the mechanical means provided, and then separating
+them to the working distance, which is approximately 1/8 inch. They must
+then be maintained at that distance by 'feeding' as they slowly burn away,
+and this 'feeding' in arc lamps for lantern work is usually done by hand,
+as in the lamp illustrated in Fig. 26, but may be done by an automatic
+arrangement, as will be described later.
+
+[Illustration: FIG. 27.]
+
+The current is really carried across the arc by _convection_, or in other
+words conducted by a bridge of white hot carbon particles, which
+continually stream across from the positive carbon to the negative, and
+this bridge, while conducting the current, interposes a very considerable
+_resistance_ (otherwise it would not of course become hot).
+
+A certain potential or tension is therefore necessary if a given current is
+to be maintained, and this potential has to be greater the longer the arc
+and also (though not in direct proportion) the smaller the carbons.
+
+When, however, everything is in the best proportion, _i.e._ length of arc,
+size of carbons, and current passing, the potential at the arc lamp
+terminals required is approximately 45 volts, and this may be taken as a
+fixed figure for any current.
+
+The length of arc to give the best results may also be taken {47} as
+approximately fixed at 1/8 inch, and the _variable_ factor for different
+currents as required is provided by altering the sizes of carbons employed.
+
+The error must not be made, however, of assuming that an E.M.F. of 45 volts
+is sufficient to work an arc lamp, as the minimum in practice is at least
+65 volts, and 100 or even 200 volts are advantageous.
+
+I have come across more than one private generating installation where the
+innocent owner has put in a dynamo for 45 or 50 volts, depending upon some
+carelessly written statement that this is sufficient.
+
+_Why_ a higher E.M.F. is required can be simply explained.
+
+Take for instance an average hand-fed arc lamp as used for lantern work and
+consuming, say, 10 ampères.
+
+Take also, as a fact, the statement given above that the necessary E.M.F.
+at the actual terminals of the arc lamp may be accepted as a constant at 45
+volts, and reverting to the equation given on page 40, C = E / R, and
+substituting these figures we get--
+
+  Current (10 ampères) = E (45 volts) / R (Resistance of Arc).
+
+It is therefore obvious that under these exact conditions the resistance or
+back E.M.F. of the arc, as it is termed, must equal 4.5 ohms.
+
+Now suppose the lamp left for a few seconds unattended, while the carbons
+are burning away and the arc is lengthening; in a very few moments the
+resistance will have increased, owing to the greater distance between the
+carbons, and we will suppose it to have become 5 ohms instead of 4.5.
+
+The current passing will now be 45 / 5 = 9 ampères only.
+
+In other words, a very slight lengthening of the arc has reduced the
+current, and therefore the light, by 10 per cent.
+
+Not only so, but 45 volts being needed to maintain an arc of {48} normal
+length, it is insufficient to maintain a longer one, and in practice the
+effect of leaving an arc under these conditions to itself for even a few
+seconds is that it _goes out_, to the annoyance of the lecturer and the
+confusion of the operator.
+
+It is just _possible_ to work an arc lamp with a total E.M.F. of 45 volts
+by giving one's whole attention to it and never taking the hand off the
+feeding handle; but in practice no one with any experience would attempt
+it. The arc would almost certainly go out several times during the
+exhibition.
+
+Now, take an example of a similar arc lamp consuming 10 ampères but worked
+from a supply of 200 volts.
+
+Our equation C = E / R must then obviously become
+
+  C (10 ampères) = E (200 volts) / Total Resistance (20 ohms).
+
+The resistance of the arc itself being the same as before, viz. 4.5 ohms,
+it is obviously necessary to put an _extra_ fixed resistance equal to 15.5
+ohms in series with it in order to make up the total of 20 ohms.
+
+_Now_ leave the arc unattended until the resistance of 4.5 ohms has again
+become 5 ohms; the only effect is that our current, instead of remaining at
+10 ampères, has become 200 / 20.5 or 9.8 nearly, a difference which is
+imperceptible.
+
+This is not all, for it is an elementary rule in electrical science that
+the total E.M.F. of any circuit distributes itself along that circuit in
+proportion to the distribution of resistance.
+
+In other words, our original E.M.F. of 200 volts will so distribute itself
+as to reserve, so to speak, an E.M.F. of 45 volts for the arc, while the
+resistance of this remains at 4.5 ohms, but directly this resistance
+increases, the E.M.F. at the arc lamp terminals automatically rises, and
+therefore the actual diminution in current is even less than the figures
+above quoted. {49}
+
+Should the arc tend to 'break' or go out, the resistance across it
+automatically becomes infinite and the _whole_ 200 volts is at that moment
+available to prevent the occurrence.
+
+Under these conditions, therefore, the operator can safely leave the arc
+for many minutes at a time. In carrying out experimental work I have often
+left the lantern, walked up to the screen, discussed results with a friend,
+and walked back, and the arc has shown no signs of misbehaviour whatever.
+
+[Illustration: Fig. 28.--Resistance.]
+
+In practice any current from 100 volts to 250 volts may be considered as
+satisfactory for lantern work with a suitable resistance. Less than this
+involves feeding the arc rather frequently, and more may give a nasty
+shock, should the operator inadvertently touch a live wire, though I have
+worked an arc lamp on a current of as much as 500 volts.
+
+The _resistance_ usually consists of a suitable length of wire of high
+resistance (Iron, German Silver, or those alloys known as Platinoid,
+Eureka, Manganin, Beacon, &c., are most commonly used) wound in spirals on
+a frame, and is generally supplied adjustable (Fig. 28), so that more or
+less current may be used as desired. These resistances get pretty hot in
+use, and care must be taken that they are placed where they cannot scorch
+woodwork, &c., and in cases where the lantern is a fixture it is a good
+plan to have the resistance bolted up against a wall once and for all. The
+resistance may be placed anywhere in the circuit, so long as the current
+passes through it, then through the arc lamp (or _vice versâ_), and back to
+the other {50} pole of the supply main; it does not matter in the least
+whereabouts it comes.
+
+In cases, however, where one pole of the supply main is _earthed_, it is a
+good thing to place the resistance in the 'live' side, as this keeps the
+arc lamp within 45 volts of earth potential while it is working, to the
+comfort of the operator should he touch a terminal or wire, though with an
+ordinary lighting main there is no real fear of a dangerous shock in any
+case.
+
+The _amount of current required_ depends of course on the size of the
+sheet, length of the hall, and density or otherwise of the slides; but it
+is usually accepted in practice that the efficient light from a continuous
+current arc lamp equals 100 candles per ampère, and therefore a 10-ampère
+arc will give 1000 candles. This is sufficient for all ordinary halls and
+slides, but where these latter are very dense, as for example with the
+Lumière three-colour process, as much as 20 or 25 ampères may be required.
+
+In these cases some special precautions must be taken for keeping the
+slides cool, or the result may be disastrous, but this is a question that
+will be referred to in a later chapter. A current of 10 ampères is pretty
+safe for all ordinary slides, and may be taken as the normal current used
+in large halls, though in arranging for the wiring it is as well to
+stipulate for at least 12 or even 15 ampères, especially as there must
+necessarily be a momentary increase of current at the instant the arc is
+'struck.'
+
+VARIETIES OF HAND-FED ARC LAMPS.--The pattern of hand-fed arc lamp
+illustrated in Fig. 26 is only typical of many of the same general design,
+and there are others in which the design itself is fundamentally different.
+Of these the 'Scissors' arc lamp made by several firms deserves mention on
+account of its simplicity and cheapness. As its name implies, the mechanism
+resembles a pair of scissors, the carbons being attached to the ends of a
+pair of levers hinged together {51} (Fig. 29). In this lamp centring
+movements are usually dispensed with, the arc being clamped on to a tray
+pin as in the case of a limelight jet. This is not, of course, so
+convenient, and a further disadvantage of this pattern arc lamp is that the
+feeding process gradually alters the position and angle of the carbons. In
+fact, the one great merit of the lamp is cheapness, and where expense is an
+object, it should certainly be considered.
+
+[Illustration: FIG. 29.--'Scissors' Arc Lamp.]
+
+Yet another arc lamp deserving of mention is the 'Parallel,' a name again
+very aptly chosen, as the two carbons are either exactly parallel to each
+other or very slightly inclined. In the former case the arc has to be
+'struck' by touching the ends of the carbon rods with a piece of metal or
+carbon. Of the actual manipulation of this lamp I have had very little
+practical experience, but I have heard it well spoken of, though I believe
+it has so far only been made for currents of 5 ampères or so.
+
+Yet another type which must not be ignored is the 'Right-angled' pattern
+(Fig. 30), a name again self-descriptive. The horizontal carbon is the
+positive, and the vertical the {52} negative, and this lamp again is made
+by several manufacturers in slightly different forms.
+
+This pattern lamp is in my experience the best of all for _small_ currents,
+say, of 5 ampères or so, but inferior to Fig. 26 for currents of 10 ampères
+or more. This last remark perhaps hardly applies to _alternating_ currents,
+which, however, I have not yet discussed. I cannot conclude this brief
+category of arc lamps without referring to the _enclosed_ pattern, of which
+the 'Westminster' is perhaps the best-known and most popular (Fig. 31).
+
+[Illustration: FIG. 30.--'Right-angled' Arc Lamp.]
+
+This is a lamp of the right-angled type, but the arc burns in a cylindrical
+glass chamber, not air-tight, but partially so. After burning a few minutes
+the oxygen in this chamber becomes used up and its place is taken by
+carbonic-acid gas and other products of combustion, after which the carbons
+burn away very much more slowly, and therefore require feeding at much
+greater intervals.
+
+This lamp again is chiefly made for small currents not exceeding 5 ampères
+(and can therefore be used from any ordinary lamp socket), and for a
+moderate-sized hall is on the {53} whole as cheap, efficient and simple a
+lamp as any I am acquainted with. It can be supplied with or without
+mechanical centring movements as required, and is usually sent out with its
+own resistance for the particular current on which it is to be used, so
+that it only requires connecting up to the nearest lamp socket, and is
+ready for use.
+
+[Illustration: FIG. 31.--'Westminster' Arc Lamp.]
+
+It is _not_ sufficient for anything larger than a 12-foot sheet or for
+working at a greater distance than, say, 40 feet, but within these limits
+the lamp, and in fact _any_ good 5-ampère arc lamp, will be found quite
+satisfactory and saves the expense of putting in a special cable.
+
+AUTOMATIC ARC LAMPS.--Arc lamps for lantern work in which the feeding is
+done automatically are also made. Like hand-fed lamps, they vary in exact
+design, but all, or practically all, are so designed that the carbons are
+brought together by means of springs or weights, and some form of 'brake'
+controlled by a system of electro-magnets checks the {54} movement. As the
+carbons burn away the arc lengthens, the current weakens, the
+electro-magnets lose their grip, and the carbons move together until the
+increasing current puts on the brake again. Some of these lamps are
+'semi-automatic' only, that is to say, the arc has to be struck by hand,
+while others perform this operation automatically as well, usually by an
+additional magnet which draws back the carbons by the correct amount after
+the arc is struck.
+
+My frank advice to intending lanternists is to leave these lamps alone.
+Some of them are satisfactory up to a point, but they are all apt to be
+'jumpy,' and on the whole the hand-fed type is in my opinion to be
+preferred.
+
+ARC LAMPS ON ALTERNATING CURRENTS.--The alternating current is not so good
+as the continuous for lantern work with arc lamps: the light per ampère is
+not so great, the light has an irritating habit of travelling round the
+carbons and there is always a slight 'hum.'
+
+The sum total of these drawbacks is nothing very serious, provided that
+proper arrangements are adopted, and I have frequently manipulated arc
+lamps on alternating circuits with such good results that professional
+lecturers have at first refused to believe that the circuit really _was_
+alternating.
+
+As it is frequently stated that to obtain a steady light with an
+alternating current is impossible, I can understand their surprise, and I
+can also understand the statement in question, as the problem is usually
+tackled on entirely wrong lines.
+
+It is almost always stated that arc lamps for alternating currents should
+be arranged with the carbons _vertical_, and many makers actually so
+construct their lamps as to allow of this.
+
+To obtain a steady light under these conditions _is_ impossible and I pity
+anyone who attempts it; but the statement that this is the best method of
+working has been repeated so often that it seems to have been taken for
+granted.
+
+The best arrangement (in my hands at any rate) is to {55} slant the carbons
+as for the continuous current, and also to have the upper carbon cored and
+the lower one solid, but to use a rather larger lower carbon than would be
+correct if the main were continuous.
+
+Also the upper carbon should not be _quite_ so far back as with D.C.; to
+have the front edges of the two carbons practically in line is about
+correct, but the _exact_ position should be carefully adjusted to obtain
+the steadiest light, and it will be found that a slight alteration makes a
+considerable difference.
+
+It is also a great help to have a weak electro-magnet, or its equivalent,
+so arranged that it tends by its influence to keep the arc to the front. On
+some lamps this is provided for, as even with a continuous current it is
+quite harmless and, if anything, beneficial; but, if not, any competent
+mechanic can easily fit an 'Induction Ring,' consisting of a single turn of
+stout copper wire, which has sufficient magnetic influence to do all that
+is required (Fig. 32).
+
+This ring must be wired in series with the arc itself, and as the current
+passing in it automatically reverses in synchronism with the arc, its
+effect is _always_ to deflect the arc in the same direction, and care must
+of course be taken that it is so wired that the deflection is forward and
+not backward. This is the exact arrangement I have myself adopted, and I
+never experience any difficulty on the score of the arc wandering.
+
+Right-angled arc lamps, as described on pages 52 and 53, are also very
+efficient on A.C. mains, and frequently these lamps are already equipped
+with electro-magnets for the purpose required. The 'hum' of an alternating
+current cannot be altogether eliminated, but can be reduced to a minimum
+_by reducing the voltage as far as possible_.
+
+As has been already said, the A.C. lends itself readily to transformation
+of voltage, and I find in practice 90-100 to {56} be ideal. More than this
+is inclined to be noisy, and less is apt to result in an unsteady arc.
+
+The arrangement, therefore, which I recommend from long experience is to
+employ a transformer to reduce the E.M.F. to 100 volts or thereabouts, and
+then work with a resistance in the usual way (if the original current is
+100 volts, of course _no_ transformer is required) with a properly
+constructed arc lamp fitted with an induction ring or electro-magnet. No
+difficulty should then be experienced in obtaining a good, steady, and
+fairly quiet light.
+
+[Illustration: FIG. 32.--Arc Lamp with Induction Ring.]
+
+Any little 'hum' remaining can be silenced to a very considerable extent by
+placing the entire lantern on a thick block of saddlers' felt, but in
+practice I have never found this necessary with ordinary currents, though a
+few abnormal circuits where the 'periodicity' is very high are noisier than
+others.
+
+[Illustration: FIG. 33.--THE OPTICAL SYSTEM OF A LANTERN.]
+
+{57}
+
+The following table gives the sizes and particulars of carbons for various
+currents that I have found best in actual practice:
+
+                    CONTINUOUS CURRENT
+
+  Ampères.        + Carbon _Cored_.     - Carbon _Solid_.
+
+    7-10                   12 mm.                   7 mm.
+   10-15                   13 ,,                    8 ,,
+   15-20                   16 ,,                   10 ,,
+
+                    ALTERNATING CURRENT
+
+  Ampères.    Upper Carbon _Cored_.    Lower Carbon _Solid_.
+
+    7-10                   12 mm.                  10 mm.
+   10-15                   13 ,,                   11 ,,
+   15-20                   16 ,,                   13 ,,
+
+       *       *       *       *       *
+
+
+CHAPTER VII
+
+THE OPTICAL SYSTEM OF A LANTERN
+
+[Illustration: FIG. 33A.--Optical System of Lantern.]
+
+As previously noted, the essential parts of an Optical Lantern are, in
+order from rear to front: (1) The illuminant; (2) the condenser; (3) the
+slide and slide stage; (4) the objective, to which must be added, (5) the
+body or framework which holds the whole together. Fig. 33 is a diagrammatic
+representation of the entire optical system and Fig. 33A shows all the
+various parts _in situ_: A being the illuminant, shown in Fig. 33 {58} as
+an arc lamp, B the condenser, C the slide stage, and D the objective. The
+foundation, so to speak, of the whole instrument is of course the slide,
+which, as made in this country, consists of a square of glass 3¼ inches
+diameter, the slide itself being somewhat less than this on account of the
+binding, &c.; in making calculations it is usually taken as a 3-inch
+circle. Slides are usually made by binding together with strips of paper or
+cloth two such squares, on one of which is the photographic film or
+painting forming the picture, the other being simply a plain cover glass
+placed over the slide surface to protect it, and between the two being
+placed a paper mask with an aperture of whatever size or shape is required,
+that of the aforesaid 3-inch circle being usually taken as the standard or
+normal dimension for this aperture.
+
+The slide being illuminated by one of the various methods discussed in the
+previous chapters, is focussed on the screen by the objective, which must
+be selected according to the size of picture required and the distance
+between lantern and screen.
+
+These points will be gone into later, and also details as to various types
+of objectives and their respective advantages; but it may be said here that
+a lantern objective consists usually of a combination of lenses of 2 inches
+or 2½ inches diameter mounted in a rackwork focussing system at a distance
+from the slide of 6 inches to 18 inches, according to the length of its
+'focus.' As our slide is from 3 to 3¼ inches diameter, it is evident that
+all the light radiating from this cannot possibly get through the objective
+unless it is _converged_ upon it, and to do this is the function of the
+condenser. The following two diagrams, Figs. 34 and 35, will make the
+matter clear.
+
+S represents our glass slide of 3 inches clear diameter, R the radiant or
+illuminant, and L our objective, shown here for the sake of simplicity as a
+single lens.
+
+The slide is well illuminated by the light emanating from {59} R, but it is
+obvious that the bulk of this light will never pass through the lens, and,
+in fact, only the very centre of the slide will under these circumstances
+appear upon the screen at all.
+
+[Illustration: FIG. 34.--Optical System without Condenser.]
+
+[Illustration: FIG. 35.--Action of Condenser.]
+
+What is evidently wanted is to _converge_ these outer rays, or in other
+words to bend them in so that they also pass through the objective, and
+this is the function of the condenser as illustrated in Fig. 35. The
+condenser is here represented also by a single lens, but in practice it
+also is invariably constructed of two or even three lenses, for both
+optical and mechanical reasons. It is evident from the above diagrams that
+the condenser must be somewhat larger in diameter than the slide itself,
+and condensers for ordinary lantern work are usually 4 inches to 4½ inches
+diameter. The former size {60} will suffice if the condenser is placed very
+close to the slide, but it is often advisable to leave a little intervening
+space, especially if the illuminant is a powerful one, in order to allow
+any condensation of moisture readily to evaporate and escape. Hence
+lanterns for long range work (which involve, of course, good illumination)
+are usually made with condensers of 4½ inches diameter. Lantern condensers
+of to-day usually take one of the two forms shown in Fig. 36, but the exact
+curve must be left to the manufacturer, as the focus of the condenser must
+have a definite relation to that of the objective. Taking, however, the
+design of E, the most common of all, the two lenses should not be exactly
+similar unless the objective is pretty short in focus, or, in other words,
+unless the distance of the illuminant on the one hand and that of the
+objective on the other are approximately equal. If the lantern is intended
+for long range work, that is equipped with a long focus objective, the
+front component of the condenser should also be constructed longer in focus
+(that is to say, with a shallower curve) than the rear one, and it is
+amazing how careless manufacturers are in this respect. If, as is often the
+case, the lantern is fitted with several objectives of different foci, it
+is usually necessary to supply alternative condensers also, or at least to
+supply an interchangeable front component.
+
+[Illustration: FIG. 36.--Forms of Condensers.]
+
+If the entire condenser is too long in focus, light is lost; if too short,
+it is impossible to obtain an even disc, as there is invariably a dark
+patch either in the centre or round the edges.
+
+The mounting of the condenser also varies with different makers; but it
+must be remembered in any case that it gets {61} extremely hot, especially
+the back component, and hence the glass must be mounted _loose_ in its
+cell, otherwise there is great danger of it cracking. Also the space
+between the components should be well ventilated, in order to provide for
+the escape of moisture which usually at the start of a lantern exhibition
+is deposited upon the glass, and should be got rid of before the actual
+lecture commences.
+
+Even with all care, the back component of a condenser will sometimes crack,
+though such an accident should be a rare occurrence; and hence a
+professional operator will usually provide himself with a spare lens, and
+the condenser should be so constructed that it can readily be changed, and
+with as little delay as possible.
+
+Condenser lenses as made in this country are usually ground from the glass
+known as 'English Crown,' and comparatively rarely crack; but they are very
+slightly green in colour. French condensers, on the other hand, are whiter,
+but the glass is more brittle, and a fracture a more common occurrence. The
+French variety are (or were before the war) cheaper and generally met with
+in cheaper instruments. More expensive lanterns are usually fitted with
+English condensers, as the tinge of green is almost imperceptible, and the
+advantage as regards greater security pretty considerable.
+
+THE SLIDE CARRIER AND SLIDE STAGE.--Taking still the optical system of the
+lantern in order from back to front, we now come to the slide, slide
+carrier, and slide stage. The slide itself has already been described, and
+the carrier is simply a mechanical contrivance, usually of wood, designed
+for the purpose of readily changing the pictures and which in its turn fits
+into the stage of the lantern. It may be asked why, if slides are now
+always made to a standard size, the slide carrier should not itself be
+built into the lantern and form the stage; but the answer is, in the first
+place, that slides of a different size, _i.e._ American or Continental,
+_may_ be met with, {62} and also that there are various mechanical slides
+on the market--for example, chromotropes or scientific models, such for
+instance as are made to illustrate the movements of the planetary
+bodies--and these slides are permanently mounted in wooden frames which
+could not be put into a carrier. The commonest form of carrier is that
+known as the 'Double Sliding' pattern (Fig. 37), which consists of a frame
+with two apertures for the slide, and an outer frame through which this
+itself slides and which fits the stage of the lantern.
+
+[Illustration: FIG. 37.--Double Sliding Carrier.]
+
+This carrier, as will be seen, allows the next picture to be placed in
+position in the second aperture while the former one is being projected,
+and at a signal from the lecturer, the inner frame slides smoothly through
+the outer, and the slides are thereby changed. This carrier is simple,
+cheap, and quiet in its action; its one disadvantage is that each alternate
+slide has to be inserted from opposite sides of the lantern, and unless the
+operator is fairly tall this almost necessitates an assistant.
+Nevertheless, the carrier is the most popular of any, its other advantages,
+especially as regards price, being so great. It is usually constructed in
+such a way that the slide, as it moves out from the central position,
+automatically rises in its groove in order to facilitate quick removal.
+
+Another pattern deservedly popular is that known as {63} 'Beard's
+Dissolving Carrier' and is shown in Fig. 38. In this ingenious carrier all
+the slides are inserted from the same side, the oncoming slide being pushed
+_in front_ of its predecessor, and being therefore somewhat out of focus it
+produces a blur on the screen.
+
+The movement is performed by pushing in a projecting handle, and on
+withdrawing this the slide which is finished with comes with it, and the
+finish of the movement presses the new slide back until it is in its proper
+position and in focus.
+
+[Illustration: FIG. 38.--Beard's Dissolving Carrier.]
+
+The entire action is simpler than it sounds, and the temporary blurring of
+the image on the screen during the process of changing is supposed to give
+somewhat the effect of 'Dissolving Views,' and hence the name 'Dissolving
+Carrier.'
+
+This appliance is three times the price of the 'Double Sliding' pattern,
+but the fact that it is worked from one side only is a decided advantage,
+though on the other hand it is not (unless great care is used) quite so
+silent in its action as the 'Double Sliding' type.
+
+A further modification of this carrier adapts it to take any of the
+recognised 'foreign' sizes of slides, so that if a few American ones, for
+instance, are met with among a collection of English manufacture, there is
+no need to change the carrier. {64}
+
+There are other varieties of carriers on the market which there is no need
+particularly to describe, such as, for example, carriers fitted with roller
+curtains to give the effect of a curtain rolling up, magazine carriers to
+hold twenty-four or more slides and exhibit them in rotation, and other
+patterns too numerous to mention. Of these the reader must be left to judge
+for himself, but, generally speaking, _simplicity_ in a carrier is the most
+important point to be looked for, and complications, however ingenious,
+should be avoided.
+
+[Illustration: FIG. 39.--Focussing Action of Lens.]
+
+The lantern stage must also receive consideration, but it will be better to
+discuss it as part of the mechanical construction of the lantern.
+
+THE OBJECTIVE is really the most vital part of a lantern, as the definition
+of the picture almost entirely depends upon the excellence or otherwise of
+this lens. This will be obvious at once when it is realised that the
+objective has to project on to the distant screen a greatly magnified image
+of the comparatively small lantern slide, and the intending purchaser is
+strongly advised to economise almost anywhere rather than on this item.
+
+The action of a lens in focussing the image is perhaps best explained by a
+simple diagram (Fig. 39), from which it will be seen that all the rays
+proceeding from any one point on the object are re-converged (when the lens
+is in focus) to a definite point on the image, and the perfection of the
+picture depends upon the lens performing this function accurately. {65}
+
+The imperfections are chiefly two, viz. those known as chromatic and
+spherical aberration respectively. Chromatic aberration simply means that
+all the colours composing the original beam of, say, white light are not
+equally refracted or converged, and therefore do not meet again at the same
+spot (the well-known prism or lustre effect), and reveals itself by
+coloured fringes round the edges of the various details in the picture.
+
+[Illustration: FIG. 40.--Achromatic Lens.]
+
+By spherical aberration we mean that the light falling upon the centre of a
+lens is not brought to a focus at exactly the same spot as the marginal
+rays, and a general want of definition is the result, usually accompanied
+also by a want of 'flatness' in the image, that is to say the edges of the
+picture do not focus at the same time as the centre.
+
+Chromatic aberration is easily cured by using an achromatic or compound
+lens made by cementing together two lenses of crown and flint glass
+respectively, as in Fig. 40.
+
+It will be seen that the flint glass component by itself is a _concave_
+lens and therefore neutralises in part, or in whole, the convex crown lens.
+Flint glass has both greater dispersive power and also greater refractive
+power than crown glass, but fortunately not to the same _degree_; hence a
+compound lens made in this way and with curves carefully worked out may
+have its chromatic effect entirely neutralised while retaining very
+considerable refractive or 'focussing' power, and simple achromatic
+objectives of this type are quite inexpensive.
+
+In lanterns intended for Science demonstration, as distinct from the mere
+projection of slides, lenses of this pattern are very frequently used, as
+they will project the latter when required reasonably well, and for the
+demonstration of {66} experiments or of apparatus on the screen have
+advantages that need not be discussed here.
+
+For very long focus lenses also they are sometimes employed, as the trouble
+from spherical aberration is much less apparent with lenses of long focus
+than with short, and the difference in expense is much more in the former
+case than in the latter. For short focus lenses, however, as used in
+moderate-sized halls, they are not good enough, and the type of lens almost
+universally employed is that known as the 'Petzval' combination (Fig. 41).
+
+[Illustration: FIG. 41.--Petzval Combination.]
+
+This lens really consists of two achromatic combinations, the pair at the
+front being cemented together, and that at the rear having an air space
+between. The combination is so designed that the spherical aberration of
+the one pair neutralises that of the other, and the result is or should be
+a lens corrected both for chromatic and spherical aberration.
+
+These lenses, however, vary very much in the perfection of their results,
+and as they are at present usually imported in bulk from France, the
+customer does well to insist upon a demonstration of his own particular
+lantern before acceptance.
+
+The magnifying power of a lens depends upon its 'focus' multiplied by its
+distance from the screen, and the focus in the case of a simple lens is
+easily determined by the familiar 'burning-glass' experiment, that is by
+focussing an image of the sun upon a piece of paper and measuring
+accurately {67} the distance the lens must be away to produce the most
+concentrated spot.
+
+In practice it is sufficiently accurate to focus a distant window, or other
+luminous object, upon the paper, any error obtained by this method being
+for ordinary purposes a negligible one.
+
+With a compound lens, such as a 'Petzval' combination, this method does not
+hold good, as the optical centre of such a lens is not necessarily midway
+between its two components.
+
+The actual focus can be got pretty approximately by focussing a window or
+other object as before and measuring the distance from one definite point
+(say the front edge of one of the lens cells) to the paper, then turning it
+round and taking a second measurement from the _same_ point, the mean
+between the two measurements giving the actual focus.
+
+In practice the 'simple equivalent focus,' as it is termed, of a lantern
+lens is usually determined by measuring the magnification of the image
+thrown upon the screen, when, by knowing the original size of the slide (a
+'standard' slide of 3 inches diameter is usually taken) and the distance
+between lantern and screen, we get the focus from the following very simple
+equation:
+
+  Diameter of picture on screen    Distance between lens and screen
+           (in feet)                        (in feet)
+  ----------------------------- = ---------------------------------
+  Diameter of slide (in inches)      Focus of lens (in inches)
+
+or perhaps more simply still:
+
+  Distance between lens  × Diameter of slide
+  and screen (in feet)      (in inches)
+  ------------------------------------------  = Focus of lens in inches;
+          Diameter of picture (in feet)
+
+or, if we know the focus of the lens but want to know how far from the
+screen we must go to produce a given-sized picture, the formula will be:
+{68}
+
+  Diameter of picture  × Focus of lens
+        (in feet)         (in inches)
+  ------------------------------------ = Distance required (in feet).
+      Diameter of slide (in inches)
+
+It is handy for the lanternist to remember that, dealing with a standard
+3-inch slide, a 6-inch lens will _always_ give a picture whose diameter is
+_one-half_ the distance from lens to screen, a 12-inch lens half this again
+or _one-quarter_, and a 9-inch lens half-way between the two.
+
+Bearing these simple figures in mind, the approximate distance can usually
+be _guessed_ sufficiently near for the first trial, and then the lantern
+shifted a little nearer or the reverse as required.
+
+The following table may, however, be useful, as showing readily the
+magnification produced at different distances by lenses of given foci:
+
+  +------+---------+--------+--------+--------+--------+--------+--------+
+  | Disc |  Focus  |  Focus |  Focus |  Focus |  Focus |  Focus |  Focus |
+  |wanted| 4½ in.  |  6 in. |  8 in. |  10 in.|  12 in.|  15 in.|  18 in.|
+  +------+---------+--------+--------+--------+--------+--------+--------+
+  | feet.| ft. in. | ft. in.| ft. in.| ft. in.| ft. in.| ft. in.| ft. in.|
+  |   9  | 13   6  | 18   0 | 24   0 | 30   0 | 36   0 | 45   0 | 54   0 |
+  |  12  | 18   0  | 24   0 | 32   0 | 40   0 | 48   0 | 60   0 | 72   0 |
+  |  15  | 22   6  | 30   0 | 40   0 | 50   0 | 60   0 | 75   0 | 90   0 |
+  |  18  | 27   0  | 36   0 | 48   0 | 60   0 | 72   0 | 90   0 |108   0 |
+  |  20  | 30   0  | 40   0 | 53   4 | 66   8 | 80   0 |100   0 |120   0 |
+  |  25  | 37   6  | 50   0 | 66   8 | 83   4 |100   0 |125   0 |150   0 |
+  |  30  | 45   0  | 60   0 | 80   0 |100   0 |120   0 |150   0 |180   0 |
+  +------+---------+--------+--------+--------+--------+--------+--------+
+
+THE DIAMETER OF THE OBJECTIVE.--The diameter of the objective must depend
+to a certain extent upon its focus in the case of a double combination such
+as a Petzval. These lenses consist, as has already been said, of two
+achromatic components some distance apart, and for technical
+considerations, which need not be discussed here, the _distance_ between
+these components is usually about two-thirds of the focal length. This is
+not a universal rule, as the lenses of different makers vary a good deal;
+but it is generally a fact {69} that the longer the focus of the lens the
+greater is usually the separation between the two lens systems.
+
+The entire lens therefore mounted in its tube resembles a _tunnel_ of
+varying length according to its focus, and through this tunnel a _cone_ of
+light rays have to be passed. It is plain, therefore, that a lens of long
+focus, which in practice means a long tube length, must be made also of
+large diameter, or a portion of the cone will be cut off, with a consequent
+loss of light.
+
+In practice lenses up to 6 inches focus are usually made of 2 inches
+diameter, and there is no advantage in a larger size. With a lens of 8
+inches focus there is a slight gain in increasing the diameter to 2-3/8
+(the next 'standard' size), and lenses of longer focus than this should
+certainly be 2-3/8 inches up to, say, 12 inches focus, when a lens of 3
+inches diameter is preferable. These large lenses are, however, very
+expensive, both in themselves and also on account of the fact that their
+weight entails heavy and expensive brass mounting, and hence lenses up to
+14 or 15 inches focus are often supplied in the 2-3/8 size for reasons of
+economy.
+
+To sum up, _short-range_ lanterns, as they are called, are usually fitted
+with lenses of 2 inches diameter, and _long-range_ instruments either with
+3-inch lenses or the intermediate size of 2-3/8 inches. If a lantern is
+purchased for either long or short-range work, it is usually fitted with a
+brass front for a large lens, and so arranged that a shorter focus lens of
+2 inches diameter can easily be interchanged, utilising the same brass
+mounting.
+
+Lenses of _variable_ focus have also been designed, in which an additional
+lens can be added or subtracted to increase or decrease the focal length;
+but nothing very practical has yet been achieved in this direction, and
+therefore these 'Omnifocal' lenses have never come into general favour.
+
+Objectives like condensers want cleaning at times, and care must be taken
+not to scratch the glass, as the concave lens of each component is of flint
+glass, and very soft. A {70} clean chamois leather is the best thing to
+use, but a soft cloth, or even a handkerchief, may be employed with care.
+It is very important that a lens be reassembled, after cleaning, the
+correct way, as a single lens reversed would utterly spoil the definition.
+The front component is usually balsamed together, and therefore all that is
+needed is to see that the whole combination is not reversed. In the Petzval
+system this lens should have its convex constituent towards the screen
+(Fig. 41). The back combination is usually loose, and the two lenses are
+sometimes separated by a thin brass ring. In the Petzval lens the concave
+element should be inside, with its concave surface outwards, the deep curve
+of the other lens should fit into this concavity, and the flatter curve
+face towards the condenser. One or two makers, however, have introduced a
+modification of the Petzval system in which the whole of this back
+combination is reversed, and the exact arrangement should therefore be
+noted very carefully when taking the lens to pieces.
+
+       *       *       *       *       *
+
+
+CHAPTER VIII
+
+THE BODY OF THE LANTERN
+
+We now come to the mechanical construction of the optical lantern, and a
+great variety of design presents itself, according to price, type (_i.e._
+short range or long range), and the individual ideas of the various makers.
+
+Lantern bodies as a rule are now made of metal, although up till quite
+recently the better class instruments were more usually made of polished
+mahogany lined internally with iron; but there has of late been a consensus
+of opinion in favour of metal only.
+
+In the cheaper lanterns this metal body is usually made either of Russian
+iron or of sheet-iron tinned and japanned, {71} there being little to
+choose either in price or quality between the two varieties, and in all but
+the very cheapest instruments the front is usually of brass.
+
+In better lanterns the body is more often made of enamelled steel, the
+front as before being of brass; but brass, copper, or aluminium are also
+used occasionally for the body of the lantern.
+
+[Illustration: FIG. 42.--Hughes' Short-Range Lantern.]
+
+In deciding upon the type of body to be purchased the main considerations
+to be borne in mind are: (1) The type or types of illuminant to be used, a
+powerful arc lamp for example requiring a larger body than is necessary for
+a weaker radiant; (2) the size and position of the lens to be carried, a
+Petzval objective of say 3 inches diameter which has to be supported at the
+end of a long brass mount for long-range work obviously demanding a body of
+greater strength and rigidity than is required with a 6-inch focus lens of
+2 inches diameter; (3) price.
+
+Fig. 42 shows an extremely good lantern body for short-range work made by
+Messrs. Hughes, the illustration depicting the instrument complete with a
+'Luna' methylated spirit lamp, though, of course, any other illuminant
+suitable for a small lantern could be used instead. {72}
+
+This lantern illustrates well one point that has already been emphasised as
+important, viz. the ventilation of the condenser. It will be noticed that
+this is placed _outside_ the body of the instrument instead of inside as is
+usual with larger bodies, and that wide slots are cut in the condenser
+mount to allow free escape of steam.
+
+Other points of this excellent design are the screw adjustment to the slide
+stage (facilitating the use of special slides, such, for example, as those
+illustrating the movements of the planetary bodies which sometimes involve
+the use of extra thick frames) and a simple but efficient tilting
+arrangement to the base.
+
+[Illustration: FIG. 43.--Long-Range Lantern.]
+
+Such a lantern is hardly suitable for a powerful arc lamp or limelight jet,
+or for heavy long-range lenses, but is a very good typical instrument for
+use in moderate-sized halls, and a lantern of this general type is usually
+found in lantern catalogues, though, of course, the exact designs vary
+according to the ideas of the manufacturer. Of lanterns for long-range work
+a good example is perhaps Messrs. Newton & Co.'s 'Intermediate' pattern
+(Fig. 43).
+
+This again is only typical of many others by the various makers, but the
+principal points are common to all. These are: (1) The large and
+well-ventilated body; (2) the long {73} baseboard; (3) the strong and
+massive brass front necessary to carry the large long-range lenses; (4) the
+velvet curtain at the back to close in any stray light from a powerful arc
+lamp.
+
+[Illustration: FIG. 44.--Connections for a Bi-unial Lantern.]
+
+The two foregoing designs are perhaps sufficiently typical of lantern
+bodies in general to make further detailed description of individual
+designs unnecessary; but reference should be made to features which special
+requirements may render advisable.
+
+Under this heading mention must be made of _Bi-unials_ or Double Lanterns,
+as used for the once famous 'Dissolving Views.'
+
+A bi-unial lantern consists essentially of two different instruments, each
+complete with its limelight jet or other illuminant--front, condensers,
+objective, &c., usually mounted on one body--and with some arrangement for
+'dissolving' or turning the light in each lantern gradually on and off.
+
+Fig. 44 shows the back view of such a lantern with two limelight jets and
+dissolving tap, this piece of mechanism (shown below in the illustration)
+being so arranged that when the lever is horizontal _both_ lanterns are on
+full, but moving the lever either way cuts off the gas supply to one
+lantern. In the case of limelight the tap should always operate by cutting
+off the oxygen supply in advance of the coal gas (in order to avoid a
+'snap'), and the latter should never be cut off entirely, but a small bead
+of flame left to keep the jet alight, until the lantern is required for the
+next slide. {74}
+
+This is usually arranged for by means of a bye-pass, and a bye-pass is
+sometimes provided on the oxygen side as well, but is usually discarded in
+practice.
+
+A bi-unial lantern can be worked in the same way with acetylene gas, but
+with the electric arc it is impossible to turn the light on and off
+gradually, and in practice dissolving must be done by keeping both lanterns
+fully alight, and using a dissolving shutter, that is a movable shutter
+that covers each objective alternately. The same arrangement must be used
+with other illuminants, such as oil, only in this case the lanterns must be
+mounted side by side, on account of the tall chimneys. With oil lamps the
+arrangement answers fairly well, the dissolving fan, as it is termed, being
+made with serrated edges which give the _gradual_ obliteration required;
+but with the electric arc the extremely sharp definition becomes a serious
+difficulty, and a good dissolver for this illuminant has never yet been
+found, though, in view of the fact that dissolving views are more or less a
+thing of the past, the matter cannot be regarded as important.
+
+The advantages claimed for a double lantern are two: first, a 'Dissolving'
+effect by which one picture fades gradually into the next, and which is
+supposed to be more pleasing than the movement of a carrier; and second,
+'Dissolving Effects' can be shown, such as exhibiting a landscape by day
+and changing it into a moonlight scene, or bringing on the appearance of a
+snowstorm, which can easily be done by means of a roller slide, with minute
+perforations shown in motion by the second lantern while the landscape
+remains on the screen from the first. In the days when dissolving views
+were all the vogue, a third or even a fourth lantern has been added for
+more complicated effects, and at the famous Polytechnic demonstrations of
+years ago, I believe that as many as six were sometimes employed.
+
+In these days of the cinematograph it is doubtful how far interest in such
+effects could be revived, and a lantern has {75} gradually come to be
+looked on more as an instrument for showing illustrations as required by
+the lecturer rather than as a pleasing exhibition in itself, and as
+dissolving views have lost their attraction, the double or triple lantern
+has been relegated to the limbo of antiquity.
+
+[Illustration: FIG. 45.--Beard's Circulating Water Tank.]
+
+Among other 'special' lanterns should be mentioned models made with
+water-cooled stages, for use with very delicate slides. This elaboration is
+not necessary with ordinary slides and illuminants of moderate power, but
+where very delicate slides, such as specimens of natural colour
+photography, have to be shown, it is an advisable precaution to pass the
+beam of light first through a tank of water in order to absorb the heat.
+Lanterns intended for this work are usually constructed with a kind of
+double stage, a glass trough of water fitting into the rear aperture and
+the slide-carrier into the front one. Such an arrangement answers quite
+well for most purposes, but for extreme cases lanterns are equipped with a
+trough connected to a large outside tank and complete circulatory system,
+after the manner of the cooling tank of a gas engine.
+
+Such a lantern, constructed by Messrs. Beard, is illustrated in Fig. 45,
+and it will be seen that in this instrument the water trough is placed
+between the lenses of the condenser. {76} This is a very good position, as
+the beam of light at this point is, or should be, parallel, whereas between
+the condenser and the slide it is convergent, and therefore a condenser of
+a larger diameter than the slide must be employed in the latter case if the
+trough is of considerable width.
+
+While dealing with 'Special' lantern bodies, we should perhaps just mention
+here the numerous pattern lanterns made for the demonstration both of
+lantern slides and of Scientific Phenomena, such as the projection of
+insect life or other microscopic objects, polarised light experiments,
+electrical apparatus, opaque objects, &c. A detailed description of these
+lanterns and how to use them belongs to the second part of this work, as
+also does the popular cinematograph; but educational institutes, and even
+boys' clubs, when considering the purchase of a lantern, might well reflect
+whether it would be advisable to spend a little more money in the
+acquisition of an instrument which can be utilised for a variety of
+purposes.
+
+       *       *       *       *       *
+
+
+CHAPTER IX
+
+LANTERN BOXES, STANDS, READING LAMPS, ETC.
+
+Having now discussed all the essential parts of a lantern, the next points
+to be considered are those of lantern boxes and stands. It is best to take
+these together, as more often than not a lantern is arranged to stand upon
+its box during use, and the plan is both convenient and simple. The whole
+question is one to be settled upon its own merits in each individual case.
+Sometimes neither box nor stand is wanted at all. The lantern is put away
+into a locked-up cupboard or other safe place, and used upon a permanent
+support or (as is often the case in a church) from a gallery at the back.
+{77}
+
+In most cases, however, a box of some sort is desirable, and the two main
+considerations are strength and simplicity.
+
+All patent arrangements, such, for example, as those in which the sides of
+the box fall down and provide trays for the slides, are beautiful in
+theory, but cannot be recommended in practice. A good, simple and
+substantial box is what is required, preferably painted black, and provided
+with strong handles.
+
+One addition may be permitted, viz. a tilting top. Some means for tilting
+the lantern is always advisable, as it is seldom convenient to raise the
+instrument to the level of the centre of the screen, and a slight upward
+elevation does not appreciably distort the image. This arrangement for
+tilting may be either embodied in the lantern itself, as for instance in
+the instrument shown in Fig. 42, or may be provided for on the box or on
+the stand, if a stand is used.
+
+It is, perhaps, an elaboration that may be regarded as not strictly
+necessary, as a book or two or other article may be placed under the
+lantern base as required; but a tilting arrangement is so convenient that
+it can be strongly recommended, and the addition is not expensive.
+
+For large, long-range lanterns a strong deal box, on which the lantern can
+stand, is usually all that it is desirable to purchase in the way of a
+support. A good solid table can usually be found, which will do all the
+rest, as it must be remembered that a slight tilt at a long range means a
+good deal of total elevation.
+
+Where this is not procurable a stand must be provided, and this for a large
+lantern should be strong and rigid. Anything in the way of a collapsible
+tripod should be avoided, but such an arrangement as Fig. 46 is quite good
+and rigid enough for all practical purposes.
+
+For a _small_ lantern a tripod stand is quite suitable, though care must be
+taken that one of the legs does not get kicked, either by accident or
+design, or the result may be a catastrophe. {78}
+
+_Slide Boxes._--On this subject not much need be said. The variety of
+patterns on the market is endless, some being designed from the point of
+view of safe transit by post, others for convenience of storage and
+classification. It is essentially a case where each individual user must
+use his or her taste, and in any case the question of the box is one for
+the owner of the slides rather than for the lanternist.
+
+[Illustration: FIG. 46.--Quadruple Lantern Stand.]
+
+READING-DESKS, LAMPS, AND SIGNALS.--Some form of reading lamp for the
+lecturer is generally considered to be part of a lanternist's equipment,
+and the most usual pattern is fitted with a candle, after the manner of a
+carriage lamp, or else constructed to burn colza or other vegetable oil,
+such as supplied for cycle lamps. Oil gives the brighter light, but is apt
+to get spilled in transit, hence a candle lamp is the more {79} convenient
+for a travelling lecturer, while oil is to be preferred if transport is not
+a factor to be considered.
+
+These lamps are usually constructed with a red flashing signal at the rear,
+actuated by a simple lever, by which the lecturer can communicate his wish
+for a change of slide, &c., to the lanternist (Fig. 47).
+
+[Illustration: FIG. 47.--Reading Lamp.]
+
+There are various other devices used for the same purpose, such as a
+castanet, to be held in the lecturer's hand and clicked when necessary, an
+electric bell to ring in the lantern box, &c. If this latter is used it is
+usual to remove the gong, the buzz of the hammer being sufficiently loud
+without it. Some lecturers again prefer to use no such apparatus at all,
+but simply to say 'Next slide' as required, or to tap on the floor with a
+pointer, and the choice of a suitable means of communication between
+lecturer and lanternist must be largely a matter of individual selection.
+More elaborate _reading-desks_ are also supplied by most makers, but here
+again judgment must largely come into play in what is hardly a technical
+matter.
+
+       *       *       *       *       *
+
+
+CHAPTER X
+
+SCREENS AND SCREEN STANDS
+
+The best of all screens for lantern purposes is undoubtedly a smooth
+whitewashed wall, and this is now provided in many halls where lantern
+exhibitions are usual. In places where this is not practicable the next
+best substitute is a canvas {80} screen, which rolls up and down (Fig. 48).
+This can be obtained from any good maker, but again can only really be used
+as a _fixture_ in the hall where the lantern is to be used. It can,
+however, be fitted into a wooden box which can be painted or varnished to
+suit the other architecture, and the provision of such a screen is to be
+strongly recommended whenever possible. If portability is required, a linen
+or calico sheet that can be folded up is necessary, but this can never be
+hung absolutely flat, and also loses a considerable amount of light by
+transmission.
+
+[Illustration: FIG. 48.--Roller Screen.]
+
+A so-called 'transparent' sheet is made of very thin linen, and intended to
+work with the lantern _behind_ it, showing the picture through the linen to
+the audience on the other side, but this is seldom used except in the open
+air for religious or political meetings, &c.
+
+An _opaque_ sheet can be had in one piece up to 9 feet square; larger sizes
+than this must have at least one seam, and most skilful sewing is
+necessary, especially with large sheets consisting of several strips sewn
+together.
+
+Sheets such as these are usually supplied with either eyelet holes round
+the edges or else linen tapes sewn on, and the exact method of hanging must
+be left to circumstances. {81}
+
+In the case of a small sheet it will be sufficient to stretch it at the
+four corners, and this can often be done by screwing into the walls or some
+convenient girder two screw eyes and similar eyes into the floor, all four
+being considerably farther apart than the size of the sheet.
+
+[Illustration: FIG. 49.--Portable Screen Stand.]
+
+A stout cord being then passed through the two upper eyes, long enough for
+both ends to reach near the floor, one end of each can be fastened to the
+two top corners of the sheet and the latter drawn up, the two bottom
+corners being afterwards stretched and tied down tightly to the lower eyes.
+In the case of large sheets this hardly suffices, and it will be found
+necessary to fasten the sheet at intervals all round or it will exhibit
+awkward creases, and this again is a matter where the lanternist must use
+his own initiative according to the possibilities.
+
+In some halls the erection of a sheet in the way above described is a sheer
+impossibility, and in such cases a frame must be made by nailing strips of
+wood together, or better by utilising a portable screen stand (Fig. 49).
+
+These stands are usually made of bamboo, with short brass connecting tubes,
+and the method of using them is so obvious that a description need hardly
+be given. The screen frames are supplied by all the leading opticians, but
+an intending purchaser would be well advised to see one erected before
+ordering. I have actually seen a 12-foot screen frame offered for sale that
+was too weak to carry its own weight, let alone the weight of the sheet!
+
+       *       *       *       *       *
+
+
+{82}
+
+CHAPTER XI
+
+THE PRACTICAL MANIPULATION OF A LANTERN
+
+Having now described the optical lantern in its various forms and the more
+important accessories, we come to the question of practical manipulation.
+In making arrangements for an exhibition the first thing to be seen to is
+to ensure that every accessory that will be required will be there, and the
+best plan is to make a complete list of all sundries to be provided. Such
+items as string (for the sheet), lime tongs if limelight is used, pliers
+for changing carbons if the arc is to be the illuminant, screw-driver,
+matches, the _key of the lantern box_, and other similar items, are likely
+to be left behind unless such a list is made and carefully checked. On
+arriving at the hall, the first thing to be done as a rule is to get up the
+sheet, after which the professional operator generally begins to feel happy
+again.
+
+The next thing, if it has not been done first, is to determine the position
+of the lantern, and this, as has been explained in Chapter VII, is a matter
+of the size of picture to be shown and the focus of the objective.
+
+It is a mistake to show too large a picture; a little 'white' round the
+edges is a good thing, and it is better to have a small disc well
+illuminated than a large one less bright. Convenience, however, must also
+be considered, and it is often justifiable to go back a few feet farther
+than other considerations would dictate in order to place the lantern in a
+gallery or other spot where it is out of the way.
+
+Having fixed the position of the lantern, it should be got into place, the
+cable or tubing connected or whatever else is necessary, according to the
+illuminant to be used. It should then be lit up, the flasher of the lens
+opened, and the light {83} centred sufficiently to produce some sort of
+disc upon the screen. (It is, of course, presumed that the lenses, &c.,
+have previously been cleaned.)
+
+A carrier should now be placed in the stage and a slide inserted into it,
+and the method of doing so requires a little explanation. The slide must be
+placed in the carrier upside down, as will be obvious to anyone who has
+studied Chapter VII, but in addition to this it must be turned the correct
+way, otherwise the picture will be reversed from left to right. This in the
+case of certain subjects, such as a copy of a picture, may not greatly
+matter; but in slides depicting buildings or landscapes with which the
+audience may be familiar, or worse still, printing or writing, is a serious
+blunder.
+
+Slides made by a commercial firm will usually be 'spotted,' that is to say,
+will have two white spots on the face of the slide when the latter is
+viewed in its correct position, and at the top. The slides should be turned
+upside down and placed in the carrier with the spots, of course, now at the
+bottom and _towards the condenser_.
+
+If a slide is not spotted it should be viewed as it is to appear on the
+screen, and then placed in the carrier with the face that was towards the
+operator as he viewed it turned to the condenser, and of course inverted.
+
+The above remarks apply only in cases where the image is thrown _on_ the
+screen; in the comparatively rare instances where it is shown _through_ the
+latter the slides must be turned round laterally, but of course still
+inverted. The slide having been placed in the stage it should be 'focussed'
+by racking the objective in or out, and if necessary pulling out the draw
+tube as well until the image on the screen is sharply defined. So far the
+light has only been roughly centred, sufficiently so to enable the slide to
+be focussed, and to complete the operation both slide and carrier should
+next be taken out of the lantern, leaving a clear disc on the screen, and
+this disc may resemble any of the appearances shown in Fig. 50. {84}
+
+If it resembles A the light must be moved to the left, if like B to the
+right, like C it must be lowered, like D it must be raised, always moving
+it to the side opposite to the dark shade until this is central on the
+disc. If it now resembles E, the light must be moved nearer the condenser;
+if, on the contrary, the centre is dark, it must be drawn back until
+finally the circle should be as nearly as possible clear and bright all
+over, as at F.
+
+[Illustration: FIG. 50.--Adjustment of the Light.]
+
+It is important to note that this adjustment _cannot_ be properly made
+while a slide is in position, and neither can it be made until the lantern
+has been focussed, so the above procedure is the only way to get a
+satisfactory result. With some of the larger illuminants, such as a
+paraffin-oil lamp, there are no centering adjustments, the size of the
+radiant rendering exact centering unnecessary, and generally speaking the
+smaller the luminous point, the more exact must the operation be.
+
+In the case of such illuminants as acetylene or limelight {85} care must be
+taken that they are turned fully on before centering, otherwise turning on
+the fuller amount afterwards will raise the position of the luminous spot.
+
+The centering achieved, the slide carrier may be replaced, the first slides
+placed in position, the remainder arranged in their proper order, the
+system of signalling with the lecturer determined, and all is ready.
+
+If there is still an interval before commencing, the light may be switched
+off or turned out, or in the case say of limelight, turned down very low
+until wanted.
+
+It is of extreme importance to see that all the slides are in their right
+order, though the duty of seeing to this usually rests with the lecturer
+rather than with the operator. I remember hearing of one lecture on the
+life of Queen Victoria, when the lecturer announced, 'The next picture will
+be a photograph of the Royal Prince who for many years shared the Throne
+with our gracious Sovereign.' At the words the operator brought on the next
+slide, which proved to be _a restored specimen of a prehistoric monster_
+(tableaux!). Such mistakes 'bring down the house,' but in serious lectures,
+and especially at religious services, cannot be too carefully guarded
+against.
+
+Mention has already been made of the liability of moisture to condense on
+the surfaces of the condensers or slides, and to avoid this, so far as the
+condensers are concerned, it is well to light up say ten minutes before the
+lantern is actually wanted, or alternatively to take out the condensers and
+thoroughly warm them in front of a stove, or to place them wrapped in a
+cloth on hot-water pipes. The slides should in the same manner be warmed
+before using and should be finally held above the lantern or placed on the
+top, if this is flat, the last thing before being placed in the carrier. If
+these precautions are omitted, on a cold night the first surface of the
+condenser will become so covered with moisture as to almost obscure the
+slide, and this will quickly disappear {86} with the heat of the lantern.
+Next, the two inner surfaces of the condensers will behave in turn in the
+same way, and will take considerably longer to clear, especially if the
+ventilation of the condenser is poor; then the fourth surface will take up
+the running, and finally, when the lanternist is congratulating himself
+that the trouble is over, each successive slide will become affected in the
+same way. With an operator who knows his business, none of these troubles
+should occur.
+
+ACCIDENTS.--These will occur sometimes, even in the best managed
+exhibition; the rubber tubing feeding a limelight jet gets kinked or
+trodden on, or a fuse melts if electric light is being used, &c., and out
+goes the light. In such cases a loud request such as, 'Would you mind
+turning up the light for a minute, please,' accompanied by a good-humoured
+laugh, usually allays the fears of 'nervy' people. An operator must never
+get 'nervy' himself. I have known of more than one fiasco because some
+little hitch occurred, and two or three timid ladies crowded round and
+asked anxious questions, till the lanternist lost his head. In one such
+case the cautious superintendent at a children's entertainment decided that
+it would be safer not to have the exhibition at all, simply because a
+regulator was not screwed tightly enough into a cylinder to prevent an
+escape of gas, only the operator (a somewhat youthful one) had been driven
+to the verge of lunacy by continual questions of the standard type, 'Are
+you sure it is safe?' 'Will it blow up?' 'Are you certain you understand
+it?' &c., &c. More serious accidents, such as the entire lantern getting
+upset, ought never to occur, and it is up to the lanternist to take
+whatever precautions he deems necessary to safeguard his instrument. With a
+juvenile audience, for example, it is often a good thing to arrange a
+barricade of forms round the lantern and to see that no one comes within
+it.
+
+Finally, 'whatever is worth doing at all is worth doing well,' and this is
+as true of lantern exhibiting as of anything {87} else. There are a
+deplorable number of lantern exhibitions given with the sheet hanging in
+creases, dirty lenses, light poorly adjusted and centred, and occasionally
+slides shown upside down. A conscientious lanternist should see to _every_
+detail; slipshod methods, as in everything else, mean poor results.
+
+  Printed by SPOTTISWOODE, BALLANTYNE & CO. LTD.
+  Colchester, London & Eton, England
+
+
+
+
+
+
+End of the Project Gutenberg EBook of Optical Projection, by 
+Lewis Wright and Russell S. Wright
+
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+
+<pre>
+
+Project Gutenberg's Optical Projection, by Lewis Wright and Russell S. Wright
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Optical Projection
+       Part 1: Projection of Lantern Slides
+
+Author: Lewis Wright
+        Russell S. Wright
+
+Release Date: October 31, 2010 [EBook #33899]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK OPTICAL PROJECTION ***
+
+
+
+
+Produced by Chris Curnow, Keith Edkins and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+</pre>
+
+
+<h1>OPTICAL PROJECTION</h1>
+
+<p class="cenhead">A TREATISE ON THE USE OF THE LANTERN IN<br />
+EXHIBITION AND SCIENTIFIC DEMONSTRATION</p>
+
+<p class="cenhead"><span class="scac">BY</span></p>
+
+<h3>LEWIS WRIGHT</h3>
+
+<p class="cenhead"><span class="scac">AUTHOR OF 'LIGHT: A COURSE OF EXPERIMENTAL OPTICS'</span></p>
+
+<p class="cenhead">5<span class="scac">TH</span> EDITION</p>
+
+<p class="cenhead"><span class="scac">RE-WRITTEN AND BROUGHT UP-TO-DATE BY</span></p>
+
+<h3>RUSSELL S. WRIGHT, M.I.E.E.</h3>
+
+<h3>IN TWO PARTS</h3>
+
+<h3>PART I</h3>
+
+<h3><i>THE PROJECTION OF LANTERN SLIDES</i></h3>
+
+  <p>&nbsp;</p>
+
+<p class="cenhead"><i>WITH ILLUSTRATIONS</i></p>
+
+  <p>&nbsp;</p>
+
+<h3>L<span class="gsp">&nbsp;</span>O<span class="gsp">&nbsp;</span>N<span class="gsp">&nbsp;</span>G<span class="gsp">&nbsp;</span>M<span class="gsp">&nbsp;</span>A<span class="gsp">&nbsp;</span>N<span class="gsp">&nbsp;</span>S<span class="gsp">&nbsp;</span>,<span class="gsp">&nbsp;</span> <span class="gsp">&nbsp;</span>G<span class="gsp">&nbsp;</span>R<span class="gsp">&nbsp;</span>E<span class="gsp">&nbsp;</span>E<span class="gsp">&nbsp;</span>N<span class="gsp">&nbsp;</span>,<span class="gsp">&nbsp;</span> <span class="gsp">&nbsp;</span>A<span class="gsp">&nbsp;</span>N<span class="gsp">&nbsp;</span>D<span class="gsp">&nbsp;</span> <span class="gsp">&nbsp;</span>C<span class="gsp">&nbsp;</span>O.<br />
+39 PATERNOSTER ROW, LONDON, E.C.</h3>
+<p class="cenhead">FOURTH AVENUE &amp; 30TH STREET, NEW YORK<br />
+BOMBAY, CALCUTTA, AND MADRAS</p>
+
+<h3>1920</h3>
+
+<p class="cenhead">(<i>A<span class="gsp">&nbsp;</span>l<span class="gsp">&nbsp;</span>l<span class="gsp">&nbsp;</span> <span class="gsp">&nbsp;</span>r<span class="gsp">&nbsp;</span>i<span class="gsp">&nbsp;</span>g<span class="gsp">&nbsp;</span>h<span class="gsp">&nbsp;</span>t<span class="gsp">&nbsp;</span>s<span class="gsp">&nbsp;</span> <span class="gsp">&nbsp;</span>r<span class="gsp">&nbsp;</span>e<span class="gsp">&nbsp;</span>s<span class="gsp">&nbsp;</span>e<span class="gsp">&nbsp;</span>r<span class="gsp">&nbsp;</span>v<span class="gsp">&nbsp;</span>e<span class="gsp">&nbsp;</span>d</i>)</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>{v}</span></p>
+
+<h2>PREFACE</h2>
+<p class="cenhead">TO</p>
+<h3>THE FIFTH EDITION</h3>
+
+  <p>The first edition of this work was written by my father, the late Mr.
+  Lewis Wright, and was published in 1890.</p>
+
+  <p>The reception that it received testified to the fact that it met a
+  long-felt want, and successive editions were published in 1895, 1901, and
+  1906.</p>
+
+  <p>My father, unfortunately, met his death in a railway accident in 1905,
+  and the corrections and additions to the last edition, which had been to
+  a certain extent prepared by him, were completed and written by myself,
+  and the work as published then was again reprinted in 1911.</p>
+
+  <p>As the original text is now thirty years old, it has seemed better
+  entirely to re-write the whole book rather than make fresh revisions, the
+  more so as the last ten years have seen great advances in the science of
+  Lantern Projection, and especially in the developments of Acetylene and
+  Electric Lighting.</p>
+
+  <p>It has also seemed best at the present juncture to issue the book in
+  two parts, the first dealing with the Projection of Lantern Slides only,
+  and the second with the Demonstration of Opaque and Microscopic Objects,
+  Scientific Phenomena and accessory apparatus, including Cinematograph
+  Projection.</p>
+
+  <p>It must of necessity be many months before this second volume can be
+  produced, for the simple reason that Optical <!-- Page vi --><span
+  class="pagenum"><a name="pagevi"></a>{vi}</span>Instrument Makers have as
+  yet hardly had time to turn round after the war and produce their new
+  models, and therefore any such book written now could do little more than
+  describe apparatus that was on the market prior to 1914.</p>
+
+  <p>The present work, therefore, deals solely with the exhibition of
+  Lantern Slides in the Optical Lantern, and as such I trust will be found
+  of value to Schoolmasters, Social Workers, Lecturers, and, in fact, to
+  all who use the lantern as a means of illustration.</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>RUSSELL S. WRIGHT.</p>
+      <p><i>January 1920.</i></p>
+    </div>
+  </div>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page vii --><span class="pagenum"><a name="pagevii"></a>{vii}</span></p>
+
+<h3>CONTENTS</h3>
+
+<table class="nobctr" summary="Table of Contents" title="Table of Contents">
+<tr><td class="spacsingle"> CHAPTER </td><td class="spacsingle" style="text-align:right;"> PAGE</td></tr>
+<tr><td class="spacsingle"> I. <span class="sc">Introductory</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page1">1</a></td></tr>
+<tr><td class="spacsingle"> II. <span class="sc">The Illuminant</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page3">3</a></td></tr>
+<tr><td class="spacsingle"> III. <span class="sc">Paraffin-oil Lamps, Incandescent Gas and Spirit Burners</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page6">6</a></td></tr>
+<tr><td class="spacsingle"> IV. <span class="sc">The Acetylene Light</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page11">11</a></td></tr>
+<tr><td class="spacsingle"> V. <span class="sc">Limelight and the Acetylene Blast</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page16">16</a></td></tr>
+<tr><td class="spacsingle"> VI. <span class="sc">The Electric Light</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page39">39</a></td></tr>
+<tr><td class="spacsingle"> VII. <span class="sc">The Optical System of a Lantern</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page57">57</a></td></tr>
+<tr><td class="spacsingle"> VIII. <span class="sc">The Body of the Lantern</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page70">70</a></td></tr>
+<tr><td class="spacsingle"> IX. <span class="sc">Lantern Boxes, Stands, Reading Lamps, etc.</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page76">76</a></td></tr>
+<tr><td class="spacsingle"> X. <span class="sc">Screens and Screen Stands</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page79">79</a></td></tr>
+<tr><td class="spacsingle"> XI. <span class="sc">The Practical Manipulation of a Lantern</span> </td><td class="spacsingle" style="text-align:right;"> <a href="#page82">82</a></td></tr>
+</table>
+
+<p><!-- Page viii --><span class="pagenum"><a name="pageviii"></a>{viii}</span></p>
+
+<h3>ILLUSTRATIONS</h3>
+
+<table class="nobctr" summary="Illustrations" title="Illustrations">
+<tr><td class="spacsingle"> FIG. </td><td class="spacsingle" style="text-align:right;"> PAGE</td></tr>
+<tr><td class="spacsingle"> 1. Oil Lamp </td><td class="spacsingle" style="text-align:right;"> <a href="#page6">6</a></td></tr>
+<tr><td class="spacsingle"> 2. Inverted Incandescent Lamp </td><td class="spacsingle" style="text-align:right;"> <a href="#page8">8</a></td></tr>
+<tr><td class="spacsingle"> 3. Methylated Spirit Burner </td><td class="spacsingle" style="text-align:right;"> <a href="#page9">9</a></td></tr>
+<tr><td class="spacsingle"> 4. Luna Lamp </td><td class="spacsingle" style="text-align:right;"> <a href="#page10">10</a></td></tr>
+<tr><td class="spacsingle"> 5. The Moss Generator </td><td class="spacsingle" style="text-align:right;"> <a href="#page12">12</a></td></tr>
+<tr><td class="spacsingle"> 6. The A.L. or 'Popular' Model </td><td class="spacsingle" style="text-align:right;"> <a href="#page14">14</a></td></tr>
+<tr><td class="spacsingle"> 7. Acetylene Jet </td><td class="spacsingle" style="text-align:right;"> <a href="#page15">15</a></td></tr>
+<tr><td class="spacsingle"> 8. Oxygen Cylinder in hemp cover </td><td class="spacsingle" style="text-align:right;"> <a href="#page17">17</a></td></tr>
+<tr><td class="spacsingle"> 9. Double Lever Key </td><td class="spacsingle" style="text-align:right;"> <a href="#page18">18</a></td></tr>
+<tr><td class="spacsingle"> 10. Fine Adjustment Valve </td><td class="spacsingle" style="text-align:right;"> <a href="#page19">19</a></td></tr>
+<tr><td class="spacsingle"> 11. Construction of Beard's Regulator </td><td class="spacsingle" style="text-align:right;"> <a href="#page20">20</a></td></tr>
+<tr><td class="spacsingle"> 12. Beard's Regulator </td><td class="spacsingle" style="text-align:right;"> <a href="#page21">21</a></td></tr>
+<tr><td class="spacsingle"> 13. Regulator and Gauge </td><td class="spacsingle" style="text-align:right;"> <a href="#page22">22</a></td></tr>
+<tr><td class="spacsingle"> 14. Gas-bags </td><td class="spacsingle" style="text-align:right;"> <a href="#page24">24</a></td></tr>
+<tr><td class="spacsingle"> 15. 'Blow-through' Nozzles </td><td class="spacsingle" style="text-align:right;"> <a href="#page25">25</a></td></tr>
+<tr><td class="spacsingle"> 16. 'Blow-through' Jet </td><td class="spacsingle" style="text-align:right;"> <a href="#page25">25</a></td></tr>
+<tr><td class="spacsingle"> 17. Mixed Jet </td><td class="spacsingle" style="text-align:right;"> <a href="#page27">27</a></td></tr>
+<tr><td class="spacsingle"> 18. Mixed Jet, Gwyer pattern </td><td class="spacsingle" style="text-align:right;"> <a href="#page27">27</a></td></tr>
+<tr><td class="spacsingle"> 19. Mixing Chamber of Jet </td><td class="spacsingle" style="text-align:right;"> <a href="#page28">28</a></td></tr>
+<tr><td class="spacsingle"> 20. 'Injector' Jet </td><td class="spacsingle" style="text-align:right;"> <a href="#page30">30</a></td></tr>
+<tr><td class="spacsingle"> 21. 'Gridiron' Saturator </td><td class="spacsingle" style="text-align:right;"> <a href="#page32">32</a></td></tr>
+<tr><td class="spacsingle"> 22. 'Pendant' Saturator </td><td class="spacsingle" style="text-align:right;"> <a href="#page33">33</a></td></tr>
+<tr><td class="spacsingle"> 23. Fallot Air Blast </td><td class="spacsingle" style="text-align:right;"> <a href="#page37">37</a></td></tr>
+<tr><td class="spacsingle"> 24. Fallot Air Blast, and Cylinder </td><td class="spacsingle" style="text-align:right;"> <a href="#page37">37</a></td></tr>
+<tr><td class="spacsingle"> 25. Lime-tongs </td><td class="spacsingle" style="text-align:right;"> <a href="#page39">39</a></td></tr>
+<tr><td class="spacsingle"> 26. Universal Hand-fed Arc Lamp </td><td class="spacsingle" style="text-align:right;"> <a href="#page45">45</a></td></tr>
+<tr><td class="spacsingle"> 27. </td><td class="spacsingle" style="text-align:right;"> <a href="#page46">46</a></td></tr>
+<tr><td class="spacsingle"> 28. Resistance </td><td class="spacsingle" style="text-align:right;"> <a href="#page49">49</a></td></tr>
+<tr><td class="spacsingle"> 29. 'Scissors' Arc Lamp </td><td class="spacsingle" style="text-align:right;"> <a href="#page51">51</a></td></tr>
+<tr><td class="spacsingle"> 30. 'Right-angled' Arc Lamp </td><td class="spacsingle" style="text-align:right;"> <a href="#page52">52</a></td></tr>
+<tr><td class="spacsingle"> 31. 'Westminster' Arc Lamp </td><td class="spacsingle" style="text-align:right;"> <a href="#page53">53</a></td></tr>
+<tr><td class="spacsingle"> 32. Arc Lamp with Induction Ring </td><td class="spacsingle" style="text-align:right;"> <a href="#page56">56</a></td></tr>
+<tr><td class="spacsingle"> 33. The Optical System of a Lantern <i>facing p.</i> </td><td class="spacsingle" style="text-align:right;"> <a href="#page57">57</a></td></tr>
+<tr><td class="spacsingle"> 33<span class="scac">A</span>. Optical System of Lantern </td><td class="spacsingle" style="text-align:right;"> <a href="#page57">57</a></td></tr>
+<tr><td class="spacsingle"> 34. Optical System without Condenser </td><td class="spacsingle" style="text-align:right;"> <a href="#page59">59</a></td></tr>
+<tr><td class="spacsingle"> 35. Action of Condenser </td><td class="spacsingle" style="text-align:right;"> <a href="#page59">59</a></td></tr>
+<tr><td class="spacsingle"> 36. Forms of Condensers </td><td class="spacsingle" style="text-align:right;"> <a href="#page60">60</a></td></tr>
+<tr><td class="spacsingle"> 37. Double Sliding Carrier </td><td class="spacsingle" style="text-align:right;"> <a href="#page62">62</a></td></tr>
+<tr><td class="spacsingle"> 38. Beard's Dissolving Carrier </td><td class="spacsingle" style="text-align:right;"> <a href="#page63">63</a></td></tr>
+<tr><td class="spacsingle"> 39. Focussing Action of Lens </td><td class="spacsingle" style="text-align:right;"> <a href="#page64">64</a></td></tr>
+<tr><td class="spacsingle"> 40. Achromatic Lens </td><td class="spacsingle" style="text-align:right;"> <a href="#page65">65</a></td></tr>
+<tr><td class="spacsingle"> 41. Petzval Combination </td><td class="spacsingle" style="text-align:right;"> <a href="#page66">66</a></td></tr>
+<tr><td class="spacsingle"> 42. Hughes' Short-Range Lantern </td><td class="spacsingle" style="text-align:right;"> <a href="#page71">71</a></td></tr>
+<tr><td class="spacsingle"> 43. Long-Range Lantern </td><td class="spacsingle" style="text-align:right;"> <a href="#page72">72</a></td></tr>
+<tr><td class="spacsingle"> 44. Connections for a Bi-unial Lantern </td><td class="spacsingle" style="text-align:right;"> <a href="#page73">73</a></td></tr>
+<tr><td class="spacsingle"> 45. Beard's Circulating Water Tank </td><td class="spacsingle" style="text-align:right;"> <a href="#page75">75</a></td></tr>
+<tr><td class="spacsingle"> 46. Quadruple Lantern Stand </td><td class="spacsingle" style="text-align:right;"> <a href="#page78">78</a></td></tr>
+<tr><td class="spacsingle"> 47. Reading Lamp </td><td class="spacsingle" style="text-align:right;"> <a href="#page79">79</a></td></tr>
+<tr><td class="spacsingle"> 48. Roller Screen </td><td class="spacsingle" style="text-align:right;"> <a href="#page80">80</a></td></tr>
+<tr><td class="spacsingle"> 49. Portable Screen Stand </td><td class="spacsingle" style="text-align:right;"> <a href="#page81">81</a></td></tr>
+<tr><td class="spacsingle"> 50. Adjustment of the Light </td><td class="spacsingle" style="text-align:right;"> <a href="#page84">84</a></td></tr>
+</table>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>{1}</span></p>
+
+<h2>OPTICAL PROJECTION</h2>
+
+<h3>A TREATISE ON THE USE OF THE OPTICAL
+LANTERN</h3>
+
+<h3>CHAPTER I</h3>
+
+<p class="cenhead">INTRODUCTORY</p>
+
+  <p>Lantern Projection, as commonly understood, may be broadly subdivided
+  into two branches: (A) The Projection of Lantern Slides, and (B) The
+  Projection of Scientific Phenomena, Opaque Objects, Microscopic
+  Specimens, &amp;c., usually referred to broadly under the heading of
+  'Scientific Demonstration.'</p>
+
+  <p>To these two classes may perhaps now be added a third, viz. The
+  Projection of So-called Living Pictures, or, in other words, the
+  Cinematograph. In the earlier editions of this work both A and B were
+  dealt with in the same volume, but, as there are thousands who require to
+  use a lantern for the demonstration of lantern slides only, and who have
+  no interest or concern with Science Projection, it has seemed to the
+  writer that the work might, with advantage, be divided into two portions,
+  Vol. I. dealing with slides only, and Vol. II. with the various
+  adaptations of the science lantern. This present book therefore only
+  deals with the exhibition of lantern slides, and as such it will, I
+  trust, be found to be of real assistance to the ordinary user of the
+  optical lantern, including clergymen, schoolmasters, army and cadet
+  officers, and others <!-- Page 2 --><span class="pagenum"><a
+  name="page2"></a>{2}</span>who require advice and instruction in the
+  purchase or use of a lantern.</p>
+
+  <p>The essential parts of a lantern are: (<i>a</i>) A <i>slide-holder</i>
+  or <i>carrier</i> to hold the slide; (<i>b</i>) a <i>lens</i> to 'focus'
+  it on the screen; (<i>c</i>) a <i>condenser</i> to converge the light
+  upon slide and lens; (<i>d</i>) a source of light or <i>radiant</i> to
+  provide the necessary illumination; and (<i>e</i>) a <i>body</i> or
+  framework to hold the whole together. All possible variations in choice
+  of a suitable lantern relate to one or another of the above parts, and
+  will be treated of in turn; but, fortunately, we have this all-important
+  simplification that every ordinary English lantern slide is the same
+  <i>standard size</i>, viz. 3¼ inches square. Some Continental and
+  American slides differ in one dimension from the above, but not enough to
+  cause any serious difficulty, and the convenient English standard is
+  being gradually adopted throughout the world.</p>
+
+  <p>The varieties of slide-holders or carriers are therefore comparatively
+  few and are chiefly concerned with the question of rapidly and easily
+  changing the slides. The choice of a focussing lens or objective is
+  mainly a matter of the size of picture required, and the most convenient
+  distance from the screen for the lantern to be placed. Variations in
+  condensers, which are comparatively small, are usually only a matter of
+  conforming these with the size or type of objective to be used, and
+  should be left to the manufacturer's judgment. The question of a suitable
+  radiant is partly a matter of the amount of illumination required, and
+  partly that of the practical possibilities; for example, if electric
+  current is available some form of electric light is usually the most
+  convenient, as well as the least expensive, but where this is not the
+  case, paraffin-oil, methylated spirit, incandescent gas, acetylene,
+  limelight, &amp;c., are alternatives which all have their uses and must
+  be considered on their own merits.</p>
+
+  <p>Sometimes, as for example in the case of a travelling lecturer, a
+  lantern is required fitted with a range of lenses for <!-- Page 3
+  --><span class="pagenum"><a name="page3"></a>{3}</span>halls of different
+  size, and also with a variety of illuminants, and this in most lanterns
+  can be easily provided for.</p>
+
+  <p>The body is usually a matter of taste and price only, and may range
+  from a simple but efficient shell of Russian iron to an elaborate
+  mahogany instrument with a brass front, screw tilting arrangements and
+  other adornments; but of late years there has been a wholesome reaction
+  against unnecessary finish, and a simple metal body of some description
+  is now chiefly the order of the day. In the foregoing remarks the various
+  parts of a lantern have been mentioned in what I should consider the
+  correct order, starting from the slide and slide-holder, and so to speak
+  building up the rest of the instrument round these items; but I now
+  propose somewhat to vary the procedure and for convenience deal in detail
+  first with the Radiant, or <i>Illuminant</i>.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h3>CHAPTER II</h3>
+
+<p class="cenhead">THE ILLUMINANT</p>
+
+  <p>The first necessity for lantern projection is a strong light, and this
+  can be obtained from a variety of sources, the principal means in common
+  use being approximately in order of excellence as follows: paraffin-oil,
+  incandescent spirit, incandescent gas, acetylene, acetylene air blast,
+  oxyhydrogen (limelight), oxyether, and electric light in its various
+  forms. The ideal characteristics to be sought for are (1) great intrinsic
+  brilliancy; (2) minimum <i>size</i> of luminous spot; (3) freedom from
+  flicker; (4) freedom from smell; (5) absence of any preponderating
+  colour; (6) cheapness; and (7) convenience. There is no question whatever
+  as to which of the available sources of light most perfectly combines all
+  the above if it is available, viz. the electric arc. If a current supply
+  is in <!-- Page 4 --><span class="pagenum"><a
+  name="page4"></a>{4}</span>the building, this form of lighting easily
+  excels all others, except possibly in the matter of flicker, and even in
+  this respect there is very little fault to be found with it.</p>
+
+  <p>From all other points of view it is wellnigh perfect, inasmuch as it
+  provides an extremely concentrated and intensely luminous spot, of almost
+  perfect whiteness (if anything slightly bluish), no smell, comparatively
+  little heat, convenient and inexpensive. So great is the advantage of the
+  electric arc that attempts have been made to use it from accumulators in
+  places where a current supply is not available, but this cannot be
+  seriously recommended, except in special cases. Where an electric supply
+  is, however, available there can be no real choice, whether the lantern
+  is required for use in a large hall or a small class-room. The advantages
+  of using the arc are so great that no other method need be seriously
+  considered.</p>
+
+  <p>The one real objection that I have heard urged against it is due,
+  curiously enough, to its very perfection, and that is, that it lends
+  itself to such exceedingly sharp definition that any slight imperfection
+  in the slide is too faithfully reproduced on the screen, for which reason
+  it is sometimes recommended that the operator shall work with the
+  objective the least fraction out of focus; but this is a matter for
+  individual taste and judgment.</p>
+
+  <p>If, however, there is no possibility of using the electric current,
+  one of the other sources of illumination must perforce be adopted, and
+  for a <i>large</i> hall this can only be limelight in one of its many
+  forms, viz. oxyhydrogen, oxyether, oxyacetylene, &amp;c. As regards
+  results on the screen, this light compares well even against the electric
+  arc, but it involves the expense and trouble of compressed gas cylinders,
+  or the infinitely worse recourse to the now obsolete method of filling
+  gas-bags.</p>
+
+  <p>Limelight is therefore now but little used in this country, as the
+  majority of large halls are equipped with the electric <!-- Page 5
+  --><span class="pagenum"><a name="page5"></a>{5}</span>current, and for
+  smaller buildings it is deemed unnecessary and too expensive.</p>
+
+  <p><b>Acetylene</b> is undoubtedly the illuminant most in favour next to
+  electric light, as the light is brilliant enough to illuminate a picture
+  12 feet in diameter at a distance up to, say, 30 feet from the screen,
+  and this suffices in a large majority of cases, and acetylene is
+  comparatively cheap, and reasonably simple to work.</p>
+
+  <p><b>Incandescent-gas</b> is often employed for small class-rooms and is
+  fairly effective for a picture not exceeding 9 or 10 feet in diameter,
+  and the same can be said of the same type of burner heated by methylated
+  spirit.</p>
+
+  <p><b>Paraffin-oil</b> is the poorest of all present-day forms of lantern
+  illuminants. The flame is large, impairing the definition, yellow in
+  colour, uneven in illumination, liable to smoke and smell, and barely
+  equal to incandescent gas in illuminating power.</p>
+
+  <p>It is therefore going gradually out of use in this country, but in
+  out-of-the-way places, especially abroad, it is sometimes the only
+  practicable light, and is therefore still employed from the best of all
+  reasons, necessity.</p>
+
+  <p>It is not the intention of the author to give precise working
+  instruction for all and every variety of the above illuminants as
+  manufactured by different firms. For such the reader must be referred to
+  the directions usually issued by the makers themselves, but a general
+  description of the various types offered for choice will not be out of
+  place, and it will be more convenient to begin with the poorest, viz.
+  paraffin-oil, and finish with the most perfect, the electric arc.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 6 --><span class="pagenum"><a name="page6"></a>{6}</span></p>
+
+<h3>CHAPTER III</h3>
+
+<p class="cenhead">PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND
+SPIRIT BURNERS</p>
+
+  <div class="figcenter" style="width:47%;">
+      <a href="images/Fig01.jpg"><img style="width:100%" src="images/Fig01.jpg"
+      alt="Fig. 1.--Oil Lamp" title="Fig. 1.--Oil Lamp" /></a>
+    <span class="sc">Fig. 1.</span>&mdash;Oil Lamp.
+  </div>
+
+  <p>There are several varieties of oil lamps on the market, but in
+  practically every case they take the same general form, a metal reservoir
+  sliding in grooves in the lantern body and holding approximately a pint
+  of oil with (usually) four wicks <i>nearly</i> parallel, but slightly
+  converging from rear to front, these enclosed in a flame chamber of
+  Russian iron, with <i>loose</i> well-annealed ends of sheet glass and an
+  adjustable reflector at the back, or sometimes the reflector itself forms
+  the rear end of the flame chamber. The chimney must be tall and is now
+  usually made adjustable, though I have never been able to trace any real
+  advantage from this complication <!-- Page 7 --><span class="pagenum"><a
+  name="page7"></a>{7}</span>(Fig. 1). The whole secret of obtaining the
+  best results from these lamps may be summed up&mdash;<i>good oil and
+  perfect cleanliness</i>; and it is wonderful what can be done when these
+  points are properly attended to.</p>
+
+  <p>Care should be taken in trimming the wicks to see that no charred
+  parts fall down between the wick holders, but it makes little difference
+  whether the trimming is done with scissors or by rubbing with the finger.
+  Special lamp scissors are sold by all makers with a large flat on one
+  side to catch the portions cut off.</p>
+
+  <p>These lamps should be well rubbed over the last thing before use, as
+  paraffin-oil is apt to 'creep,' and the operator does not want to be told
+  that his apparatus is suggestive of a fried fish shop. In working with
+  these lamps it is difficult to avoid a dark streak down the centre of the
+  sheet, representing the space between the two centre wicks; to a certain
+  extent this can be obviated by adjusting the reflector, and in any case
+  is not very obvious when the slide is in place. Lamps constructed with
+  either three or five wicks are better in this respect, but the former are
+  usually considered to be too poor in illuminating power, and the latter
+  are apt to crack the sheet-glass ends by excessive heat.</p>
+
+  <p><b>Incandescent Gas.</b>&mdash;Incandescent gas burners do not need
+  much description, as they are practically similar to those in general use
+  for house lighting. They may be either of the erect or inverted forms,
+  the latter being preferable owing to the light being more concentrated,
+  and a reflector is provided to increase the illumination (Fig. 2).</p>
+
+  <p>These reflectors should be <i>spherical</i> and so adjusted that the
+  radiant is in the centre of curvature, thus ensuring that the light from
+  the reflector passes again through the original source. If this point is
+  not attended to, we shall be dealing with essentially two sources of
+  light instead of one, to the detriment of the definition.</p>
+
+  <p>The same remark applies to every lantern illuminant <!-- Page 8
+  --><span class="pagenum"><a name="page8"></a>{8}</span>which is
+  supplemented by a reflector, and it is extraordinary how often it is
+  neglected by the manufacturer. Of course the opacity of the illuminant
+  destroys much of the efficiency of the reflector, and hence in the case
+  of incandescent gas mantles there is not much real gain in making use of
+  them, but with these comparatively weak illuminants every fraction tells,
+  and the reflector does not add much to the cost.</p>
+
+  <div class="figcenter" style="width:44%;">
+      <a href="images/Fig02.jpg"><img style="width:100%" src="images/Fig02.jpg"
+      alt="Fig. 2.--Inverted Incandescent Lamp" title="Fig. 2.--Inverted Incandescent Lamp" /></a>
+    <span class="sc">Fig. 2.</span>&mdash;Inverted Incandescent Lamp.
+  </div>
+
+  <p>In light the inverted gas burner is very little superior to oil, but
+  it is whiter, slightly more concentrated, and freer from smell, and
+  therefore to be regarded as preferable if a supply of gas is
+  available.</p>
+
+  <p><b>Methylated Spirit Burners.</b>&mdash;Incandescent mantles heated by
+  methylated spirit are also largely used, and provide a light decidedly
+  superior to gas and nearly equal to acetylene. Some arrangement must be
+  made for volatilising the spirit and driving the vapour out under
+  pressure, and the most usual contrivance is somewhat as illustrated in
+  Fig. 3.</p>
+
+  <p>In this apparatus the spirit is contained in a metal reservoir at the
+  rear and air pressure is provided by a pair of rubber balls and valves
+  after the manner of a medical spray. Sufficient <!-- Page 9 --><span
+  class="pagenum"><a name="page9"></a>{9}</span>pressure having been
+  obtained, the liquid spirit is forced into a vaporising chamber
+  immediately behind the mantle, and a kind of miniature pitchfork, with
+  its prongs wrapped in asbestos wool, is soaked in spirit, and pushed over
+  the brass fitting of the burner in such a way that when lighted the flame
+  heats the chamber and volatilises the spirit. The burner can now be lit,
+  and although the fork burns out in the course of a minute or so, the heat
+  from the mantle itself is thereafter sufficient to vaporise the spirit as
+  rapidly as required. This lamp works exceedingly well in practice, but
+  has one drawback, viz. that it is possible to obtain too much pressure
+  and squirt <i>liquid</i> spirit through the burner, when it naturally
+  catches fire and may even run on to the floor.</p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/Fig03.jpg"><img style="width:100%" src="images/Fig03.jpg"
+      alt="Fig. 3.--Methylated Spirit Burner" title="Fig. 3.--Methylated Spirit Burner" /></a>
+    <span class="sc">Fig. 3.</span>&mdash;Methylated Spirit Burner.
+  </div>
+
+  <p>An accident of this sort is rare and usually harmless even if it does
+  occur, but an audience is easily frightened, and hence this burner should
+  only be used by <i>an operator with experience</i>. An altogether better
+  arrangement is that made by Messrs. Hughes of Kingsland and known as the
+  'Luna' Lamp (Fig. 4).</p>
+
+  <p>In this burner there is no pump and no volatilising chamber; <!-- Page
+  10 --><span class="pagenum"><a name="page10"></a>{10}</span>the spirit is
+  contained as before in a metal reservoir and a separate burner underneath
+  is used to keep this sufficiently hot to both vaporise the spirit and
+  provide the necessary pressure. The heat can be regulated by means of an
+  adjustable sheath to the burner, and a simple safety valve provides
+  against an excess of vapour.</p>
+
+  <p>I do not say that an accident of the sort previously referred to is
+  impossible even with this burner, but I have never heard of it happening,
+  and the lamp is certainly the best apparatus of its kind that I am
+  acquainted with.</p>
+
+  <div class="figcenter" style="width:29%;">
+      <a href="images/Fig04.jpg"><img style="width:100%" src="images/Fig04.jpg"
+      alt="Fig. 4.--Luna Lamp" title="Fig. 4.--Luna Lamp" /></a>
+    <span class="sc">Fig. 4.</span>&mdash;Luna Lamp.
+  </div>
+
+  <p><b>Incandescent Electric Lamps.</b>&mdash;Incandescent electric lamps
+  of the ordinary metal or carbon filament type are also frequently used in
+  small class-rooms, and should be mentioned here, as they provide
+  approximately the same illumination as a gas mantle, or in some cases
+  rather better. It will, however, be more convenient to deal with the
+  question of electric lighting as a whole in the chapter devoted to
+  it.</p>
+
+  <p>It will suffice here to say that lamps are made for the purpose with a
+  special filament arranged to provide a concentrated light, the ordinary
+  type being almost useless in this respect, and that small battery lamps,
+  worked by a suitable accumulator, can also be used, but except under very
+  special circumstances are hardly worth the trouble of keeping the
+  batteries charged.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 11 --><span class="pagenum"><a name="page11"></a>{11}</span></p>
+
+<h3>CHAPTER IV</h3>
+
+<p class="cenhead">ACETYLENE</p>
+
+  <p>There is no doubt that at present acetylene holds second place to
+  electric light in popularity for optical lantern work. The light is good;
+  not, it is true, <i>so</i> good as limelight or the electric arc, but
+  still sufficient for a picture up to 12 feet in diameter at a working
+  distance from the screen of not more than 30 feet, and this suffices for
+  the large majority of halls.</p>
+
+  <p>It has great advantages over limelight in convenience and cheapness,
+  although on both these points it must yield place to the electric arc,
+  always providing that current is available, and therefore it is chiefly
+  used in country districts and in gas-lit halls in large towns.</p>
+
+  <p>Acetylene gas is formed, as is well known, by the action of water upon
+  carbide of calcium, and the generators constructed for lantern work are
+  essentially the same in construction as for other purposes.</p>
+
+  <p>The alterations introduced are chiefly directed towards obtaining a
+  light as <i>steady</i> as possible from a comparatively small generator,
+  and, secondly, towards the entire elimination of smell, which obviously
+  is far more serious in a lecture hall than, for instance, on a motor car.
+  The generators in most common use may be divided into two classes, i.e.
+  those on the gasometer principle in which the carbide is gradually
+  lowered into the water, and those in which the water is allowed slowly to
+  gain access to the carbide. A good example of the former is perhaps that
+  made by Messrs. Moss of Birmingham, though there are several others
+  equally good, and clear and explicit directions for working should be
+  supplied by the makers. The Moss Generator (Fig. 5) consists of a tall
+  iron vessel A fitted with a gas tap at bottom, this communicating <!--
+  Page 12 --><span class="pagenum"><a name="page12"></a>{12}</span>with a
+  vertical iron tube within the vessel. Into this container fits the inner
+  bell or container B, divided internally into two concentric portions
+  entirely separated from each other, but connected by the pipe <span
+  class="scac">P P</span> and the tap <span class="scac">T</span>.</p>
+
+  <p>A guide inside the bell encircles the iron tube in the outer tank and
+  prevents rotation. Into the inner portion fits again the
+  carbide-container (shown separately on the left), which is locked when in
+  place by giving it a half turn, when a hook inside the bell engages with
+  the lower edge of the carbide container and prevents it from falling.</p>
+
+  <p>The carbide container is fitted with a series of shelves, and the
+  contents of a 2 lb. tin of carbide should be roughly divided among them;
+  there is no need to make any accurate division. The carbide used should
+  be that known as ½ inch mesh, and should be <i>pure</i>. That described
+  as 'chemically' treated is apt to give trouble by over-generation in
+  these gasometers and should be scrupulously avoided.</p>
+
+  <div class="figcenter" style="width:34%;">
+      <a href="images/Fig05.jpg"><img style="width:100%" src="images/Fig05.jpg"
+      alt="Fig. 5.--The Moss Generator" title="Fig. 5.--The Moss Generator" /></a>
+    <span class="sc">Fig. 5.</span>&mdash;The Moss Generator.
+  </div>
+
+  <p>The carbide having been placed in the receptacles, these should be
+  closed by means of the loose flap and the whole pushed into the bell and
+  secured.</p>
+
+  <p>Water should be poured into the outer vessel up to a mark on the iron
+  tube, and the bell placed in position. The lower tap being then turned on
+  and the upper one closed, air from the outer portion of the bell can
+  gradually escape by means of the iron tube and lower tap, and the bell
+  gradually sinks by its own weight until it is on the bottom, but still
+  <!-- Page 13 --><span class="pagenum"><a name="page13"></a>{13}</span>no
+  water can reach the carbide, the air imprisoned in the inner portion of
+  the bell effectually excluding it.</p>
+
+  <p>The lower tap should now be connected by means of india-rubber or
+  flexible metallic tubing to the burner in the lantern (of which more
+  anon), and the upper tap on the generator turned on, the tap or taps on
+  the burner being likewise opened. The air from the inner portion of the
+  bell can now escape by the pipe <span class="scac">P P</span> into the
+  outer part, and thence through the iron tube, and out through tubing and
+  jet, and as it does so water will rise in the interior and attack the
+  carbide.</p>
+
+  <p>In a few moments the burner can be lit; but the gas, being generated
+  far in excess of requirements, and filling both the inner and outer
+  portions of the bell faster than it can escape, lifts the latter until
+  the carbide is entirely out of the water, when in a few minutes
+  generation ceases.</p>
+
+  <p>If the jet is left burning the bell will gradually sink again as the
+  gas is used up, and should thereafter maintain an automatic balance
+  without attention.</p>
+
+  <p>It can be turned off at any moment by simply closing the taps at the
+  jet or, better, the lower tap at the generator, when the bell rises
+  sufficiently to take the carbide out of the water; but if it is required
+  to leave the generator unlit for a considerable time, it is better to
+  turn off the tap on the top first. This causes the inner portion of the
+  bell to fill with gas which cannot escape, and as that in the outer part
+  burns out, the bell sinks to the bottom and remains there, the gas itself
+  imprisoned in the inner chamber excluding the water from the carbide. The
+  exact arrangement varies in different patterns of generator, but the
+  above may be taken as roughly indicating the action, and further
+  information may always be obtained from the maker or dealer.</p>
+
+  <p><i>Emptying</i> should always be done out of doors, as the odour of
+  acetylene gas is most objectionable, and for the same reason rubber
+  tubes, &amp;c., should be securely tied on, so that the slightest escape
+  may be avoided. <!-- Page 14 --><span class="pagenum"><a
+  name="page14"></a>{14}</span></p>
+
+  <p>If the exhibition has been a short one it will often be found that the
+  upper cells have not been affected by the water, in which case they may
+  be put back in the tin and used again, but it is not generally advisable
+  to put in less than the full charge to begin with as the weight of the
+  carbide plays a definite part in securing the smooth action of the
+  apparatus. The sludge should be thrown away (it forms a good manure for
+  the garden) and the entire generator thoroughly dried, otherwise rust
+  will quickly appear.</p>
+
+  <div class="figcenter" style="width:21%;">
+      <a href="images/Fig06.jpg"><img style="width:100%" src="images/Fig06.jpg"
+      alt="Fig. 6.--The A.L. or 'Popular' Model" title="Fig. 6.--The A.L. or 'Popular' Model" /></a>
+    <span class="sc">Fig. 6.</span>&mdash;The A.L. or 'Popular' Model.
+  </div>
+
+  <p>Theoretically one of these generators may be filled and left standing
+  indefinitely, but in practice it is not advisable, as the damp in the
+  atmosphere is apt to produce a very slow generation of gas, sufficient
+  often to cause a decided smell.</p>
+
+  <p>Of generators which act by admitting water to the carbide perhaps the
+  best known is the A.L. or 'Popular' Model (Fig. 6), this being, in fact,
+  a pattern designed for motor-car head-lights, but which answers well for
+  lantern work.</p>
+
+  <p>Its exact operation need hardly be described here in full detail. It
+  will suffice to say that the water gains access to the carbide by
+  'creeping' up between two concentric copper cones, and in the event of
+  over-generation the pressure of the gas automatically checks the
+  flow.</p>
+
+  <p>This generator is smaller than the gasometer pattern, and hence can be
+  recommended for portability; but in my experience the light is not quite
+  so steady, and the control rather less delicate, thereby causing on
+  occasions a perceptible smell, especially if left standing for a
+  considerable time.</p>
+
+  <p>There are other types of generators, such as the 'Water <!-- Page 15
+  --><span class="pagenum"><a name="page15"></a>{15}</span>dropping'
+  variety, in which the water drips on to the carbide, and the reverse, in
+  which fine granulated carbide drops a little at a time into water; but
+  these types are not very frequently met with and need hardly be
+  described.</p>
+
+  <p>It should never be forgotten that acetylene is an explosive gas and
+  should be treated as such. Searching for a leak with a lighted match,
+  though perhaps permissible when the operator knows his business, may be a
+  dangerous proceeding when the contrary is the case.</p>
+
+  <div class="figcenter" style="width:44%;">
+      <a href="images/Fig07.jpg"><img style="width:100%" src="images/Fig07.jpg"
+      alt="Fig. 7.--Acetylene Jet" title="Fig. 7.--Acetylene Jet" /></a>
+    <span class="sc">Fig. 7.</span>&mdash;Acetylene Jet.
+  </div>
+
+  <p>Acetylene burners are generally of the 'Batswing' type, and are as a
+  rule four in number, mounted in a row with a reflector behind, each
+  burner being separately controlled by its own tap (Fig. 7). An acetylene
+  flame is very smoky, and care must be taken that the burners are not
+  turned too high. A nipple cleaner, consisting of a fine wire in a short
+  handle, can usually be obtained from any dealer, and is very handy.</p>
+
+  <p>Acetylene gas can also be used for lantern illumination in quite
+  another way, viz. by a blast from a blowpipe, in combination with either
+  air or oxygen, on to a special 'Pastille' provided for the purpose, or an
+  ordinary limelight jet can be used. These methods entail the use of
+  acetylene <i>under pressure</i>, and are so analogous to limelight that I
+  shall for convenience deal with them in the chapter devoted to that
+  illuminant.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 16 --><span class="pagenum"><a name="page16"></a>{16}</span></p>
+
+<h3>CHAPTER V</h3>
+
+<p class="cenhead">LIMELIGHT AND THE ACETYLENE BLAST</p>
+
+  <p>The illumination possible with this light is almost unlimited, and for
+  really large halls it is, as remarked before, the <i>only</i> substitute
+  for the electric arc. It consists essentially of a blowpipe flame,
+  composed of oxyhydrogen, oxyether, oxyspirit, oxy-acetylene, &amp;c., or
+  acetylene air blast, heating to incandescence a block of lime, or other
+  refractory material, and the essential feature is that one at least of
+  these gases must be under <i>pressure</i>. Thirty years ago this was
+  usually achieved by storing the gas in rubber bags, and obtaining the
+  requisite pressure by means of heavy weights; but except in a very few
+  outlying districts this method has now been completely superseded by the
+  use of compressed gas cylinders. The earlier editions of this work
+  contained very full directions for manufacturing gas for storage in bags,
+  but it is so exceptional now to find an operator who uses this method
+  that it seems hardly necessary to devote much space to it, and the same
+  may be said of automatic oxygen 'generators.' The present work will
+  therefore deal chiefly with compressed gas cylinders.</p>
+
+  <p>Most elaborate precautions are now enforced by the Board of Trade to
+  ensure the absolute safety of these, and any doubt existing from
+  occasional accidents of years ago may be promptly dismissed. Humanly
+  speaking, an accident nowadays <i>cannot</i> happen, except by such
+  wilful negligence on the part of the maker or filler as would almost
+  render the culprit subject to criminal proceedings.</p>
+
+  <p>Compressed gas cylinders are painted a distinctive colour, oxygen for
+  example being black and coal gas or hydrogen red; the screw connections
+  to the pumps, and all nozzle <!-- Page 17 --><span class="pagenum"><a
+  name="page17"></a>{17}</span>and regulator fittings, are made with a
+  totally different screw and therefore cannot be interchanged; the
+  cylinders themselves are bound by law to be reannealed and retested under
+  hydraulic pressure at regular intervals; the steel itself has to be of a
+  guaranteed quality; and, in fact, every possible risk is guarded
+  against.</p>
+
+  <p>The most usual sizes of cylinders supplied for lantern exhibitions are
+  those containing 6, 12, 20, or 40 cubic feet, and are usually sent out in
+  wooden or hemp cases.</p>
+
+  <div class="figcenter" style="width:37%;">
+      <a href="images/Fig08.jpg"><img style="width:100%" src="images/Fig08.jpg"
+      alt="Fig. 8.--Oxygen Cylinder in hemp cover" title="Fig. 8.--Oxygen Cylinder in hemp cover" /></a>
+    <span class="sc">Fig. 8.</span>&mdash;Oxygen Cylinder in hemp cover.
+  </div>
+
+  <p>Fig. 8 shows a 12-foot cylinder in its hemp case, the approximate size
+  without case being 22 in. by 4 in. This size cylinder will supply an
+  average limelight jet for just over two hours. The extra powerful jets as
+  used for cinematograph work or for illuminating a very large screen take
+  a good deal more, but for the usual apparatus as supplied for ordinary
+  lantern purposes this is a pretty safe figure.</p>
+
+  <p>A 12-foot cylinder is therefore the favourite size for a lantern
+  exhibition lasting from an hour to one and a half hours, as it leaves a
+  fair margin for gas used in adjusting the instrument, &amp;c., and a
+  20-foot cylinder will usually suffice for <i>two</i> such
+  exhibitions.</p>
+
+  <p>The price of gas per cubic foot varies with the size of the cylinder,
+  being less for large cylinders than for small ones, and the cost of
+  transit is also less in proportion&mdash;hence it is frequently an
+  economy to hire a large cylinder and retain it for several exhibitions.
+  On the other hand most suppliers charge a small rent if a cylinder is
+  retained beyond a definite <!-- Page 18 --><span class="pagenum"><a
+  name="page18"></a>{18}</span>time, so this is a question to be decided by
+  each user on its own merits.</p>
+
+  <p>Alternatively, of course, cylinders can be <i>purchased</i>, and the
+  question of rent does not then come in; also gas is supplied a little
+  cheaper in a customer's own cylinder than if sent on hire. If purchase is
+  decided on it is frequently an economy to buy <i>two</i>, or two of each
+  gas, if coal gas cylinders are required as well.</p>
+
+  <div class="figcenter" style="width:37%;">
+      <a href="images/Fig09.jpg"><img style="width:100%" src="images/Fig09.jpg"
+      alt="Fig. 9.--Double Lever Key" title="Fig. 9.--Double Lever Key" /></a>
+    <span class="sc">Fig. 9</span>.&mdash;Double Lever Key.
+  </div>
+
+  <p>The whole contents of the cylinders can then be used up without waste,
+  as if a cylinder should become exhausted during the course of a lecture,
+  it is only a matter of a minute or two to change over to the spare one,
+  whereas the compressors are required by law to empty out every cylinder
+  returned to them for refilling, and any remaining gas is thereby
+  wasted.</p>
+
+  <p>It is extremely tantalising, to say the least of it, to find the
+  pressure gauge indicating that there is, say, 8 feet of gas remaining in
+  a cylinder, and to be compelled to waste this or else risk running short
+  for the next exhibition, and duplicate cylinders are the only way of
+  avoiding the loss.</p>
+
+  <p>The cylinders are filled to a pressure of 120 atmospheres, or 1800 lb.
+  per square inch, and are closed by strong screw nozzles. The keys for
+  opening or closing these are of three types, viz. the 'T' pattern,
+  'Spanner' pattern, and that known as the 'Double Lever' type. This latter
+  is so made that in closing the valve it shuts up to half its length and
+  <!-- Page 19 --><span class="pagenum"><a
+  name="page19"></a>{19}</span>opens out to double the leverage when being
+  used to <i>open</i> the cylinder (Fig. 9). The idea is to avoid the
+  possibility, which has been known to occur, of the cylinder valve being
+  screwed down by a powerful wrist and defying the efforts of the
+  despairing lanternist to open it.</p>
+
+  <div class="figcenter" style="width:29%;">
+      <a href="images/Fig10.jpg"><img style="width:100%" src="images/Fig10.jpg"
+      alt="Fig. 10.--Fine Adjustment Valve" title="Fig. 10.--Fine Adjustment Valve" /></a>
+    <span class="sc">Fig. 10.</span>&mdash;Fine Adjustment Valve.
+  </div>
+
+  <p>Cylinder nozzles are unfortunately not yet standardised, but those
+  most frequently met with in this country are those adopted by the British
+  Oxygen Company, both oxygen and coal gas cylinders being fitted with
+  corresponding <i>internal</i> screws &#x215E; inch diameter, those for
+  oxygen being <i>right-handed</i>, and those for coal gas
+  <i>left-handed</i>, and in each case terminated at the bottom by a hollow
+  metal cone.</p>
+
+  <p>As such an internal screw cannot obviously be connected to a piece of
+  rubber tubing, some type of screw connector must be employed, and this
+  may take one of three forms: (1) A simple connecting nozzle, (2) a fine
+  adjustment valve, or (3) a regulator. The first is seldom used in
+  practice for lantern work, for the reason that the direct pressure of a
+  full cylinder (120 atmospheres) cannot be checked or controlled by a tap
+  on the jet, as the intervening rubber tubing would either burst or blow
+  off, and must therefore be regulated at the cylinder nozzle itself, and
+  gradually readjusted as the pressure diminishes.</p>
+
+  <p>To achieve this regulation with the ordinary cylinder key is
+  difficult, though possible to a careful operator, but for a slight extra
+  expense a combined nozzle and <i>fine adjustment valve</i> (Fig. 10) can
+  be obtained, and regulation with this is <!-- Page 20 --><span
+  class="pagenum"><a name="page20"></a>{20}</span>infinitely easier. The
+  best plan of all, however, is to use an automatic regulator, which not
+  only reduces the pressure so as to permit of the required adjustments
+  being made at the jet-taps, but also maintains a practically steady
+  supply as the cylinder empties, thereby obviating continual
+  readjustments. Regulators are now so inexpensive that they have come into
+  almost universal use, and are generally reckoned an indispensable part of
+  a limelight lantern equipment. The form of regulator in most common use
+  is that usually known as 'Beard's,' having been originally designed and
+  patented by Messrs. R. Beard &amp; Sons, though as the patent has now
+  expired it is open to any firm to make the same article if they
+  desire.</p>
+
+  <div class="figcenter" style="width:33%;">
+      <a href="images/Fig11.jpg"><img style="width:100%" src="images/Fig11.jpg"
+      alt="Fig. 11.--Construction of Beard's Regulator" title="Fig. 11.--Construction of Beard's Regulator" /></a>
+    <span class="sc">Fig. 11.</span>&mdash;Construction of Beard's
+    Regulator.
+  </div>
+
+  <p>The construction of Beard's Regulator is shown in Fig. 11. The gas
+  enters from below into a rubber bag, C, from which it can emerge through
+  the nozzle.</p>
+
+  <p>Any accumulation of gas raises the bellows against the pressure of a
+  spiral spring pressing it down, and this brings into action an
+  arrangement of so-called 'Lazy Levers,' which in turn presses down a
+  small conical valve and closes the supply from the cylinder, this valve
+  re-opening immediately the pressure diminishes.</p>
+
+  <p>The outward form of this regulator is shown in Fig. 12, <!-- Page 21
+  --><span class="pagenum"><a name="page21"></a>{21}</span>which
+  incidentally also illustrates the usual form of connection to the
+  cylinder, referred to later on.</p>
+
+  <p>In Beard's Regulator the pressure at which the gas can be delivered is
+  determined by the strength of the spiral spring, and can only be altered
+  by changing this spring.</p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/Fig12.jpg"><img style="width:100%" src="images/Fig12.jpg"
+      alt="Fig. 12.--Beard's Regulator" title="Fig. 12.--Beard's Regulator" /></a>
+    <span class="sc">Fig. 12.</span>&mdash;Beard's Regulator.
+  </div>
+
+  <p>In practice Beard's Regulators are supplied set to a low pressure for
+  ordinary mixed or 'blow-through' jets and for a higher pressure (14-16
+  inches) for 'injector' jets. At this latter pressure the rubber tubing
+  used must be fairly thick and strong and well tied on, and even so the
+  taps of the jet should not be turned entirely off unless the gas at the
+  cylinder is likewise turned off immediately afterwards. The British
+  Oxygen Company make a regulator which can be set to any desired pressure,
+  but it is not quite so delicate in its action as Beard's, and Messrs.
+  Clarkson also make a pattern regulator which is widely used and well
+  spoken of. The attachment of any of these fittings to the cylinder is a
+  somewhat peculiar one, as will be seen on reference to Fig. 10 or Fig.
+  12. The regulator or nozzle ends at its lower extremity in a screw and
+  cone, the latter being intended to make a gas-tight connection with the
+  internal cone on the cylinder, and over this screws a loose wing piece
+  with another outer screw, this latter fitting the thread in the
+  cylinder.</p>
+
+  <p>In making the connection care must be taken that the wing piece is not
+  screwed too far down the inner screw, or the cone will not reach down and
+  make a tight fit on its <!-- Page 22 --><span class="pagenum"><a
+  name="page22"></a>{22}</span>seating; in its correct position the wing
+  piece when clamped down should leave a turn or two of its thread exposed,
+  in order to ensure that the cone does bed properly.</p>
+
+  <div class="figcenter" style="width:40%;">
+      <a href="images/Fig13.jpg"><img style="width:100%" src="images/Fig13.jpg"
+      alt="Fig. 13.--Regulator and Gauge" title="Fig. 13.--Regulator and Gauge" /></a>
+    <span class="sc">Fig. 13.</span>&mdash;Regulator and Gauge.
+  </div>
+
+  <p>Care should be taken that the nozzle of the cylinder is free from dust
+  before attaching any of these fittings: the best plan is first to blow
+  into it, and finally wipe it round with the finger. Most professional
+  operators <i>hammer</i> the wing piece home with a spanner or other
+  convenient implement a barbarous method and really unnecessary if the
+  cones are in good condition, but, nevertheless, almost always adopted in
+  practice.</p>
+
+  <p><b>Pressure Gauges.</b>&mdash;These are useful in determining the
+  amount of gas remaining in a cylinder and are of a very usual type; they
+  may either be screwed on to the cylinder before commencing to work and
+  taken off again to screw on the regulator, or they can be supplied fitted
+  to the regulator itself, in which case they can be observed during the
+  course of the exhibition (Fig. 13). As the same gauge may be used for
+  cylinders of different sizes (though <i>never</i> for those containing
+  <!-- Page 23 --><span class="pagenum"><a
+  name="page23"></a>{23}</span>different gases), they simply register in
+  atmospheres, and knowing that a full cylinder shows a pressure of 120
+  atmospheres, the requisite calculation must be made to determine how many
+  cubic feet are unused.</p>
+
+  <p>In the case of oxygen cylinders an approximate idea of the amount of
+  gas remaining can be got by <i>weighing</i> it carefully when known to be
+  either absolutely full or absolutely empty, and re-weighing it when
+  information is required. Oxygen weighs approximately 1.4 oz. per cubic
+  foot, and this is easily detected by an average scale. Coal gas is too
+  light to be gauged in this way.</p>
+
+  <p><b>Gas-Bags and Generators.</b>&mdash;It has already been remarked
+  that there are two alternative methods of obtaining gas under pressure
+  for limelight purposes, viz. gas-bags and generators (the latter for
+  oxygen alone: there is no good hydrogen generator that I know of). In
+  both these cases the oxygen is generated by heating a mixture of chlorate
+  of potash and manganese black oxide. In the case of gas-bags the gas is
+  prepared beforehand and passed through suitable purifiers into a rubber
+  gas-bag. With a generator the oxygen is evolved during the exhibition
+  itself; but this method has never come into very general use.</p>
+
+  <p>Coal gas or hydrogen is very seldom home generated; a gas-bag can, if
+  necessary, be filled a few miles away and brought full to the place of
+  exhibition, or filled on the spot if gas is laid on; or, failing this,
+  acetylene or ether, or even methylated spirit may be utilised
+  instead.</p>
+
+  <p>The bags in use are placed between double pressure boards (if
+  <i>both</i> gases are required under pressure) and weights sufficiently
+  heavy placed on the top (Fig. 14), or with a 'blow-through' jet only the
+  oxygen need be stored in a bag and the coal gas used from the supply
+  main.</p>
+
+  <p>Cylinders have, however, so universally superseded these appliances,
+  that space is hardly warranted in fully describing them, especially as
+  any operator wishing to adopt <!-- Page 24 --><span class="pagenum"><a
+  name="page24"></a>{24}</span>the process can obtain full directions from
+  any responsible dealer.</p>
+
+  <p><b>Limelight Jets.</b>&mdash;These are of three general types, viz.
+  the 'Blow-through,' the 'Mixed,' and the 'Injector.'</p>
+
+  <p>Of these the 'Blow-through' is now very little made, having been
+  largely superseded by the 'Injector' pattern, but, as there are hundreds
+  in common use in this country, they cannot yet be regarded as a thing of
+  the past.</p>
+
+  <div class="figcenter" style="width:50%;">
+      <a href="images/Fig14.jpg"><img style="width:100%" src="images/Fig14.jpg"
+      alt="Fig. 14.--Gas-bags" title="Fig. 14.--Gas-bags" /></a>
+    <span class="sc">Fig. 14.</span>&mdash;Gas-bags.
+  </div>
+
+  <p>The exact design of this jet varies considerably, but all are alike in
+  this, that a jet of coal gas is burned at the orifice of a more or less
+  open nozzle, and a stream of oxygen <i>blown</i> <i>through</i> it on to
+  a cylinder of lime which it thereby renders incandescent. Fig. 15
+  represents the various designs chiefly adopted for this jet, that marked
+  <span class="scac">A</span> being perhaps the most usual, though <span
+  class="scac">C</span> is also frequently met with.</p>
+
+  <p>In light-giving power there is not much to choose between the various
+  types; probably <span class="scac">D</span> on the whole is the best in
+  this respect, but so much depends upon the exact position of the two
+  nozzles, and the <i>smoothness</i> or otherwise of that <!-- Page 25
+  --><span class="pagenum"><a name="page25"></a>{25}</span>provided for the
+  oxygen blast, that exact comparisons are difficult.</p>
+
+  <div class="figcenter" style="width:59%;">
+      <a href="images/Fig15.jpg"><img style="width:100%" src="images/Fig15.jpg"
+      alt="Fig. 15.--Blow-through Nozzles" title="Fig. 15.--Blow-through Nozzles" /></a>
+    <span class="sc">Fig. 15.</span>&mdash;'Blow-through' Nozzles.
+  </div>
+
+  <p>'Blow-through' jets are the weakest form of limelight as used at the
+  present day, and may be taken roughly as some 50 per cent. better than
+  acetylene, or in other words, sufficient to illuminate a 12-foot picture
+  at a distance of some 40 to 50 feet; but their advantage is, or was, that
+  they only required one gas (oxygen) under pressure, the coal gas supply
+  being obtained from the ordinary house main.</p>
+
+  <div class="figcenter" style="width:60%;">
+      <a href="images/Fig16.jpg"><img style="width:100%" src="images/Fig16.jpg"
+      alt="Fig. 16.--Blow-through Jet" title="Fig. 16.--Blow-through Jet" /></a>
+    <span class="sc">Fig. 16.</span>&mdash;'Blow-through' Jet.
+  </div>
+
+  <p>This advantage is now shared by the more recently introduced
+  'Injector' jets, which give a far better light, and have therefore
+  rendered the 'Blow-through' type nearly extinct.</p>
+
+  <p>The general construction of a 'Blow-through' jet is shown in Fig. 16,
+  and it will be seen that a short vertical spindle is <!-- Page 26
+  --><span class="pagenum"><a name="page26"></a>{26}</span>provided to
+  carry the lime cylinder, and that this can be rotated from the back by
+  means of bevelled gear wheels, which at the same time screw the spindle
+  up and down. A lime cylinder of the usual pattern being placed on this
+  spindle can be rotated from time to time to expose a fresh surface, as
+  that in use gradually becomes 'pitted' by the blast, while the screw
+  provides sufficient vertical movement to ensure that a complete rotation
+  does not bring round the same position again.</p>
+
+  <p>Some arrangement is also generally provided by which the distance
+  between the lime spindle and the jet can be adjusted. The exact position
+  of this does not matter within a reasonable margin, but limes vary in
+  size, and 'Pastilles,' and other substitutes for limes, which will be
+  referred to later, vary still more, at any rate as regards this
+  adjustment. The average distance which gives the best result is usually
+  about half an inch, and once set need not be altered with that particular
+  jet unless a lime of different size is employed; minor variations due to
+  limes being drilled slightly out of centre, &amp;c., do not seriously
+  matter.</p>
+
+  <p>There is no accepted rule for colouring jet-taps in accordance with
+  the cylinders, and although jets are sometimes met with painted in this
+  way, <i>i.e.</i> red for coal gas and black for oxygen, it is more usual
+  to find coal gas taps <i>black</i> and oxygen <i>bright</i>, or sometimes
+  both black or both bright. Care must therefore be taken that the right
+  cylinder is connected to the right tap on the jet, but there should be no
+  difficulty in telling which is which, and fortunately any mistake, even
+  if it be made, is quite harmless.</p>
+
+  <p><b>The Mixed-Gas or Double-Pressure Jet.</b>&mdash;This jet is
+  fundamentally different from the 'blow-through' form, inasmuch as the two
+  gases are combined in one mixing chamber before combustion, and burn in
+  their correct proportions at one nipple.</p>
+
+  <p>It is usually stated that this jet necessitates both gases being under
+  equal or approximately equal pressure, but this <!-- Page 27 --><span
+  class="pagenum"><a name="page27"></a>{27}</span>is not literally
+  accurate, and I have given many a lantern exhibition with one of these
+  jets, using coal gas from the ordinary supply, and oxygen from a
+  cylinder. To use a mixed jet in this way needs care, as a very slight
+  excess of oxygen puts the light out with a 'pop' which, although not
+  dangerous, is disconcerting, while the light obtained under these
+  conditions is very little better than with a 'blow-through' jet, and far
+  inferior to the 'Injector' jets to be described next.</p>
+
+  <div class="figcenter" style="width:60%;">
+      <a href="images/Fig17.jpg"><img style="width:100%" src="images/Fig17.jpg"
+      alt="Fig. 17.--Mixed Jet" title="Fig. 17.--Mixed Jet" /></a>
+    <span class="sc">Fig. 17.</span>&mdash;Mixed Jet.
+  </div>
+
+  <div class="figcenter" style="width:39%;">
+      <a href="images/Fig18.jpg"><img style="width:100%" src="images/Fig18.jpg"
+      alt="Fig. 18.--Mixed Jet, Gwyer pattern" title="Fig. 18.--Mixed Jet, Gwyer pattern" /></a>
+    <span class="sc">Fig. 18.</span>&mdash;Mixed Jet, Gwyer pattern.
+  </div>
+
+  <p>The mixed-gas jet is intended then to be used with both gases under
+  pressure, and is the <i>only</i> jet to be seriously <!-- Page 28
+  --><span class="pagenum"><a name="page28"></a>{28}</span>considered in
+  cases where a really powerful light is required. The power of this jet is
+  indeed almost unlimited, and those made with large bores, such for
+  example as used for cinematograph work, provide a light amounting often
+  to some two or three thousand candles, and consume an enormous amount of
+  gas; but the ordinary pattern, with a nipple of one-twentieth to
+  one-sixteenth of an inch bore, and using some 5 feet of each gas per
+  hour, or perhaps slightly more for the coal gas, will suffice for all
+  ordinary work.</p>
+
+  <div class="figcenter" style="width:30%;">
+      <a href="images/Fig19.jpg"><img style="width:100%" src="images/Fig19.jpg"
+      alt="Fig. 19.--Mixing Chamber of Jet" title="Fig. 19.--Mixing Chamber of Jet" /></a>
+    <span class="sc">Fig. 19.</span>&mdash;Mixing Chamber of Jet.
+  </div>
+
+  <p>The mixed-gas jet, like the 'blow-through,' is made in many forms, but
+  these may be roughly divided into two main types, viz. those with small
+  mixing chambers immediately below the nipple (Fig. 17), and those with
+  larger chambers in the horizontal part of the jet as in the 'Gwyer'
+  pattern (Fig. 18).</p>
+
+  <p>The construction of the mixing chamber itself varies also, but that
+  advocated by my father, the original author of this work, is generally
+  followed, the chamber being packed with alternate discs pierced as in
+  Fig. 19, which ensures a thorough mixture of the gases. A layer or two of
+  gauze is often introduced as well by way of further improvement. The
+  distance between the lime and nipple is much less than with the
+  'blow-through' jet, and the adjustment has to be more exactly made. About
+  &#x215B; inch is approximately correct for a jet of moderate power, and
+  rather more for a bigger bore; also care must be taken to turn the lime
+  frequently, as the latter 'pits' pretty quickly with these jets, and if
+  it is neglected the jet may spurt back out of the hole, which is
+  gradually formed, and crack the condenser. <!-- Page 29 --><span
+  class="pagenum"><a name="page29"></a>{29}</span></p>
+
+  <p>There is still an erroneous opinion extant that these jets are
+  dangerous, and if the operator is working with the now obsolete gas-bags
+  it is certainly a fact that an accident in careless hands is
+  <i>possible</i>; but with cylinders there is, so far as I know, no
+  possibility even of an accident under ordinary conditions.</p>
+
+  <p>It is true that if too much oxygen is turned on the jet may suddenly
+  go out with a loud snap or pop, and this is in reality a miniature
+  explosion in the mixing chamber; but it can in any case hardly be serious
+  enough to matter, though I have found after such a snap that the gauze
+  packing, inside the chamber above referred to, has been pierced right
+  through, and, when first lit afterwards, the jet has for a few minutes
+  burnt with a characteristic green flame, denoting the presence in the gas
+  of fine copper or brass particles.</p>
+
+  <p>To obtain a good light with these jets, and in fact with <i>all</i>
+  jets, great care must be taken that the nipple is absolutely smooth,
+  otherwise the flame is bound to hiss. The simplest plan is to slightly
+  roughen a suitable sized needle with emery paper and to burnish the
+  inside of the nipple from time to time with this. Especially if there has
+  been one of the 'snaps' referred to is it desirable to see that the
+  inside of the nipple is thoroughly smooth and polished.</p>
+
+  <p><b>Manipulation of the Mixed-Gas Jet.</b>&mdash;On this point there is
+  not much to be said. A good hard stone lime must be used&mdash;'soft'
+  limes are useless for this jet&mdash;and the coal gas flame should be lit
+  first, and the lime thoroughly heated with this before the oxygen is
+  slowly turned on. As the oxygen increases the flame will gradually
+  disappear and the light increase, until it is at a maximum for that
+  particular amount of coal gas. This latter can then be turned on a little
+  more, and more oxygen passed to balance it until the jet begins to
+  'roar,' when we are getting the maximum light for that particular sized
+  nipple. When the two gases are, however, in the proper proportion to give
+  the best light, there will always be a slight excess of coal gas flame
+  visible playing about the lime. <!-- Page 30 --><span class="pagenum"><a
+  name="page30"></a>{30}</span></p>
+
+  <p><b>The Injector Jet.</b>&mdash;This is essentially a mixed jet, and in
+  outward appearance differs but little from one of the ordinary type (Fig.
+  20), but is so constructed that the pressure of oxygen 'sucks' coal gas
+  into the mixing chamber, and so obviates all necessity for the latter
+  being under pressure.</p>
+
+  <p>With this jet there is little or no danger of the jet 'snapping' out
+  through a surplus of oxygen, as the greater the flow of this gas, the
+  greater the suction on the coal gas side.</p>
+
+  <p>The light is not quite equal to a good mixed jet, but very nearly so,
+  and therefore this jet is deservedly gaining in favour every day.</p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/Fig20.jpg"><img style="width:100%" src="images/Fig20.jpg"
+      alt="Fig. 20.--Injector Jet" title="Fig. 20.--Injector Jet" /></a>
+    <span class="sc">Fig. 20.</span>&mdash;'Injector' Jet.
+  </div>
+
+  <p>One point must be noted: the oxygen itself must be under greater
+  pressure than with the ordinary mixed jet if the best light is to be
+  obtained, and therefore a special regulator must be used, or one of
+  ordinary type modified (which can easily be done by the maker), and
+  rubber connections must be securely tied both on to jet and regulator, as
+  the pressure required to work this jet to advantage, while not enough to
+  burst a rubber tube, is enough to blow it off an easy fitting
+  connection.</p>
+
+  <p><b>The Oxyether Light.</b>&mdash;This is practically similar to the
+  oxyhydrogen, except that ether vapour is used in place of the hydrogen or
+  coal gas. The method adopted consists essentially of passing a stream of
+  oxygen through a vessel packed with some porous material (such as cotton
+  wool or cotton gauze) which is saturated with ether. The oxygen <!-- Page
+  31 --><span class="pagenum"><a name="page31"></a>{31}</span>becomes
+  saturated with ether vapour, and the mixture is then used in place of the
+  coal gas supply in a double-pressure jet, an additional supply of free
+  oxygen being still required through the ordinary oxygen tap.</p>
+
+  <p>The arrangement is cheap, as it dispenses with the necessity for a
+  coal gas cylinder, and effective, as the light is little, if at all
+  inferior to the oxyhydrogen, but differs from the latter in this, that
+  with careless handling an accident is <i>possible</i>.</p>
+
+  <p>In competent hands there is no danger, and I have used ether
+  saturators myself scores of times without one single contretemps; but it
+  should <i>not</i> be entrusted to any chance amateur.</p>
+
+  <p>The use of the ether light has a curious history. In the earlier days
+  before the proper construction of ether saturators was understood, and
+  gas-bags were still in vogue, it was largely condemned on the score of
+  danger. Modern improvements in apparatus rendered it perfectly safe
+  against anything but gross carelessness or bungling, and the London
+  County Council and other similar bodies immediately supplied it broadcast
+  to elementary schools (in disregard of warnings offered by myself and
+  others), where it was often entrusted to incompetent operators or even
+  senior boys. So far as I know no serious accident ever resulted, a pretty
+  conclusive proof that the light is really safe, but in time the London
+  County Council realised that the universal adoption of this illuminant
+  was not advisable, and I believe <i>now</i> prohibit it altogether in
+  halls licensed by them for entertainments.</p>
+
+  <p>In time, no doubt, they will learn to adopt a sane policy between the
+  two extremes, but at present the official attitude in many localities has
+  placed ether saturators out of the running, and before purchasing one the
+  would-be operator should ascertain that he will be allowed to use it.</p>
+
+  <p>Ether saturators as made at the present day may be divided into two
+  principal patterns, viz. those in which saturator and jet are combined in
+  one piece of apparatus <!-- Page 32 --><span class="pagenum"><a
+  name="page32"></a>{32}</span>which fits bodily into the lantern, and
+  saturators which are used outside and connected by means of tubing to any
+  ordinary oxyhydrogen double-pressure jet.</p>
+
+  <p>Both forms have their advantages and disadvantages; the first pattern
+  tends to become too warm from its position in the lantern and generates
+  ether vapour too quickly, while the second has the fault of becoming too
+  cold (owing to evaporation of the ether) and therefore not vaporising
+  quickly <i>enough</i>.</p>
+
+  <div class="figcenter" style="width:48%;">
+      <a href="images/Fig21.jpg"><img style="width:100%" src="images/Fig21.jpg"
+      alt="Fig. 21.--Gridiron Saturator" title="Fig. 21.--Gridiron Saturator" /></a>
+    <span class="sc">Fig. 21.</span>&mdash;'Gridiron' Saturator.
+  </div>
+
+  <p>Writing at the present date, when manufacturers are slowly beginning
+  to resume their normal occupations after the stress of war work, it is
+  impossible to say exactly what models will or will not be made, but I
+  will mention one typical example of each pattern as made in pre-war
+  days.</p>
+
+  <p>The first of these is the 'Gridiron' (Fig. 21), adopted largely by the
+  London County Council in the days I have referred to, and certainly one
+  of the best designed saturators ever put on the market.</p>
+
+  <p>In the 'Gridiron' saturator there are three taps: two at the <!-- Page
+  33 --><span class="pagenum"><a name="page33"></a>{33}</span>rear and one
+  in front, between the saturator and the mixing chamber. Between the rear
+  taps is the inlet for the oxygen, which divides into two channels, that
+  on the left passing upwards through the <b>U</b> tube shown in the
+  illustration (the corresponding tube on the right is merely a dummy), and
+  thence through the saturator and out through the horizontal tube and tap
+  into the mixing chamber, whence the saturated stream of oxygen finally
+  passes to the nipple, and the combination burns with a whitish flame
+  closely resembling that produced by coal gas.</p>
+
+  <div class="figcenter" style="width:25%;">
+      <a href="images/Fig22.jpg"><img style="width:100%" src="images/Fig22.jpg"
+      alt="Fig. 22.--Pendant Saturator" title="Fig. 22.--Pendant Saturator" /></a>
+    <span class="sc">Fig. 22.</span>&mdash;'Pendant' Saturator.
+  </div>
+
+  <p>The other channel for the oxygen is to the right, down the vertical
+  tube shown there (the lower vertical tube on the left is also a dummy),
+  underneath the saturator, and finally coming up into the mixing chamber
+  from below, transforming the white flame into an intensely hot blowpipe
+  exactly as it does with a coal gas jet. The front tap controls the supply
+  of saturated ether to the mixing chamber, and whereas at first a good
+  stream of oxygen is needed to pick up enough ether, by degrees as the
+  instrument warms in the lantern, the oxygen passing through the saturator
+  can be cut off entirely, and even then the front tap must be gradually
+  closed down to prevent the hot ether coming off too fast.</p>
+
+  <p>There is a disagreeable feeling of 'sitting on the safety-valve' in
+  doing this, but in reality the pressure is never likely to become great
+  enough to cause danger.</p>
+
+  <p>Of saturators for use outside the lantern the best-known is probably
+  the 'Pendant' (Fig. 22). With this instrument the oxygen supply is
+  connected to the inlet marked <span class="scac">A</span>; <span
+  class="scac">B</span> goes <!-- Page 34 --><span class="pagenum"><a
+  name="page34"></a>{34}</span>direct to the oxygen tap of any ordinary
+  mixed-gas jet; while <span class="scac">C</span>, from whence issues the
+  saturated stream, is connected to the coal gas tap of the jet. Whichever
+  pattern is used, the essential thing is to keep a good supply of oxygen
+  well saturated. If the lime becomes incandescent without any free oxygen,
+  or it is found that this requires gradually turning off, it indicates
+  that the saturation is becoming defective, and to continue is to risk the
+  jet snapping out. In the case of an outside saturator such as the
+  'Pendant,' this may even blow off the connecting tubes with a loud
+  report, though no worse accident is likely to happen, and for this reason
+  an outside saturator should be placed <i>as close</i> to the jet as
+  possible, so that the rubber tube may be kept short, and incidentally
+  this keeps the saturator warm and accelerates vaporisation.</p>
+
+  <p>As ether vapour usually contains a certain amount of moisture which
+  does not vaporise to any great extent, this gradually accumulates and the
+  capacity of the instrument becomes reduced. It is therefore usually
+  necessary to return a saturator to the makers every now and again for
+  repacking.</p>
+
+  <p>The only real danger with a modern saturator is not in using but in
+  <i>filling</i>. This should be done if possible in the open air, and at
+  any rate never near a light. Ordinary sulphuric ether of specific gravity
+  720-730 is usually considered the best, and a quarter of a pint will keep
+  an ordinary small-bore jet going for nearly two hours.</p>
+
+  <p>More precise directions are usually sent out by the makers, and as the
+  various patterns of saturator in use are pretty numerous, it would be
+  useless here to attempt more detailed instructions for working.</p>
+
+  <p><b>Oxy-Acetylene Jets.</b>&mdash;Any good mixed gas jet may be used
+  with acetylene instead of coal gas, provided that it is under pressure
+  more or less corresponding to that from an oxygen cylinder, and at the
+  present day there is no difficulty in obtaining this, in civilised
+  countries at all events, by <!-- Page 35 --><span class="pagenum"><a
+  name="page35"></a>{35}</span>means of compressed or, to speak more
+  correctly, 'dissolved' acetylene cylinders, referred to later on.</p>
+
+  <p>With an 'Injector' jet there is no need for the acetylene gas to be
+  under pressure at all, and a simple generator such as described on page
+  <a href="#page12">12</a> will answer perfectly, though in practice very
+  seldom used. With such a generator the pressure is so low that in many
+  cases the jet will not even burn until <i>some</i> oxygen is turned on;
+  but this introduces no real difficulty, as with a good 'Injector' a snap
+  is practically impossible, provided the generator is large enough to
+  evolve sufficient acetylene. It is far better in every way, however, to
+  use the acetylene from a cylinder, just as with coal gas. Only in this
+  case the cylinder is completely filled with a porous material, and this
+  again filled with liquid acetone or other suitable fluid, in which the
+  acetylene is dissolved as rapidly as it is pumped into the cylinder.</p>
+
+  <p>To compress acetylene in the ordinary way is neither safe nor
+  practicable; but these 'dissolved' cylinders are now used extensively for
+  both oxy-acetylene welding and motor car lighting, and may be entirely
+  relied upon.</p>
+
+  <p>The D.A. (Dissolved Acetylene) Company were the pioneers in this
+  country of the industry, and their methods of business are peculiar and
+  ingenious. The user is requested in the first place to purchase a
+  cylinder, and he then becomes the owner of <i>a</i> cylinder, but not of
+  one <i>particular</i> cylinder. A list is supplied to him of various
+  depots in the country where the Company's cylinders are stored, and when
+  empty he can, on payment of a fixed sum, exchange his empty cylinder for
+  a full one, which then becomes <i>his</i> cylinder <i>pro tem</i>.</p>
+
+  <p>This saves the delay and expense of returning a cylinder to London,
+  and incidentally clears the customer of any question of deterioration,
+  this being obviously covered by degrees with each individual exchange.
+  The system was first introduced in connection with the lighting of cars
+  and only applies to the standard size for this purpose, viz. 20 cubic
+  feet capacity, <!-- Page 36 --><span class="pagenum"><a
+  name="page36"></a>{36}</span>but as this is, on the whole, the most
+  convenient size for lantern work also, the limitation is not a
+  disadvantage. The arrangement is also in vogue to a less extent with
+  cylinders of 6 feet capacity (a size sometimes used for motor
+  <i>cycles</i>), but the depots of exchange are at present far fewer for
+  this size.</p>
+
+  <p>The oxy-acetylene blast is much <i>hotter</i> than the ordinary
+  oxyhydrogen, and therefore produces a more intense light. I have
+  therefore used it with success on occasions when even the ordinary
+  limelight would fail, and the choice has lain between an oxyhydrogen jet
+  of enormous bore (and, of course, corresponding consumption of gas), and
+  the oxy-acetylene.</p>
+
+  <p>For this very reason great care must be taken only to use the hardest
+  limes, and even then to use the lime-turning movement frequently, or the
+  lime will pit or crack and a broken condenser follow.</p>
+
+  <p><b>The Fallot Acetylene Light.</b>&mdash;This light consists of a jet
+  of acetylene under pressure, without oxygen, but producing its own <i>air
+  blast</i> from the atmosphere by suction, much as the 'Injector' jet
+  does, but the reverse way round.</p>
+
+  <p>The light is better than with an ordinary acetylene jet, though not
+  quite so good as with a 'blow-through' jet; but as it only requires a
+  cylinder of dissolved acetylene, or even a 'Pressure' generator, it is
+  fast coming into favour.</p>
+
+  <p>The peculiarity of the Fallot apparatus is that, instead of providing
+  a direct beam of light in the direction of the screen, it projects the
+  beam <i>backwards</i> on to a concave mirror, and it is the reflected
+  light from this that is used (Fig. 23).</p>
+
+  <p>Instead of a lime is used a spherical 'Pastille' of peculiar
+  composition, and before use each pastille must be burnt off exactly like
+  an incandescent gas mantle, after which it is extremely fragile and
+  difficult to handle.</p>
+
+  <p>To use this illuminant one lens of the condenser must be removed, the
+  curvature of the mirror taking its place, and it will be seen at once
+  that the pastille itself will get in its own <!-- Page 37 --><span
+  class="pagenum"><a name="page37"></a>{37}</span>light and throw a shadow,
+  which actually happens, but it is hardly perceptible unless specially
+  looked for.</p>
+
+  <div class="figcenter" style="width:54%;">
+      <a href="images/Fig23.jpg"><img style="width:100%" src="images/Fig23.jpg"
+      alt="Fig. 23.--Fallot Air Blast" title="Fig. 23.--Fallot Air Blast" /></a>
+    <span class="sc">Fig. 23.</span>&mdash;Fallot Air Blast.
+  </div>
+
+  <p>A complete Fallot Air Blast Outfit, with cylinder, fine adjustment
+  valve, pressure gauge and burner, with two spare pastilles, is shown in
+  Fig. 24, but if preferred a regulator, such as previously described for
+  oxygen, can be used instead of the fine adjustment valve.</p>
+
+  <div class="figcenter" style="width:55%;">
+      <a href="images/Fig24.jpg"><img style="width:100%" src="images/Fig24.jpg"
+      alt="Fig. 24.--Fallot Air Blast, and Cylinder" title="Fig. 24.--Fallot Air Blast, and Cylinder" /></a>
+    <span class="sc">Fig. 24.</span>&mdash;Fallot Air Blast, and Cylinder.
+  </div>
+
+<p><!-- Page 38 --><span class="pagenum"><a name="page38"></a>{38}</span></p>
+
+  <p><b>Fallot Oxy-Acetylene Blast.</b>&mdash;This is similar to the
+  foregoing, utilising oxygen from a cylinder instead of air, and the light
+  is equal to a powerful limelight, and may be considered as an efficient
+  substitute, though for <i>long range</i> work the shadow before alluded
+  to becomes more noticeable (for optical reasons which need not be here
+  discussed). The Fallot Company also make a special 'Pressure Generator'
+  which can be used instead of a D.A. Cylinder; but my experience of this
+  so far is that, although perfectly safe, the blast from it is a little
+  unsteady as compared with a cylinder.</p>
+
+  <p><b>Limes and Accessories.</b>&mdash;Limes for Optical Lantern work are
+  usually supplied in the form of cylinders, the 'ordinary' size being
+  &#x215E; inch in diameter and about 1½ inches long, with a hole drilled
+  longitudinally to take the lime pin. Extra large limes up to 2 inches in
+  diameter are supplied for more powerful jets.</p>
+
+  <p>So-called 'soft' limes used to be recommended for 'blow-through' jets
+  as giving a better light than 'hard' limes, but the advantage, if any, is
+  very little, and these limes are now very seldom heard of, possibly
+  because 'blow-through' jets themselves are becoming less and less used,
+  and 'soft' limes will not stand the heat of a mixed or 'Injector' jet for
+  long.</p>
+
+  <p>'Hard' limes are turned out of the hardest stone lime, and must be
+  kept in sealed tins until used, as they rapidly disintegrate when exposed
+  to the air. There are one or two quarries known to provide the best lime
+  for lantern purposes, and the various good brands on the market
+  practically have the same origin as regards raw material, though called
+  by different trade names; and the 'Hardazion' (hard as iron) limes,
+  placed on the market some years ago by a well-known wholesale firm, to be
+  countered shortly after by the 'Hardastil' (harder still) brand, are, I
+  take it, legitimate though amusing instances of phonetic
+  advertisement.</p>
+
+  <p>Even the best of limes is liable to crack under the heat <!-- Page 39
+  --><span class="pagenum"><a name="page39"></a>{39}</span>of a powerful
+  jet, and so a pair of lime-tongs should always be provided, and there is
+  nothing better than the simple form shown in Fig. 25, and which is, or
+  should be, sold by all dealers.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/Fig25.jpg"><img style="width:100%" src="images/Fig25.jpg"
+      alt="Fig. 25.--Lime-tongs" title="Fig. 25.--Lime-tongs" /></a>
+    <span class="sc">Fig. 25.</span>&mdash;Lime-tongs.
+  </div>
+
+  <p><b>Substitutes for Limes.</b>&mdash;A good substitute for lime, that
+  will give the same light, stand heat equally as well, and <i>not</i>
+  deteriorate if exposed to the atmosphere, has long been sought for, and
+  some of the more recently discovered refractory materials are more or
+  less satisfactory. 'Mabor' limes, for example, belong to this class, and
+  so do some of the 'pastilles,' which before the war came chiefly from
+  France and to a less extent from Germany.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h3>CHAPTER VI</h3>
+
+<p class="cenhead">THE ELECTRIC LIGHT</p>
+
+  <p>The electric current provides <i>the</i> light for an optical lantern,
+  though it may take various forms, such as the incandescent glow-lamp in
+  some shape or other, the comparatively new Ediswan 'Pointolite' lamp, the
+  enclosed arc, and the open arc. This little book is not a treatise on
+  electricity, but a few elementary notes may not be out of place, and may
+  be of assistance to the non-technical lanternist.</p>
+
+  <p>The first point then to be considered in adopting the electric light
+  for the purpose of lantern projection is the character of the supply, and
+  the information required may be summed up thus: (1) <i>E.M.F.</i>,
+  <i>voltage</i>, or <i>tension</i> (these three expressions having exactly
+  the same meaning); (2) <i>ampèrage</i> or amount of current available;
+  (3) whether current is (<i>a</i>) <i>continuous</i>, <i>constant</i>, or
+  <i>direct</i> (again three words meaning <!-- Page 40 --><span
+  class="pagenum"><a name="page40"></a>{40}</span>the same thing), or
+  (<i>b</i>) <i>alternating</i>. The E.M.F. or tension corresponds to
+  <i>pressure</i>, to use the mechanical analogy of a water pipe, and the
+  <i>ampèrage</i> to volume, and the voltage of the supply currents in this
+  country are usually between 100 and 250 volts. Private lighting sets are
+  frequently as low as 50, and current derived from accumulators may be
+  anything from a few volts and upwards. <i>Power</i> currents, such as
+  commonly employed for tramways, &amp;c., are usually about 500 volts, but
+  the use of these currents for lighting purposes, though practicable, is
+  not to be advocated.</p>
+
+  <p>Ampères and volts are convertible terms in a sense; that is to say, a
+  current of 10 ampères at 100 volts requires the same horse-power to
+  generate it as one of 5 ampères at 200 volts, or 20 ampères at 50 volts,
+  but they are by no means convertible as regards their <i>efficient</i>
+  use for our purpose. The ampères used multiplied by the number of volts
+  give the total power consumed in <i>watts</i>, and 1000 watts used for
+  one hour represent 1 <i>unit</i> as charged for on our dreaded lighting
+  bills. The current available from a public supply may be said to be
+  unlimited so far as our purpose is concerned, and the amount actually
+  used depends only on the total electrical resistance of our circuit, and
+  this is measured in <i>ohms</i>, the three factors, viz. volts, ampères,
+  and resistance, being connected by the well-known and simple equation
+  C&nbsp;=&nbsp;E&nbsp;/&nbsp;R, C representing the current in ampères, E the tension or E.M.F.
+  (electro-motive force) in volts, and R the resistance in ohms. The total
+  current we <i>can</i> use, however, is limited by the size of the cable
+  laid on in the building, and this is automatically safeguarded (or should
+  be) by the <i>fuses</i>, which consist, as is generally known, of thin
+  wires or strips of tin or lead fixed on a fuse board in an easily
+  accessible place, and which melt directly the current exceeds a safe
+  amount in ampères. Whatever method of lighting we use therefore,
+  <i>enough</i> resistance must always be kept in the circuit to ensure
+  <!-- Page 41 --><span class="pagenum"><a
+  name="page41"></a>{41}</span>that no more current can pass than has been
+  provided for, and in the case of an arc lamp this usually means a
+  <i>resistance</i> or rheostat being retained in the circuit in addition
+  to the arc itself, through which the current is passed and absolutely
+  wasted, though fortunately the waste in money is negligible, and for
+  reasons to be discussed later such a resistance is necessary with an
+  optical lantern arc lamp in any case.</p>
+
+  <p>In the case of a glow-lamp, the entire resistance is provided by the
+  filament of the lamp itself, and that is why an ordinary metal or carbon
+  filament lamp, for say 200 volts, has to be manufactured with an
+  extremely long and slender, and therefore fragile, filament, while with
+  an ordinary pocket-torch, which is usually supplied with current from a
+  dry battery of some 3 or 4 volts only, the filament can be short and
+  thick.</p>
+
+  <p>Speaking generally, glow-lamps on a low voltage current can be made
+  more efficient than on a high one, and are also longer lived for very
+  obvious reasons; but, on the other hand, the transmission of current over
+  long distances is cheaper the higher the tension, as for a given number
+  of watts the ampèrage is less, and therefore smaller cables can be
+  employed. On the whole, then, currents of 200 to 250 volts have during
+  recent years become more common than 100, in spite of greater
+  difficulties in making the lamps; but occasionally one finds a hall where
+  two or more lamps are wired in <i>series</i>, two 100-volt lamps for
+  example being wired together in series on a 200-volt circuit. If we are
+  using current for our lantern from an ordinary lamp socket, this is a
+  possibility that must be borne in mind.</p>
+
+  <p>The same considerations, viz. the economy of transmitting power at
+  high tension and of <i>using</i> it at a lower one, have been mainly
+  responsible for the rapidly increasing number of alternating current
+  circuits now met with, especially in sparsely populated districts. An
+  alternating current main is one in which the current reverses its
+  direction, usually in this country 50, but sometimes 60, 80, 90, or even
+  100 times <!-- Page 42 --><span class="pagenum"><a
+  name="page42"></a>{42}</span>per second (there being unfortunately in
+  Great Britain no standard 'Periodicity' or number of cycles per second),
+  and for technical reasons which need not be entered upon here, with these
+  alternating currents the tension and ampèrage can be mutually converted
+  by means of <i>transformers</i>, so that current can be transmitted at so
+  high a tension, for instance, as 10,000 volts, and used at a voltage of
+  50 or 100 or whatever is required, the ampèrage available being increased
+  in inverse ratio as the tension is decreased. The same ready power of
+  transformation unfortunately does not apply to the continuous current, or
+  alternating currents would probably never have been heard of, but as it
+  is they are very common. For glow-lamps it is immaterial which current is
+  available, but for arc lamps the continuous is much to be preferred,
+  though both can be utilised.</p>
+
+  <p>With these initial remarks, I will now take in order of illumination
+  the various methods of utilising the electric current for optical lantern
+  work.</p>
+
+  <p><b>The Electric Glow-Lamp.</b>&mdash;The ordinary metal filament lamp
+  is not very suitable for lantern work, the light not being sufficiently
+  concentrated, but from what has already been said, it will be evident
+  that this method of lighting is more suitable where currents of low
+  voltage are available.</p>
+
+  <p>An extremely good and intense light can be obtained from a
+  comparatively small battery of accumulators, which can easily be carried
+  in the hand, and a short and thick metal filament lamp, similar to those
+  supplied with a powerful electric torch; and this arrangement is actually
+  used to some extent by travelling lecturers, but the mess and trouble of
+  keeping the accumulators in order have prevented the method being
+  generally adopted.</p>
+
+  <p>When <i>alternating</i> current is available such a lamp will work
+  well with a transformer to step down the voltage to the required degree,
+  and the arrangement is simple, cheap, and efficient, and produces a light
+  at least equal to that from <!-- Page 43 --><span class="pagenum"><a
+  name="page43"></a>{43}</span>acetylene. In comparatively small halls,
+  where the current is alternating, this is undoubtedly the best method of
+  working, as it is simpler than the arc and amply brilliant enough for all
+  practical purposes.</p>
+
+  <p>With the continuous current the problem is not so simple, as
+  transformation of voltage is not an easy matter, and a glow-lamp on; say,
+  a 200-volt circuit involves a long and fragile filament, which it is
+  difficult to arrange in a small space.</p>
+
+  <p>Many years ago the Ediswan Company produced a 'Focus' lamp for the
+  purpose, with the filament arranged in the form of a square grid, and
+  this lamp gave a light of about 100 candles, and was fairly successful
+  for a small room. More recently the Osram Company introduced a similar
+  lamp with a metal filament arranged somewhat in the form of a cone, and
+  this lamp also sufficed for a small class-room. It was, I believe, made
+  in Germany and was practically unobtainable during the war. I understand
+  the Osram Company are at present arranging to manufacture it in this
+  country, but up to the time of writing it has not made its
+  appearance.</p>
+
+  <p>None of these lamps worked direct on a public lighting circuit can be
+  regarded as really satisfactory, as it has been found impossible so far
+  to get a <i>concentrated</i> light; the 100-volt lamps have, of course,
+  been superior to those made for 200 or 250, but they are all for lantern
+  purposes far behind low voltage lamps, which are really good when a
+  suitable current can be obtained.</p>
+
+  <p><b>The Pointolite Lamp.</b>&mdash;This lamp produced by the Ediswan
+  Company is in reality a miniature arc with tungsten electrodes in a
+  highly exhausted vacuum bulb. To attempt a technical description would be
+  beyond the scope of this book; it will suffice to say that the action
+  depends upon the same principle as the various wireless vacuum valves or
+  the Coolidge X-ray tube.</p>
+
+  <p>This lamp requires a peculiar starting device which is supplied with
+  it, and gives a good, intense, and concentrated <!-- Page 44 --><span
+  class="pagenum"><a name="page44"></a>{44}</span>light, not equal to the
+  ordinary arc or to limelight, but comparing well with any other form of
+  illuminant. It can only be used with the continuous current.</p>
+
+  <p><b>The Nernst Lamp.</b>&mdash;This lamp at the present moment is
+  practically non-existent in this country, having been made exclusively in
+  Germany. Also as recent improvements in metal filament lamps have
+  rendered it almost obsolete for ordinary lighting purposes, it is, I
+  think, very doubtful whether it is still manufactured even in that
+  country, and hence I do not propose to waste space in an extensive
+  description.</p>
+
+  <p>It will suffice to say that the lamp consists of one or more straight
+  rods or filaments of a refractory material, which are semi-conducting to
+  the electric current when hot, but non-conducting when cold. To commence
+  with the filament must be heated, and in the lamps as supplied for
+  lantern work this is usually done by means of a spirit lamp, which can be
+  removed immediately the current begins to pass, as the filament is
+  thereafter maintained at a white heat automatically.</p>
+
+  <p>A three-filament Nernst lamp gives as much as 1000 candles, but it is
+  extremely hot, and the light rather diffuse. The filaments are also very
+  fragile, so on the whole the lamp was never very much in favour; but on
+  the other hand it consumed very little current, and could be worked from
+  any ordinary house lighting main, points which led to its adoption in
+  certain cases.</p>
+
+  <p><b>The Electric Arc.</b>&mdash;We now come to <i>the</i> light for
+  optical lantern work, the brightest, the most concentrated, the cheapest,
+  the easiest to work, in fact, the illuminant which combines all the
+  virtues and but few drawbacks, but of course requires one indispensable
+  condition, viz. electric current laid on. This current may be of any
+  voltage from 70-250, or even higher; it may be continuous or alternating,
+  though the former is to be preferred; and it requires a cable for <i>at
+  least</i> 5 ampères, and for a large hall 10 or 12 ampères.</p>
+
+  <p>The simplest form of arc lamp for lantern purposes is the <!-- Page 45
+  --><span class="pagenum"><a name="page45"></a>{45}</span>hand-fed type as
+  illustrated in Fig. 26. The essential feature is the pair of carbon rods,
+  the remainder of the apparatus consisting of mechanical adjustments to
+  'feed' these as they burn away, and to accurately maintain them in their
+  proper positions and in the optical centre of the lantern. Just because
+  the electric arc provides so small and concentrated a light, it is of
+  extreme importance that the centring should be exact; and hence
+  mechanical movements are usually provided for this which are unnecessary
+  with other illuminants.</p>
+
+  <div class="figcenter" style="width:47%;">
+      <a href="images/Fig26.jpg"><img style="width:100%" src="images/Fig26.jpg"
+      alt="Fig. 26.--Hand-fed Arc Lamp" title="Fig. 26.--Hand-fed Arc Lamp" /></a>
+    <span class="sc">Fig. 26.</span>&mdash;Hand-fed Arc Lamp.
+  </div>
+
+  <p>The whole question of optical adjustments has, however, been left over
+  for a future chapter, as it more or less applies to whatever illuminant
+  is used.</p>
+
+  <p>The illustration shows a lamp arranged for continuous current, the
+  upper carbon, which must be connected to the <i>positive</i> wire, being
+  larger than the lower (the negative), and very slightly behind it. The
+  light from a continuous current arc lamp comes chiefly from this upper or
+  positive carbon, <!-- Page 46 --><span class="pagenum"><a
+  name="page46"></a>{46}</span>which 'craters' as it is used, and this
+  arrangement has the effect of radiating the light in the direction
+  required (Fig. 27).</p>
+
+  <p>The positive carbon is usually of the 'cored' type, that is provided
+  with a core of softer carbon, as this assists the 'cratering' action,
+  while the negative is generally used 'solid,' that is homogeneous right
+  through.</p>
+
+  <p>The arc has to be 'struck' in the first place by touching the carbons
+  together for a moment by the mechanical means provided, and then
+  separating them to the working distance, which is approximately &#x215B;
+  inch. They must then be maintained at that distance by 'feeding' as they
+  slowly burn away, and this 'feeding' in arc lamps for lantern work is
+  usually done by hand, as in the lamp illustrated in Fig. 26, but may be
+  done by an automatic arrangement, as will be described later.</p>
+
+  <div class="figcenter" style="width:31%;">
+      <a href="images/Fig27.jpg"><img style="width:100%" src="images/Fig27.jpg"
+      alt="Fig. 27." title="Fig. 27." /></a>
+    <span class="sc">Fig. 27.</span>
+  </div>
+
+  <p>The current is really carried across the arc by <i>convection</i>, or
+  in other words conducted by a bridge of white hot carbon particles, which
+  continually stream across from the positive carbon to the negative, and
+  this bridge, while conducting the current, interposes a very considerable
+  <i>resistance</i> (otherwise it would not of course become hot).</p>
+
+  <p>A certain potential or tension is therefore necessary if a given
+  current is to be maintained, and this potential has to be greater the
+  longer the arc and also (though not in direct proportion) the smaller the
+  carbons.</p>
+
+  <p>When, however, everything is in the best proportion, <i>i.e.</i>
+  length of arc, size of carbons, and current passing, the potential at the
+  arc lamp terminals required is approximately 45 volts, and this may be
+  taken as a fixed figure for any current.</p>
+
+  <p>The length of arc to give the best results may also be taken <!-- Page
+  47 --><span class="pagenum"><a name="page47"></a>{47}</span>as
+  approximately fixed at &#x215B; inch, and the <i>variable</i> factor for
+  different currents as required is provided by altering the sizes of
+  carbons employed.</p>
+
+  <p>The error must not be made, however, of assuming that an E.M.F. of 45
+  volts is sufficient to work an arc lamp, as the minimum in practice is at
+  least 65 volts, and 100 or even 200 volts are advantageous.</p>
+
+  <p>I have come across more than one private generating installation where
+  the innocent owner has put in a dynamo for 45 or 50 volts, depending upon
+  some carelessly written statement that this is sufficient.</p>
+
+  <p><i>Why</i> a higher E.M.F. is required can be simply explained.</p>
+
+  <p>Take for instance an average hand-fed arc lamp as used for lantern
+  work and consuming, say, 10 ampères.</p>
+
+  <p>Take also, as a fact, the statement given above that the necessary
+  E.M.F. at the actual terminals of the arc lamp may be accepted as a
+  constant at 45 volts, and reverting to the equation given on page 40,
+  C&nbsp;=&nbsp;E&nbsp;/&nbsp;R, and substituting these figures we get&mdash;</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>Current (10 ampères) = E (45 volts) / R (Resistance of Arc).</p>
+    </div>
+  </div>
+
+  <p>It is therefore obvious that under these exact conditions the
+  resistance or back E.M.F. of the arc, as it is termed, must equal 4.5
+  ohms.</p>
+
+  <p>Now suppose the lamp left for a few seconds unattended, while the
+  carbons are burning away and the arc is lengthening; in a very few
+  moments the resistance will have increased, owing to the greater distance
+  between the carbons, and we will suppose it to have become 5 ohms instead
+  of 4.5.</p>
+
+  <p>The current passing will now be 45&nbsp;/&nbsp;5&nbsp;=&nbsp;9 ampères only.</p>
+
+  <p>In other words, a very slight lengthening of the arc has reduced the
+  current, and therefore the light, by 10 per cent.</p>
+
+  <p>Not only so, but 45 volts being needed to maintain an arc of <!-- Page
+  48 --><span class="pagenum"><a name="page48"></a>{48}</span>normal
+  length, it is insufficient to maintain a longer one, and in practice the
+  effect of leaving an arc under these conditions to itself for even a few
+  seconds is that it <i>goes out</i>, to the annoyance of the lecturer and
+  the confusion of the operator.</p>
+
+  <p>It is just <i>possible</i> to work an arc lamp with a total E.M.F. of
+  45 volts by giving one's whole attention to it and never taking the hand
+  off the feeding handle; but in practice no one with any experience would
+  attempt it. The arc would almost certainly go out several times during
+  the exhibition.</p>
+
+  <p>Now, take an example of a similar arc lamp consuming 10 ampères but
+  worked from a supply of 200 volts.</p>
+
+  <p>Our equation C&nbsp;=&nbsp;E&nbsp;/&nbsp;R must then obviously become</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>C (10 ampères) = E (200 volts) / Total Resistance (20 ohms).</p>
+    </div>
+  </div>
+
+  <p>The resistance of the arc itself being the same as before, viz. 4.5
+  ohms, it is obviously necessary to put an <i>extra</i> fixed resistance
+  equal to 15.5 ohms in series with it in order to make up the total of 20
+  ohms.</p>
+
+  <p><i>Now</i> leave the arc unattended until the resistance of 4.5 ohms
+  has again become 5 ohms; the only effect is that our current, instead of
+  remaining at 10 ampères, has become 200&nbsp;/&nbsp;20.5 or 9.8 nearly, a
+  difference which is imperceptible.</p>
+
+  <p>This is not all, for it is an elementary rule in electrical science
+  that the total E.M.F. of any circuit distributes itself along that
+  circuit in proportion to the distribution of resistance.</p>
+
+  <p>In other words, our original E.M.F. of 200 volts will so distribute
+  itself as to reserve, so to speak, an E.M.F. of 45 volts for the arc,
+  while the resistance of this remains at 4.5 ohms, but directly this
+  resistance increases, the E.M.F. at the arc lamp terminals automatically
+  rises, and therefore the actual diminution in current is even less than
+  the figures above quoted. <!-- Page 49 --><span class="pagenum"><a
+  name="page49"></a>{49}</span></p>
+
+  <p>Should the arc tend to 'break' or go out, the resistance across it
+  automatically becomes infinite and the <i>whole</i> 200 volts is at that
+  moment available to prevent the occurrence.</p>
+
+  <p>Under these conditions, therefore, the operator can safely leave the
+  arc for many minutes at a time. In carrying out experimental work I have
+  often left the lantern, walked up to the screen, discussed results with a
+  friend, and walked back, and the arc has shown no signs of misbehaviour
+  whatever.</p>
+
+  <div class="figcenter" style="width:25%;">
+      <a href="images/Fig28.jpg"><img style="width:100%" src="images/Fig28.jpg"
+      alt="Fig. 28.--Resistance" title="Fig. 28.--Resistance" /></a>
+    Fig. 28.&mdash;Resistance.
+  </div>
+
+  <p>In practice any current from 100 volts to 250 volts may be considered
+  as satisfactory for lantern work with a suitable resistance. Less than
+  this involves feeding the arc rather frequently, and more may give a
+  nasty shock, should the operator inadvertently touch a live wire, though
+  I have worked an arc lamp on a current of as much as 500 volts.</p>
+
+  <p>The <i>resistance</i> usually consists of a suitable length of wire of
+  high resistance (Iron, German Silver, or those alloys known as Platinoid,
+  Eureka, Manganin, Beacon, &amp;c., are most commonly used) wound in
+  spirals on a frame, and is generally supplied adjustable (Fig. 28), so
+  that more or less current may be used as desired. These resistances get
+  pretty hot in use, and care must be taken that they are placed where they
+  cannot scorch woodwork, &amp;c., and in cases where the lantern is a
+  fixture it is a good plan to have the resistance bolted up against a wall
+  once and for all. The resistance may be placed anywhere in the circuit,
+  so long as the current passes through it, then through the arc lamp (or
+  <i>vice versâ</i>), and back to the other <!-- Page 50 --><span
+  class="pagenum"><a name="page50"></a>{50}</span>pole of the supply main;
+  it does not matter in the least whereabouts it comes.</p>
+
+  <p>In cases, however, where one pole of the supply main is
+  <i>earthed</i>, it is a good thing to place the resistance in the 'live'
+  side, as this keeps the arc lamp within 45 volts of earth potential while
+  it is working, to the comfort of the operator should he touch a terminal
+  or wire, though with an ordinary lighting main there is no real fear of a
+  dangerous shock in any case.</p>
+
+  <p>The <i>amount of current required</i> depends of course on the size of
+  the sheet, length of the hall, and density or otherwise of the slides;
+  but it is usually accepted in practice that the efficient light from a
+  continuous current arc lamp equals 100 candles per ampère, and therefore
+  a 10-ampère arc will give 1000 candles. This is sufficient for all
+  ordinary halls and slides, but where these latter are very dense, as for
+  example with the Lumière three-colour process, as much as 20 or 25
+  ampères may be required.</p>
+
+  <p>In these cases some special precautions must be taken for keeping the
+  slides cool, or the result may be disastrous, but this is a question that
+  will be referred to in a later chapter. A current of 10 ampères is pretty
+  safe for all ordinary slides, and may be taken as the normal current used
+  in large halls, though in arranging for the wiring it is as well to
+  stipulate for at least 12 or even 15 ampères, especially as there must
+  necessarily be a momentary increase of current at the instant the arc is
+  'struck.'</p>
+
+  <p><b>Varieties of Hand-fed Arc Lamps.</b>&mdash;The pattern of hand-fed
+  arc lamp illustrated in Fig. 26 is only typical of many of the same
+  general design, and there are others in which the design itself is
+  fundamentally different. Of these the 'Scissors' arc lamp made by several
+  firms deserves mention on account of its simplicity and cheapness. As its
+  name implies, the mechanism resembles a pair of scissors, the carbons
+  being attached to the ends of a pair of levers hinged together <!-- Page
+  51 --><span class="pagenum"><a name="page51"></a>{51}</span>(Fig. 29). In
+  this lamp centring movements are usually dispensed with, the arc being
+  clamped on to a tray pin as in the case of a limelight jet. This is not,
+  of course, so convenient, and a further disadvantage of this pattern arc
+  lamp is that the feeding process gradually alters the position and angle
+  of the carbons. In fact, the one great merit of the lamp is cheapness,
+  and where expense is an object, it should certainly be considered.</p>
+
+  <div class="figcenter" style="width:41%;">
+      <a href="images/Fig29.jpg"><img style="width:100%" src="images/Fig29.jpg"
+      alt="Fig. 29.--Scissors Arc Lamp" title="Fig. 29.--Scissors Arc Lamp" /></a>
+    <span class="sc">Fig. 29.</span>&mdash;'Scissors' Arc Lamp.
+  </div>
+
+  <p>Yet another arc lamp deserving of mention is the 'Parallel,' a name
+  again very aptly chosen, as the two carbons are either exactly parallel
+  to each other or very slightly inclined. In the former case the arc has
+  to be 'struck' by touching the ends of the carbon rods with a piece of
+  metal or carbon. Of the actual manipulation of this lamp I have had very
+  little practical experience, but I have heard it well spoken of, though I
+  believe it has so far only been made for currents of 5 ampères or so.</p>
+
+  <p>Yet another type which must not be ignored is the 'Right-angled'
+  pattern (Fig. 30), a name again self-descriptive. The horizontal carbon
+  is the positive, and the vertical the <!-- Page 52 --><span
+  class="pagenum"><a name="page52"></a>{52}</span>negative, and this lamp
+  again is made by several manufacturers in slightly different forms.</p>
+
+  <p>This pattern lamp is in my experience the best of all for <i>small</i>
+  currents, say, of 5 ampères or so, but inferior to Fig. 26 for currents
+  of 10 ampères or more. This last remark perhaps hardly applies to
+  <i>alternating</i> currents, which, however, I have not yet discussed. I
+  cannot conclude this brief category of arc lamps without referring to the
+  <i>enclosed</i> pattern, of which the 'Westminster' is perhaps the
+  best-known and most popular (Fig. 31).</p>
+
+  <div class="figcenter" style="width:49%;">
+      <a href="images/Fig30.jpg"><img style="width:100%" src="images/Fig30.jpg"
+      alt="Fig. 30.--Right-angled Arc Lamp" title="Fig. 30.--Right-angled Arc Lamp" /></a>
+    <span class="sc">Fig. 30.</span>&mdash;'Right-angled' Arc Lamp.
+  </div>
+
+  <p>This is a lamp of the right-angled type, but the arc burns in a
+  cylindrical glass chamber, not air-tight, but partially so. After burning
+  a few minutes the oxygen in this chamber becomes used up and its place is
+  taken by carbonic-acid gas and other products of combustion, after which
+  the carbons burn away very much more slowly, and therefore require
+  feeding at much greater intervals.</p>
+
+  <p>This lamp again is chiefly made for small currents not exceeding 5
+  ampères (and can therefore be used from any ordinary lamp socket), and
+  for a moderate-sized hall is on the <!-- Page 53 --><span
+  class="pagenum"><a name="page53"></a>{53}</span>whole as cheap, efficient
+  and simple a lamp as any I am acquainted with. It can be supplied with or
+  without mechanical centring movements as required, and is usually sent
+  out with its own resistance for the particular current on which it is to
+  be used, so that it only requires connecting up to the nearest lamp
+  socket, and is ready for use.</p>
+
+  <div class="figcenter" style="width:50%;">
+      <a href="images/Fig31.jpg"><img style="width:100%" src="images/Fig31.jpg"
+      alt="Fig. 31.--Westminster Arc Lamp" title="Fig. 31.--Westminster Arc Lamp" /></a>
+    <span class="sc">Fig. 31.</span>&mdash;'Westminster' Arc Lamp.
+  </div>
+
+  <p>It is <i>not</i> sufficient for anything larger than a 12-foot sheet
+  or for working at a greater distance than, say, 40 feet, but within these
+  limits the lamp, and in fact <i>any</i> good 5-ampère arc lamp, will be
+  found quite satisfactory and saves the expense of putting in a special
+  cable.</p>
+
+  <p><b>Automatic Arc Lamps.</b>&mdash;Arc lamps for lantern work in which
+  the feeding is done automatically are also made. Like hand-fed lamps,
+  they vary in exact design, but all, or practically all, are so designed
+  that the carbons are brought together by means of springs or weights, and
+  some form of 'brake' controlled by a system of electro-magnets checks the
+  <!-- Page 54 --><span class="pagenum"><a
+  name="page54"></a>{54}</span>movement. As the carbons burn away the arc
+  lengthens, the current weakens, the electro-magnets lose their grip, and
+  the carbons move together until the increasing current puts on the brake
+  again. Some of these lamps are 'semi-automatic' only, that is to say, the
+  arc has to be struck by hand, while others perform this operation
+  automatically as well, usually by an additional magnet which draws back
+  the carbons by the correct amount after the arc is struck.</p>
+
+  <p>My frank advice to intending lanternists is to leave these lamps
+  alone. Some of them are satisfactory up to a point, but they are all apt
+  to be 'jumpy,' and on the whole the hand-fed type is in my opinion to be
+  preferred.</p>
+
+  <p><b>Arc Lamps on Alternating Currents.</b>&mdash;The alternating
+  current is not so good as the continuous for lantern work with arc lamps:
+  the light per ampère is not so great, the light has an irritating habit
+  of travelling round the carbons and there is always a slight 'hum.'</p>
+
+  <p>The sum total of these drawbacks is nothing very serious, provided
+  that proper arrangements are adopted, and I have frequently manipulated
+  arc lamps on alternating circuits with such good results that
+  professional lecturers have at first refused to believe that the circuit
+  really <i>was</i> alternating.</p>
+
+  <p>As it is frequently stated that to obtain a steady light with an
+  alternating current is impossible, I can understand their surprise, and I
+  can also understand the statement in question, as the problem is usually
+  tackled on entirely wrong lines.</p>
+
+  <p>It is almost always stated that arc lamps for alternating currents
+  should be arranged with the carbons <i>vertical</i>, and many makers
+  actually so construct their lamps as to allow of this.</p>
+
+  <p>To obtain a steady light under these conditions <i>is</i> impossible
+  and I pity anyone who attempts it; but the statement that this is the
+  best method of working has been repeated so often that it seems to have
+  been taken for granted.</p>
+
+  <p>The best arrangement (in my hands at any rate) is to <!-- Page 55
+  --><span class="pagenum"><a name="page55"></a>{55}</span>slant the
+  carbons as for the continuous current, and also to have the upper carbon
+  cored and the lower one solid, but to use a rather larger lower carbon
+  than would be correct if the main were continuous.</p>
+
+  <p>Also the upper carbon should not be <i>quite</i> so far back as with
+  D.C.; to have the front edges of the two carbons practically in line is
+  about correct, but the <i>exact</i> position should be carefully adjusted
+  to obtain the steadiest light, and it will be found that a slight
+  alteration makes a considerable difference.</p>
+
+  <p>It is also a great help to have a weak electro-magnet, or its
+  equivalent, so arranged that it tends by its influence to keep the arc to
+  the front. On some lamps this is provided for, as even with a continuous
+  current it is quite harmless and, if anything, beneficial; but, if not,
+  any competent mechanic can easily fit an 'Induction Ring,' consisting of
+  a single turn of stout copper wire, which has sufficient magnetic
+  influence to do all that is required (Fig. 32).</p>
+
+  <p>This ring must be wired in series with the arc itself, and as the
+  current passing in it automatically reverses in synchronism with the arc,
+  its effect is <i>always</i> to deflect the arc in the same direction, and
+  care must of course be taken that it is so wired that the deflection is
+  forward and not backward. This is the exact arrangement I have myself
+  adopted, and I never experience any difficulty on the score of the arc
+  wandering.</p>
+
+  <p>Right-angled arc lamps, as described on pages <a href="#page52">52</a>
+  and <a href="#page53">53</a>, are also very efficient on A.C. mains, and
+  frequently these lamps are already equipped with electro-magnets for the
+  purpose required. The 'hum' of an alternating current cannot be
+  altogether eliminated, but can be reduced to a minimum <i>by reducing the
+  voltage as far as possible</i>.</p>
+
+  <p>As has been already said, the A.C. lends itself readily to
+  transformation of voltage, and I find in practice 90-100 to <!-- Page 56
+  --><span class="pagenum"><a name="page56"></a>{56}</span>be ideal. More
+  than this is inclined to be noisy, and less is apt to result in an
+  unsteady arc.</p>
+
+  <p>The arrangement, therefore, which I recommend from long experience is
+  to employ a transformer to reduce the E.M.F. to 100 volts or thereabouts,
+  and then work with a resistance in the usual way (if the original current
+  is 100 volts, of course <i>no</i> transformer is required) with a
+  properly constructed arc lamp fitted with an induction ring or
+  electro-magnet. No difficulty should then be experienced in obtaining a
+  good, steady, and fairly quiet light.</p>
+
+  <div class="figcenter" style="width:46%;">
+      <a href="images/Fig32.jpg"><img style="width:100%" src="images/Fig32.jpg"
+      alt="Fig. 32.--Arc Lamp with Induction Ring" title="Fig. 32.--Arc Lamp with Induction Ring" /></a>
+    <span class="sc">Fig. 32.</span>&mdash;Arc Lamp with Induction Ring.
+  </div>
+
+  <p>Any little 'hum' remaining can be silenced to a very considerable
+  extent by placing the entire lantern on a thick block of saddlers' felt,
+  but in practice I have never found this necessary with ordinary currents,
+  though a few abnormal circuits where the 'periodicity' is very high are
+  noisier than others.</p>
+
+  <div class="figcenter" style="width:99%;">
+      <a href="images/Fig33.jpg"><img style="width:100%" src="images/Fig33.jpg"
+      alt="Fig. 33.--The Optical System of a Lantern" title="Fig. 33.--The Optical System of a Lantern" /></a>
+    <span class="sc">Fig. 33.</span>&mdash;<span class="sc">The Optical
+    System of a Lantern</span>.
+  </div>
+
+<p><!-- Page 57 --><span class="pagenum"><a name="page57"></a>{57}</span></p>
+
+  <p>The following table gives the sizes and particulars of carbons for
+  various currents that I have found best in actual practice:</p>
+
+<table class="nobctr" summary="Size of carbons for arc-lights" title="Size of carbons for arc-lights">
+<tr><td class="spacsingle" style="text-align:center; padding-top:1em;" colspan="3"> <span class="sc">Continuous Current</span></td></tr>
+<tr><td class="spacsingle" style="text-align:center;"> Ampères. </td><td class="spacsingle" style="text-align:center;"> + Carbon <i>Cored</i>. </td><td class="spacsingle" style="text-align:center;"> - Carbon <i>Solid</i>.</td></tr>
+<tr><td class="spacsingle" style="text-align:center;"> &nbsp; 7-10 </td><td class="spacsingle" style="text-align:center;"> 12 mm. </td><td class="spacsingle" style="text-align:center;"> &nbsp; 7 mm.</td></tr>
+<tr><td class="spacsingle" style="text-align:center;"> 10-15 </td><td class="spacsingle" style="text-align:center;"> 13&nbsp; &nbsp;,, &nbsp; </td><td class="spacsingle" style="text-align:center;"> &nbsp; 8&nbsp; &nbsp;,, &nbsp;</td></tr>
+<tr><td class="spacsingle" style="text-align:center;"> 15-20 </td><td class="spacsingle" style="text-align:center;"> 16&nbsp; &nbsp;,, &nbsp; </td><td class="spacsingle" style="text-align:center;"> 10&nbsp; &nbsp;,, &nbsp;</td></tr>
+<tr><td class="spacsingle" style="text-align:center; padding-top:1em;" colspan="3"> <span class="sc">Alternating Current</span></td></tr>
+<tr><td class="spacsingle" style="text-align:center;"> Ampères. </td><td class="spacsingle" style="text-align:center;"> Upper Carbon <i>Cored</i>. </td><td class="spacsingle" style="text-align:center;"> Lower Carbon <i>Solid</i>.</td></tr>
+<tr><td class="spacsingle" style="text-align:center;"> &nbsp; 7-10 </td><td class="spacsingle" style="text-align:center;"> 12 mm. </td><td class="spacsingle" style="text-align:center;"> 10 mm.</td></tr>
+<tr><td class="spacsingle" style="text-align:center;"> 10-15 </td><td class="spacsingle" style="text-align:center;"> 13&nbsp; &nbsp;,, &nbsp; </td><td class="spacsingle" style="text-align:center;"> 11&nbsp; &nbsp;,, &nbsp;</td></tr>
+<tr><td class="spacsingle" style="text-align:center;"> 15-20 </td><td class="spacsingle" style="text-align:center;"> 16&nbsp; &nbsp;,, &nbsp; </td><td class="spacsingle" style="text-align:center;"> 13&nbsp; &nbsp;,, &nbsp;</td></tr>
+</table>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h3>CHAPTER VII</h3>
+
+<p class="cenhead">THE OPTICAL SYSTEM OF A LANTERN</p>
+
+  <div class="figcenter" style="width:59%;">
+      <a href="images/Fig33A.jpg"><img style="width:100%" src="images/Fig33A.jpg"
+      alt="Fig. 33a.--Optical System of Lantern" title="Fig. 33a.--Optical System of Lantern" /></a>
+    <span class="sc">Fig. 33a.</span>&mdash;Optical System of Lantern.
+  </div>
+
+  <p>As previously noted, the essential parts of an Optical Lantern are, in
+  order from rear to front: (1) The illuminant; (2) the condenser; (3) the
+  slide and slide stage; (4) the objective, to which must be added, (5) the
+  body or framework which holds the whole together. Fig. 33 is a
+  diagrammatic representation of the entire optical system and Fig. 33<span
+  class="scac">A</span> shows all the various parts <i>in situ</i>: <span
+  class="scac">A</span> being the illuminant, shown in Fig. 33 <!-- Page 58
+  --><span class="pagenum"><a name="page58"></a>{58}</span>as an arc lamp,
+  <span class="scac">B</span> the condenser, <span class="scac">C</span>
+  the slide stage, and <span class="scac">D</span> the objective. The
+  foundation, so to speak, of the whole instrument is of course the slide,
+  which, as made in this country, consists of a square of glass 3¼ inches
+  diameter, the slide itself being somewhat less than this on account of
+  the binding, &amp;c.; in making calculations it is usually taken as a
+  3-inch circle. Slides are usually made by binding together with strips of
+  paper or cloth two such squares, on one of which is the photographic film
+  or painting forming the picture, the other being simply a plain cover
+  glass placed over the slide surface to protect it, and between the two
+  being placed a paper mask with an aperture of whatever size or shape is
+  required, that of the aforesaid 3-inch circle being usually taken as the
+  standard or normal dimension for this aperture.</p>
+
+  <p>The slide being illuminated by one of the various methods discussed in
+  the previous chapters, is focussed on the screen by the objective, which
+  must be selected according to the size of picture required and the
+  distance between lantern and screen.</p>
+
+  <p>These points will be gone into later, and also details as to various
+  types of objectives and their respective advantages; but it may be said
+  here that a lantern objective consists usually of a combination of lenses
+  of 2 inches or 2½ inches diameter mounted in a rackwork focussing system
+  at a distance from the slide of 6 inches to 18 inches, according to the
+  length of its 'focus.' As our slide is from 3 to 3¼ inches diameter, it
+  is evident that all the light radiating from this cannot possibly get
+  through the objective unless it is <i>converged</i> upon it, and to do
+  this is the function of the condenser. The following two diagrams, Figs.
+  34 and 35, will make the matter clear.</p>
+
+  <p><span class="scac">S</span> represents our glass slide of 3 inches
+  clear diameter, <span class="scac">R</span> the radiant or illuminant,
+  and <span class="scac">L</span> our objective, shown here for the sake of
+  simplicity as a single lens.</p>
+
+  <p>The slide is well illuminated by the light emanating from <!-- Page 59
+  --><span class="pagenum"><a name="page59"></a>{59}</span><span
+  class="scac">R</span>, but it is obvious that the bulk of this light will
+  never pass through the lens, and, in fact, only the very centre of the
+  slide will under these circumstances appear upon the screen at all.</p>
+
+  <div class="figcenter" style="width:56%;">
+      <a href="images/Fig34.jpg"><img style="width:100%" src="images/Fig34.jpg"
+      alt="Fig. 34.--Optical System without Condenser" title="Fig. 34.--Optical System without Condenser" /></a>
+    <span class="sc">Fig. 34.</span>&mdash;Optical System without
+    Condenser.
+  </div>
+
+  <div class="figcenter" style="width:53%;">
+      <a href="images/Fig35.jpg"><img style="width:100%" src="images/Fig35.jpg"
+      alt="Fig. 35.--Action of Condenser" title="Fig. 35.--Action of Condenser" /></a>
+    <span class="sc">Fig. 35.</span>&mdash;Action of Condenser.
+  </div>
+
+  <p>What is evidently wanted is to <i>converge</i> these outer rays, or in
+  other words to bend them in so that they also pass through the objective,
+  and this is the function of the condenser as illustrated in Fig. 35. The
+  condenser is here represented also by a single lens, but in practice it
+  also is invariably constructed of two or even three lenses, for both
+  optical and mechanical reasons. It is evident from the above diagrams
+  that the condenser must be somewhat larger in diameter than the slide
+  itself, and condensers for ordinary lantern work are usually 4 inches to
+  4½ inches diameter. The former size <!-- Page 60 --><span
+  class="pagenum"><a name="page60"></a>{60}</span>will suffice if the
+  condenser is placed very close to the slide, but it is often advisable to
+  leave a little intervening space, especially if the illuminant is a
+  powerful one, in order to allow any condensation of moisture readily to
+  evaporate and escape. Hence lanterns for long range work (which involve,
+  of course, good illumination) are usually made with condensers of 4½
+  inches diameter. Lantern condensers of to-day usually take one of the two
+  forms shown in Fig. 36, but the exact curve must be left to the
+  manufacturer, as the focus of the condenser must have a definite relation
+  to that of the objective. Taking, however, the design of <span
+  class="scac">E</span>, the most common of all, the two lenses should not
+  be exactly similar unless the objective is pretty short in focus, or, in
+  other words, unless the distance of the illuminant on the one hand and
+  that of the objective on the other are approximately equal. If the
+  lantern is intended for long range work, that is equipped with a long
+  focus objective, the front component of the condenser should also be
+  constructed longer in focus (that is to say, with a shallower curve) than
+  the rear one, and it is amazing how careless manufacturers are in this
+  respect. If, as is often the case, the lantern is fitted with several
+  objectives of different foci, it is usually necessary to supply
+  alternative condensers also, or at least to supply an interchangeable
+  front component.</p>
+
+  <div class="figcenter" style="width:25%;">
+      <a href="images/Fig36.jpg"><img style="width:100%" src="images/Fig36.jpg"
+      alt="Fig. 36.--Forms of Condensers" title="Fig. 36.--Forms of Condensers" /></a>
+    <span class="sc">Fig. 36.</span>&mdash;Forms of Condensers.
+  </div>
+
+  <p>If the entire condenser is too long in focus, light is lost; if too
+  short, it is impossible to obtain an even disc, as there is invariably a
+  dark patch either in the centre or round the edges.</p>
+
+  <p>The mounting of the condenser also varies with different makers; but
+  it must be remembered in any case that it gets <!-- Page 61 --><span
+  class="pagenum"><a name="page61"></a>{61}</span>extremely hot, especially
+  the back component, and hence the glass must be mounted <i>loose</i> in
+  its cell, otherwise there is great danger of it cracking. Also the space
+  between the components should be well ventilated, in order to provide for
+  the escape of moisture which usually at the start of a lantern exhibition
+  is deposited upon the glass, and should be got rid of before the actual
+  lecture commences.</p>
+
+  <p>Even with all care, the back component of a condenser will sometimes
+  crack, though such an accident should be a rare occurrence; and hence a
+  professional operator will usually provide himself with a spare lens, and
+  the condenser should be so constructed that it can readily be changed,
+  and with as little delay as possible.</p>
+
+  <p>Condenser lenses as made in this country are usually ground from the
+  glass known as 'English Crown,' and comparatively rarely crack; but they
+  are very slightly green in colour. French condensers, on the other hand,
+  are whiter, but the glass is more brittle, and a fracture a more common
+  occurrence. The French variety are (or were before the war) cheaper and
+  generally met with in cheaper instruments. More expensive lanterns are
+  usually fitted with English condensers, as the tinge of green is almost
+  imperceptible, and the advantage as regards greater security pretty
+  considerable.</p>
+
+  <p><b>The Slide Carrier and Slide Stage.</b>&mdash;Taking still the
+  optical system of the lantern in order from back to front, we now come to
+  the slide, slide carrier, and slide stage. The slide itself has already
+  been described, and the carrier is simply a mechanical contrivance,
+  usually of wood, designed for the purpose of readily changing the
+  pictures and which in its turn fits into the stage of the lantern. It may
+  be asked why, if slides are now always made to a standard size, the slide
+  carrier should not itself be built into the lantern and form the stage;
+  but the answer is, in the first place, that slides of a different size,
+  <i>i.e.</i> American or Continental, <i>may</i> be met with, <!-- Page 62
+  --><span class="pagenum"><a name="page62"></a>{62}</span>and also that
+  there are various mechanical slides on the market&mdash;for example,
+  chromotropes or scientific models, such for instance as are made to
+  illustrate the movements of the planetary bodies&mdash;and these slides
+  are permanently mounted in wooden frames which could not be put into a
+  carrier. The commonest form of carrier is that known as the 'Double
+  Sliding' pattern (Fig. 37), which consists of a frame with two apertures
+  for the slide, and an outer frame through which this itself slides and
+  which fits the stage of the lantern.</p>
+
+  <div class="figcenter" style="width:53%;">
+      <a href="images/Fig37.jpg"><img style="width:100%" src="images/Fig37.jpg"
+      alt="Fig. 37.--Double Sliding Carrier" title="Fig. 37.--Double Sliding Carrier" /></a>
+    <span class="sc">Fig. 37.</span>&mdash;Double Sliding Carrier.
+  </div>
+
+  <p>This carrier, as will be seen, allows the next picture to be placed in
+  position in the second aperture while the former one is being projected,
+  and at a signal from the lecturer, the inner frame slides smoothly
+  through the outer, and the slides are thereby changed. This carrier is
+  simple, cheap, and quiet in its action; its one disadvantage is that each
+  alternate slide has to be inserted from opposite sides of the lantern,
+  and unless the operator is fairly tall this almost necessitates an
+  assistant. Nevertheless, the carrier is the most popular of any, its
+  other advantages, especially as regards price, being so great. It is
+  usually constructed in such a way that the slide, as it moves out from
+  the central position, automatically rises in its groove in order to
+  facilitate quick removal.</p>
+
+  <p>Another pattern deservedly popular is that known as <!-- Page 63
+  --><span class="pagenum"><a name="page63"></a>{63}</span>'Beard's
+  Dissolving Carrier' and is shown in Fig. 38. In this ingenious carrier
+  all the slides are inserted from the same side, the oncoming slide being
+  pushed <i>in front</i> of its predecessor, and being therefore somewhat
+  out of focus it produces a blur on the screen.</p>
+
+  <p>The movement is performed by pushing in a projecting handle, and on
+  withdrawing this the slide which is finished with comes with it, and the
+  finish of the movement presses the new slide back until it is in its
+  proper position and in focus.</p>
+
+  <div class="figcenter" style="width:52%;">
+      <a href="images/Fig38.jpg"><img style="width:100%" src="images/Fig38.jpg"
+      alt="Fig. 38.--Beard's Dissolving Carrier" title="Fig. 38.--Beard's Dissolving Carrier" /></a>
+    <span class="sc">Fig. 38.</span>&mdash;Beard's Dissolving Carrier.
+  </div>
+
+  <p>The entire action is simpler than it sounds, and the temporary
+  blurring of the image on the screen during the process of changing is
+  supposed to give somewhat the effect of 'Dissolving Views,' and hence the
+  name 'Dissolving Carrier.'</p>
+
+  <p>This appliance is three times the price of the 'Double Sliding'
+  pattern, but the fact that it is worked from one side only is a decided
+  advantage, though on the other hand it is not (unless great care is used)
+  quite so silent in its action as the 'Double Sliding' type.</p>
+
+  <p>A further modification of this carrier adapts it to take any of the
+  recognised 'foreign' sizes of slides, so that if a few American ones, for
+  instance, are met with among a collection of English manufacture, there
+  is no need to change the carrier. <!-- Page 64 --><span
+  class="pagenum"><a name="page64"></a>{64}</span></p>
+
+  <p>There are other varieties of carriers on the market which there is no
+  need particularly to describe, such as, for example, carriers fitted with
+  roller curtains to give the effect of a curtain rolling up, magazine
+  carriers to hold twenty-four or more slides and exhibit them in rotation,
+  and other patterns too numerous to mention. Of these the reader must be
+  left to judge for himself, but, generally speaking, <i>simplicity</i> in
+  a carrier is the most important point to be looked for, and
+  complications, however ingenious, should be avoided.</p>
+
+  <div class="figcenter" style="width:45%;">
+      <a href="images/Fig39.jpg"><img style="width:100%" src="images/Fig39.jpg"
+      alt="Fig. 39.--Focussing Action of Lens" title="Fig. 39.--Focussing Action of Lens" /></a>
+    <span class="sc">Fig. 39.</span>&mdash;Focussing Action of Lens.
+  </div>
+
+  <p>The lantern stage must also receive consideration, but it will be
+  better to discuss it as part of the mechanical construction of the
+  lantern.</p>
+
+  <p><b>The Objective</b> is really the most vital part of a lantern, as
+  the definition of the picture almost entirely depends upon the excellence
+  or otherwise of this lens. This will be obvious at once when it is
+  realised that the objective has to project on to the distant screen a
+  greatly magnified image of the comparatively small lantern slide, and the
+  intending purchaser is strongly advised to economise almost anywhere
+  rather than on this item.</p>
+
+  <p>The action of a lens in focussing the image is perhaps best explained
+  by a simple diagram (Fig. 39), from which it will be seen that all the
+  rays proceeding from any one point on the object are re-converged (when
+  the lens is in focus) to a definite point on the image, and the
+  perfection of the picture depends upon the lens performing this function
+  accurately. <!-- Page 65 --><span class="pagenum"><a
+  name="page65"></a>{65}</span></p>
+
+  <p>The imperfections are chiefly two, viz. those known as chromatic and
+  spherical aberration respectively. Chromatic aberration simply means that
+  all the colours composing the original beam of, say, white light are not
+  equally refracted or converged, and therefore do not meet again at the
+  same spot (the well-known prism or lustre effect), and reveals itself by
+  coloured fringes round the edges of the various details in the
+  picture.</p>
+
+  <div class="figcenter" style="width:13%;">
+      <a href="images/Fig40.jpg"><img style="width:100%" src="images/Fig40.jpg"
+      alt="Fig. 40.--Achromatic Lens" title="Fig. 40.--Achromatic Lens" /></a>
+    <span class="sc">Fig. 40.</span>&mdash;Achromatic Lens.
+  </div>
+
+  <p>By spherical aberration we mean that the light falling upon the centre
+  of a lens is not brought to a focus at exactly the same spot as the
+  marginal rays, and a general want of definition is the result, usually
+  accompanied also by a want of 'flatness' in the image, that is to say the
+  edges of the picture do not focus at the same time as the centre.</p>
+
+  <p>Chromatic aberration is easily cured by using an achromatic or
+  compound lens made by cementing together two lenses of crown and flint
+  glass respectively, as in Fig. 40.</p>
+
+  <p>It will be seen that the flint glass component by itself is a
+  <i>concave</i> lens and therefore neutralises in part, or in whole, the
+  convex crown lens. Flint glass has both greater dispersive power and also
+  greater refractive power than crown glass, but fortunately not to the
+  same <i>degree</i>; hence a compound lens made in this way and with
+  curves carefully worked out may have its chromatic effect entirely
+  neutralised while retaining very considerable refractive or 'focussing'
+  power, and simple achromatic objectives of this type are quite
+  inexpensive.</p>
+
+  <p>In lanterns intended for Science demonstration, as distinct from the
+  mere projection of slides, lenses of this pattern are very frequently
+  used, as they will project the latter when required reasonably well, and
+  for the demonstration of <!-- Page 66 --><span class="pagenum"><a
+  name="page66"></a>{66}</span>experiments or of apparatus on the screen
+  have advantages that need not be discussed here.</p>
+
+  <p>For very long focus lenses also they are sometimes employed, as the
+  trouble from spherical aberration is much less apparent with lenses of
+  long focus than with short, and the difference in expense is much more in
+  the former case than in the latter. For short focus lenses, however, as
+  used in moderate-sized halls, they are not good enough, and the type of
+  lens almost universally employed is that known as the 'Petzval'
+  combination (Fig. 41).</p>
+
+  <div class="figcenter" style="width:37%;">
+      <a href="images/Fig41.jpg"><img style="width:100%" src="images/Fig41.jpg"
+      alt="Fig. 41.--Petzval Combination" title="Fig. 41.--Petzval Combination" /></a>
+    <span class="sc">Fig. 41.</span>&mdash;Petzval Combination.
+  </div>
+
+  <p>This lens really consists of two achromatic combinations, the pair at
+  the front being cemented together, and that at the rear having an air
+  space between. The combination is so designed that the spherical
+  aberration of the one pair neutralises that of the other, and the result
+  is or should be a lens corrected both for chromatic and spherical
+  aberration.</p>
+
+  <p>These lenses, however, vary very much in the perfection of their
+  results, and as they are at present usually imported in bulk from France,
+  the customer does well to insist upon a demonstration of his own
+  particular lantern before acceptance.</p>
+
+  <p>The magnifying power of a lens depends upon its 'focus' multiplied by
+  its distance from the screen, and the focus in the case of a simple lens
+  is easily determined by the familiar 'burning-glass' experiment, that is
+  by focussing an image of the sun upon a piece of paper and measuring
+  accurately <!-- Page 67 --><span class="pagenum"><a
+  name="page67"></a>{67}</span>the distance the lens must be away to
+  produce the most concentrated spot.</p>
+
+  <p>In practice it is sufficiently accurate to focus a distant window, or
+  other luminous object, upon the paper, any error obtained by this method
+  being for ordinary purposes a negligible one.</p>
+
+  <p>With a compound lens, such as a 'Petzval' combination, this method
+  does not hold good, as the optical centre of such a lens is not
+  necessarily midway between its two components.</p>
+
+  <p>The actual focus can be got pretty approximately by focussing a window
+  or other object as before and measuring the distance from one definite
+  point (say the front edge of one of the lens cells) to the paper, then
+  turning it round and taking a second measurement from the <i>same</i>
+  point, the mean between the two measurements giving the actual focus.</p>
+
+  <p>In practice the 'simple equivalent focus,' as it is termed, of a
+  lantern lens is usually determined by measuring the magnification of the
+  image thrown upon the screen, when, by knowing the original size of the
+  slide (a 'standard' slide of 3 inches diameter is usually taken) and the
+  distance between lantern and screen, we get the focus from the following
+  very simple equation:</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>Diameter of picture on screen (in feet)&nbsp;/&nbsp;Diameter of slide (in inches) = Distance between lens and screen (in feet)&nbsp;/&nbsp;Focus of lens (in inches)</p>
+    </div>
+  </div>
+
+  <p>or perhaps more simply still:</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>{Distance between lens and screen (in feet) × Diameter of slide (in inches)}&nbsp;/&nbsp;Diameter of picture (in feet) = Focus of lens in inches;</p>
+    </div>
+  </div>
+
+  <p>or, if we know the focus of the lens but want to know how far from the
+  screen we must go to produce a given-sized picture, the formula will be:
+  <!-- Page 68 --><span class="pagenum"><a
+  name="page68"></a>{68}</span></p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>{Diameter of picture (in feet) × Focus of lens (in inches)}&nbsp;/&nbsp;Diameter of slide (in inches) = Distance required (in feet).</p>
+    </div>
+  </div>
+
+  <p>It is handy for the lanternist to remember that, dealing with a
+  standard 3-inch slide, a 6-inch lens will <i>always</i> give a picture
+  whose diameter is <i>one-half</i> the distance from lens to screen, a
+  12-inch lens half this again or <i>one-quarter</i>, and a 9-inch lens
+  half-way between the two.</p>
+
+  <p>Bearing these simple figures in mind, the approximate distance can
+  usually be <i>guessed</i> sufficiently near for the first trial, and then
+  the lantern shifted a little nearer or the reverse as required.</p>
+
+  <p>The following table may, however, be useful, as showing readily the
+  magnification produced at different distances by lenses of given
+  foci:</p>
+
+<table class="allbctr" summary="Magnification table" title="Magnification table">
+<tr><td class="allb" style="text-align:center"> Disc<br />
+ wanted
+</td><td class="allb" style="text-align:center"> Focus<br />
+<b>4½</b> in.
+</td><td class="allb" style="text-align:center"> Focus<br />
+  <b>6</b> in.
+</td><td class="allb" style="text-align:center"> Focus<br />
+  <b>8</b> in.
+</td><td class="allb" style="text-align:center"> Focus<br />
+  <b>10</b> in.
+</td><td class="allb" style="text-align:center"> Focus<br />
+  <b>12</b> in.
+</td><td class="allb" style="text-align:center"> Focus<br />
+  <b>15</b> in.
+</td><td class="allb" style="text-align:center"> Focus<br />
+  <b>18</b> in. </td></tr>
+<tr><td class="vertb" style="text-align:center"> feet. </td><td class="vertb" style="text-align:center"> ft. in. </td><td class="vertb" style="text-align:center"> ft. in. </td><td class="vertb" style="text-align:center"> ft. in. </td><td class="vertb" style="text-align:center"> ft. in. </td><td class="vertb" style="text-align:center"> ft. in. </td><td class="vertb" style="text-align:center"> ft. in. </td><td class="vertb" style="text-align:center"> ft. in. </td></tr>
+<tr><td class="vertb" style="text-align:center"> <b>9</b></td><td class="vertb" style="text-align:center"> 13 6 </td><td class="vertb" style="text-align:center"> 18 0 </td><td class="vertb" style="text-align:center"> 24 0 </td><td class="vertb" style="text-align:center">&nbsp; 30 0 </td><td class="vertb" style="text-align:center">&nbsp; 36 0 </td><td class="vertb" style="text-align:center"> &nbsp; 45 0 </td><td class="vertb" style="text-align:center"> &nbsp; 54 0 </td></tr>
+<tr><td class="vertb" style="text-align:center"> <b>12</b></td><td class="vertb" style="text-align:center"> 18 0 </td><td class="vertb" style="text-align:center"> 24 0 </td><td class="vertb" style="text-align:center"> 32 0 </td><td class="vertb" style="text-align:center">&nbsp; 40 0 </td><td class="vertb" style="text-align:center">&nbsp; 48 0 </td><td class="vertb" style="text-align:center"> &nbsp; 60 0 </td><td class="vertb" style="text-align:center"> &nbsp; 72 0 </td></tr>
+<tr><td class="vertb" style="text-align:center"> <b>15</b></td><td class="vertb" style="text-align:center"> 22 6 </td><td class="vertb" style="text-align:center"> 30 0 </td><td class="vertb" style="text-align:center"> 40 0 </td><td class="vertb" style="text-align:center">&nbsp; 50 0 </td><td class="vertb" style="text-align:center">&nbsp; 60 0 </td><td class="vertb" style="text-align:center"> &nbsp; 75 0 </td><td class="vertb" style="text-align:center"> &nbsp; 90 0 </td></tr>
+<tr><td class="vertb" style="text-align:center"> <b>18</b></td><td class="vertb" style="text-align:center"> 27 0 </td><td class="vertb" style="text-align:center"> 36 0 </td><td class="vertb" style="text-align:center"> 48 0 </td><td class="vertb" style="text-align:center">&nbsp; 60 0 </td><td class="vertb" style="text-align:center">&nbsp; 72 0 </td><td class="vertb" style="text-align:center"> &nbsp; 90 0 </td><td class="vertb" style="text-align:center"> 108 0 </td></tr>
+<tr><td class="vertb" style="text-align:center"> <b>20</b></td><td class="vertb" style="text-align:center"> 30 0 </td><td class="vertb" style="text-align:center"> 40 0 </td><td class="vertb" style="text-align:center"> 53 4 </td><td class="vertb" style="text-align:center">&nbsp; 66 8 </td><td class="vertb" style="text-align:center">&nbsp; 80 0 </td><td class="vertb" style="text-align:center"> 100 0 </td><td class="vertb" style="text-align:center"> 120 0 </td></tr>
+<tr><td class="vertb" style="text-align:center"> <b>25</b></td><td class="vertb" style="text-align:center"> 37 6 </td><td class="vertb" style="text-align:center"> 50 0 </td><td class="vertb" style="text-align:center"> 66 8 </td><td class="vertb" style="text-align:center">&nbsp; 83 4 </td><td class="vertb" style="text-align:center"> 100 0 </td><td class="vertb" style="text-align:center"> 125 0 </td><td class="vertb" style="text-align:center"> 150 0 </td></tr>
+<tr><td class="vertb" style="text-align:center"> <b>30</b></td><td class="vertb" style="text-align:center"> 45 0 </td><td class="vertb" style="text-align:center"> 60 0 </td><td class="vertb" style="text-align:center"> 80 0 </td><td class="vertb" style="text-align:center"> 100 0 </td><td class="vertb" style="text-align:center"> 120 0 </td><td class="vertb" style="text-align:center"> 150 0 </td><td class="vertb" style="text-align:center"> 180 0 </td></tr>
+</table>
+
+  <p><b>The Diameter of the Objective.</b>&mdash;The diameter of the
+  objective must depend to a certain extent upon its focus in the case of a
+  double combination such as a Petzval. These lenses consist, as has
+  already been said, of two achromatic components some distance apart, and
+  for technical considerations, which need not be discussed here, the
+  <i>distance</i> between these components is usually about two-thirds of
+  the focal length. This is not a universal rule, as the lenses of
+  different makers vary a good deal; but it is generally a fact <!-- Page
+  69 --><span class="pagenum"><a name="page69"></a>{69}</span>that the
+  longer the focus of the lens the greater is usually the separation
+  between the two lens systems.</p>
+
+  <p>The entire lens therefore mounted in its tube resembles a
+  <i>tunnel</i> of varying length according to its focus, and through this
+  tunnel a <i>cone</i> of light rays have to be passed. It is plain,
+  therefore, that a lens of long focus, which in practice means a long tube
+  length, must be made also of large diameter, or a portion of the cone
+  will be cut off, with a consequent loss of light.</p>
+
+  <p>In practice lenses up to 6 inches focus are usually made of 2 inches
+  diameter, and there is no advantage in a larger size. With a lens of 8
+  inches focus there is a slight gain in increasing the diameter to
+  2&#x215C; (the next 'standard' size), and lenses of longer focus than
+  this should certainly be 2&#x215C; inches up to, say, 12 inches focus,
+  when a lens of 3 inches diameter is preferable. These large lenses are,
+  however, very expensive, both in themselves and also on account of the
+  fact that their weight entails heavy and expensive brass mounting, and
+  hence lenses up to 14 or 15 inches focus are often supplied in the
+  2&#x215C; size for reasons of economy.</p>
+
+  <p>To sum up, <i>short-range</i> lanterns, as they are called, are
+  usually fitted with lenses of 2 inches diameter, and <i>long-range</i>
+  instruments either with 3-inch lenses or the intermediate size of
+  2&#x215C; inches. If a lantern is purchased for either long or
+  short-range work, it is usually fitted with a brass front for a large
+  lens, and so arranged that a shorter focus lens of 2 inches diameter can
+  easily be interchanged, utilising the same brass mounting.</p>
+
+  <p>Lenses of <i>variable</i> focus have also been designed, in which an
+  additional lens can be added or subtracted to increase or decrease the
+  focal length; but nothing very practical has yet been achieved in this
+  direction, and therefore these 'Omnifocal' lenses have never come into
+  general favour.</p>
+
+  <p>Objectives like condensers want cleaning at times, and care must be
+  taken not to scratch the glass, as the concave lens of each component is
+  of flint glass, and very soft. A <!-- Page 70 --><span class="pagenum"><a
+  name="page70"></a>{70}</span>clean chamois leather is the best thing to
+  use, but a soft cloth, or even a handkerchief, may be employed with care.
+  It is very important that a lens be reassembled, after cleaning, the
+  correct way, as a single lens reversed would utterly spoil the
+  definition. The front component is usually balsamed together, and
+  therefore all that is needed is to see that the whole combination is not
+  reversed. In the Petzval system this lens should have its convex
+  constituent towards the screen (Fig. 41). The back combination is usually
+  loose, and the two lenses are sometimes separated by a thin brass ring.
+  In the Petzval lens the concave element should be inside, with its
+  concave surface outwards, the deep curve of the other lens should fit
+  into this concavity, and the flatter curve face towards the condenser.
+  One or two makers, however, have introduced a modification of the Petzval
+  system in which the whole of this back combination is reversed, and the
+  exact arrangement should therefore be noted very carefully when taking
+  the lens to pieces.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h3>CHAPTER VIII</h3>
+
+<p class="cenhead">THE BODY OF THE LANTERN</p>
+
+  <p>We now come to the mechanical construction of the optical lantern, and
+  a great variety of design presents itself, according to price, type
+  (<i>i.e.</i> short range or long range), and the individual ideas of the
+  various makers.</p>
+
+  <p>Lantern bodies as a rule are now made of metal, although up till quite
+  recently the better class instruments were more usually made of polished
+  mahogany lined internally with iron; but there has of late been a
+  consensus of opinion in favour of metal only.</p>
+
+  <p>In the cheaper lanterns this metal body is usually made either of
+  Russian iron or of sheet-iron tinned and japanned, <!-- Page 71 --><span
+  class="pagenum"><a name="page71"></a>{71}</span>there being little to
+  choose either in price or quality between the two varieties, and in all
+  but the very cheapest instruments the front is usually of brass.</p>
+
+  <p>In better lanterns the body is more often made of enamelled steel, the
+  front as before being of brass; but brass, copper, or aluminium are also
+  used occasionally for the body of the lantern.</p>
+
+  <div class="figcenter" style="width:56%;">
+      <a href="images/Fig42.jpg"><img style="width:100%" src="images/Fig42.jpg"
+      alt="Fig. 42.--Hughes' Short-Range Lantern" title="Fig. 42.--Hughes' Short-Range Lantern" /></a>
+    <span class="sc">Fig. 42.</span>&mdash;Hughes' Short-Range Lantern.
+  </div>
+
+  <p>In deciding upon the type of body to be purchased the main
+  considerations to be borne in mind are: (1) The type or types of
+  illuminant to be used, a powerful arc lamp for example requiring a larger
+  body than is necessary for a weaker radiant; (2) the size and position of
+  the lens to be carried, a Petzval objective of say 3 inches diameter
+  which has to be supported at the end of a long brass mount for long-range
+  work obviously demanding a body of greater strength and rigidity than is
+  required with a 6-inch focus lens of 2 inches diameter; (3) price.</p>
+
+  <p>Fig. 42 shows an extremely good lantern body for short-range work made
+  by Messrs. Hughes, the illustration depicting the instrument complete
+  with a 'Luna' methylated spirit lamp, though, of course, any other
+  illuminant suitable for a small lantern could be used instead. <!-- Page
+  72 --><span class="pagenum"><a name="page72"></a>{72}</span></p>
+
+  <p>This lantern illustrates well one point that has already been
+  emphasised as important, viz. the ventilation of the condenser. It will
+  be noticed that this is placed <i>outside</i> the body of the instrument
+  instead of inside as is usual with larger bodies, and that wide slots are
+  cut in the condenser mount to allow free escape of steam.</p>
+
+  <p>Other points of this excellent design are the screw adjustment to the
+  slide stage (facilitating the use of special slides, such, for example,
+  as those illustrating the movements of the planetary bodies which
+  sometimes involve the use of extra thick frames) and a simple but
+  efficient tilting arrangement to the base.</p>
+
+  <div class="figcenter" style="width:52%;">
+      <a href="images/Fig43.jpg"><img style="width:100%" src="images/Fig43.jpg"
+      alt="Fig. 43.--Long-Range Lantern" title="Fig. 43.--Long-Range Lantern" /></a>
+    <span class="sc">Fig. 43.</span>&mdash;Long-Range Lantern.
+  </div>
+
+  <p>Such a lantern is hardly suitable for a powerful arc lamp or limelight
+  jet, or for heavy long-range lenses, but is a very good typical
+  instrument for use in moderate-sized halls, and a lantern of this general
+  type is usually found in lantern catalogues, though, of course, the exact
+  designs vary according to the ideas of the manufacturer. Of lanterns for
+  long-range work a good example is perhaps Messrs. Newton &amp; Co.'s
+  'Intermediate' pattern (Fig. 43).</p>
+
+  <p>This again is only typical of many others by the various makers, but
+  the principal points are common to all. These are: (1) The large and
+  well-ventilated body; (2) the long <!-- Page 73 --><span
+  class="pagenum"><a name="page73"></a>{73}</span>baseboard; (3) the strong
+  and massive brass front necessary to carry the large long-range lenses;
+  (4) the velvet curtain at the back to close in any stray light from a
+  powerful arc lamp.</p>
+
+  <div class="figcenter" style="width:27%;">
+      <a href="images/Fig44.jpg"><img style="width:100%" src="images/Fig44.jpg"
+      alt="Fig. 44.--Connections for a Bi-unial Lantern" title="Fig. 44.--Connections for a Bi-unial Lantern" /></a>
+    <span class="sc">Fig. 44.</span>&mdash;Connections for a Bi-unial
+    Lantern.
+  </div>
+
+  <p>The two foregoing designs are perhaps sufficiently typical of lantern
+  bodies in general to make further detailed description of individual
+  designs unnecessary; but reference should be made to features which
+  special requirements may render advisable.</p>
+
+  <p>Under this heading mention must be made of <i>Bi-unials</i> or Double
+  Lanterns, as used for the once famous 'Dissolving Views.'</p>
+
+  <p>A bi-unial lantern consists essentially of two different instruments,
+  each complete with its limelight jet or other illuminant&mdash;front,
+  condensers, objective, &amp;c., usually mounted on one body&mdash;and
+  with some arrangement for 'dissolving' or turning the light in each
+  lantern gradually on and off.</p>
+
+  <p>Fig. 44 shows the back view of such a lantern with two limelight jets
+  and dissolving tap, this piece of mechanism (shown below in the
+  illustration) being so arranged that when the lever is horizontal
+  <i>both</i> lanterns are on full, but moving the lever either way cuts
+  off the gas supply to one lantern. In the case of limelight the tap
+  should always operate by cutting off the oxygen supply in advance of the
+  coal gas (in order to avoid a 'snap'), and the latter should never be cut
+  off entirely, but a small bead of flame left to keep the jet alight,
+  until the lantern is required for the next slide. <!-- Page 74 --><span
+  class="pagenum"><a name="page74"></a>{74}</span></p>
+
+  <p>This is usually arranged for by means of a bye-pass, and a bye-pass is
+  sometimes provided on the oxygen side as well, but is usually discarded
+  in practice.</p>
+
+  <p>A bi-unial lantern can be worked in the same way with acetylene gas,
+  but with the electric arc it is impossible to turn the light on and off
+  gradually, and in practice dissolving must be done by keeping both
+  lanterns fully alight, and using a dissolving shutter, that is a movable
+  shutter that covers each objective alternately. The same arrangement must
+  be used with other illuminants, such as oil, only in this case the
+  lanterns must be mounted side by side, on account of the tall chimneys.
+  With oil lamps the arrangement answers fairly well, the dissolving fan,
+  as it is termed, being made with serrated edges which give the
+  <i>gradual</i> obliteration required; but with the electric arc the
+  extremely sharp definition becomes a serious difficulty, and a good
+  dissolver for this illuminant has never yet been found, though, in view
+  of the fact that dissolving views are more or less a thing of the past,
+  the matter cannot be regarded as important.</p>
+
+  <p>The advantages claimed for a double lantern are two: first, a
+  'Dissolving' effect by which one picture fades gradually into the next,
+  and which is supposed to be more pleasing than the movement of a carrier;
+  and second, 'Dissolving Effects' can be shown, such as exhibiting a
+  landscape by day and changing it into a moonlight scene, or bringing on
+  the appearance of a snowstorm, which can easily be done by means of a
+  roller slide, with minute perforations shown in motion by the second
+  lantern while the landscape remains on the screen from the first. In the
+  days when dissolving views were all the vogue, a third or even a fourth
+  lantern has been added for more complicated effects, and at the famous
+  Polytechnic demonstrations of years ago, I believe that as many as six
+  were sometimes employed.</p>
+
+  <p>In these days of the cinematograph it is doubtful how far interest in
+  such effects could be revived, and a lantern has <!-- Page 75 --><span
+  class="pagenum"><a name="page75"></a>{75}</span>gradually come to be
+  looked on more as an instrument for showing illustrations as required by
+  the lecturer rather than as a pleasing exhibition in itself, and as
+  dissolving views have lost their attraction, the double or triple lantern
+  has been relegated to the limbo of antiquity.</p>
+
+  <div class="figcenter" style="width:45%;">
+      <a href="images/Fig45.jpg"><img style="width:100%" src="images/Fig45.jpg"
+      alt="Fig. 45.--Beard's Circulating Water Tank" title="Fig. 45.--Beard's Circulating Water Tank" /></a>
+    <span class="sc">Fig. 45.</span>&mdash;Beard's Circulating Water Tank.
+  </div>
+
+  <p>Among other 'special' lanterns should be mentioned models made with
+  water-cooled stages, for use with very delicate slides. This elaboration
+  is not necessary with ordinary slides and illuminants of moderate power,
+  but where very delicate slides, such as specimens of natural colour
+  photography, have to be shown, it is an advisable precaution to pass the
+  beam of light first through a tank of water in order to absorb the heat.
+  Lanterns intended for this work are usually constructed with a kind of
+  double stage, a glass trough of water fitting into the rear aperture and
+  the slide-carrier into the front one. Such an arrangement answers quite
+  well for most purposes, but for extreme cases lanterns are equipped with
+  a trough connected to a large outside tank and complete circulatory
+  system, after the manner of the cooling tank of a gas engine.</p>
+
+  <p>Such a lantern, constructed by Messrs. Beard, is illustrated in Fig.
+  45, and it will be seen that in this instrument the water trough is
+  placed between the lenses of the condenser. <!-- Page 76 --><span
+  class="pagenum"><a name="page76"></a>{76}</span>This is a very good
+  position, as the beam of light at this point is, or should be, parallel,
+  whereas between the condenser and the slide it is convergent, and
+  therefore a condenser of a larger diameter than the slide must be
+  employed in the latter case if the trough is of considerable width.</p>
+
+  <p>While dealing with 'Special' lantern bodies, we should perhaps just
+  mention here the numerous pattern lanterns made for the demonstration
+  both of lantern slides and of Scientific Phenomena, such as the
+  projection of insect life or other microscopic objects, polarised light
+  experiments, electrical apparatus, opaque objects, &amp;c. A detailed
+  description of these lanterns and how to use them belongs to the second
+  part of this work, as also does the popular cinematograph; but
+  educational institutes, and even boys' clubs, when considering the
+  purchase of a lantern, might well reflect whether it would be advisable
+  to spend a little more money in the acquisition of an instrument which
+  can be utilised for a variety of purposes.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h3>CHAPTER IX</h3>
+
+<p class="cenhead">LANTERN BOXES, STANDS, READING LAMPS, ETC.</p>
+
+  <p>Having now discussed all the essential parts of a lantern, the next
+  points to be considered are those of lantern boxes and stands. It is best
+  to take these together, as more often than not a lantern is arranged to
+  stand upon its box during use, and the plan is both convenient and
+  simple. The whole question is one to be settled upon its own merits in
+  each individual case. Sometimes neither box nor stand is wanted at all.
+  The lantern is put away into a locked-up cupboard or other safe place,
+  and used upon a permanent support or (as is often the case in a church)
+  from a gallery at the back. <!-- Page 77 --><span class="pagenum"><a
+  name="page77"></a>{77}</span></p>
+
+  <p>In most cases, however, a box of some sort is desirable, and the two
+  main considerations are strength and simplicity.</p>
+
+  <p>All patent arrangements, such, for example, as those in which the
+  sides of the box fall down and provide trays for the slides, are
+  beautiful in theory, but cannot be recommended in practice. A good,
+  simple and substantial box is what is required, preferably painted black,
+  and provided with strong handles.</p>
+
+  <p>One addition may be permitted, viz. a tilting top. Some means for
+  tilting the lantern is always advisable, as it is seldom convenient to
+  raise the instrument to the level of the centre of the screen, and a
+  slight upward elevation does not appreciably distort the image. This
+  arrangement for tilting may be either embodied in the lantern itself, as
+  for instance in the instrument shown in Fig. 42, or may be provided for
+  on the box or on the stand, if a stand is used.</p>
+
+  <p>It is, perhaps, an elaboration that may be regarded as not strictly
+  necessary, as a book or two or other article may be placed under the
+  lantern base as required; but a tilting arrangement is so convenient that
+  it can be strongly recommended, and the addition is not expensive.</p>
+
+  <p>For large, long-range lanterns a strong deal box, on which the lantern
+  can stand, is usually all that it is desirable to purchase in the way of
+  a support. A good solid table can usually be found, which will do all the
+  rest, as it must be remembered that a slight tilt at a long range means a
+  good deal of total elevation.</p>
+
+  <p>Where this is not procurable a stand must be provided, and this for a
+  large lantern should be strong and rigid. Anything in the way of a
+  collapsible tripod should be avoided, but such an arrangement as Fig. 46
+  is quite good and rigid enough for all practical purposes.</p>
+
+  <p>For a <i>small</i> lantern a tripod stand is quite suitable, though
+  care must be taken that one of the legs does not get kicked, either by
+  accident or design, or the result may be a catastrophe. <!-- Page 78
+  --><span class="pagenum"><a name="page78"></a>{78}</span></p>
+
+  <p><i>Slide Boxes.</i>&mdash;On this subject not much need be said. The
+  variety of patterns on the market is endless, some being designed from
+  the point of view of safe transit by post, others for convenience of
+  storage and classification. It is essentially a case where each
+  individual user must use his or her taste, and in any case the question
+  of the box is one for the owner of the slides rather than for the
+  lanternist.</p>
+
+  <div class="figcenter" style="width:49%;">
+      <a href="images/Fig46.jpg"><img style="width:100%" src="images/Fig46.jpg"
+      alt="Fig. 46.--Quadruple Lantern Stand" title="Fig. 46.--Quadruple Lantern Stand" /></a>
+    <span class="sc">Fig. 46.</span>&mdash;Quadruple Lantern Stand.
+  </div>
+
+  <p><b>Reading-Desks, Lamps, and Signals.</b>&mdash;Some form of reading
+  lamp for the lecturer is generally considered to be part of a
+  lanternist's equipment, and the most usual pattern is fitted with a
+  candle, after the manner of a carriage lamp, or else constructed to burn
+  colza or other vegetable oil, such as supplied for cycle lamps. Oil gives
+  the brighter light, but is apt to get spilled in transit, hence a candle
+  lamp is the more <!-- Page 79 --><span class="pagenum"><a
+  name="page79"></a>{79}</span>convenient for a travelling lecturer, while
+  oil is to be preferred if transport is not a factor to be considered.</p>
+
+  <p>These lamps are usually constructed with a red flashing signal at the
+  rear, actuated by a simple lever, by which the lecturer can communicate
+  his wish for a change of slide, &amp;c., to the lanternist (Fig. 47).</p>
+
+  <div class="figcenter" style="width:20%;">
+      <a href="images/Fig47.jpg"><img style="width:100%" src="images/Fig47.jpg"
+      alt="Fig. 47.--Reading Lamp" title="Fig. 47.--Reading Lamp" /></a>
+    <span class="sc">Fig. 47.</span>&mdash;Reading Lamp.
+  </div>
+
+  <p>There are various other devices used for the same purpose, such as a
+  castanet, to be held in the lecturer's hand and clicked when necessary,
+  an electric bell to ring in the lantern box, &amp;c. If this latter is
+  used it is usual to remove the gong, the buzz of the hammer being
+  sufficiently loud without it. Some lecturers again prefer to use no such
+  apparatus at all, but simply to say 'Next slide' as required, or to tap
+  on the floor with a pointer, and the choice of a suitable means of
+  communication between lecturer and lanternist must be largely a matter of
+  individual selection. More elaborate <i>reading-desks</i> are also
+  supplied by most makers, but here again judgment must largely come into
+  play in what is hardly a technical matter.</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<h3>CHAPTER X</h3>
+
+<p class="cenhead">SCREENS AND SCREEN STANDS</p>
+
+  <p>The best of all screens for lantern purposes is undoubtedly a smooth
+  whitewashed wall, and this is now provided in many halls where lantern
+  exhibitions are usual. In places where this is not practicable the next
+  best substitute is a canvas <!-- Page 80 --><span class="pagenum"><a
+  name="page80"></a>{80}</span>screen, which rolls up and down (Fig. 48).
+  This can be obtained from any good maker, but again can only really be
+  used as a <i>fixture</i> in the hall where the lantern is to be used. It
+  can, however, be fitted into a wooden box which can be painted or
+  varnished to suit the other architecture, and the provision of such a
+  screen is to be strongly recommended whenever possible. If portability is
+  required, a linen or calico sheet that can be folded up is necessary, but
+  this can never be hung absolutely flat, and also loses a considerable
+  amount of light by transmission.</p>
+
+  <div class="figcenter" style="width:57%;">
+      <a href="images/Fig48.jpg"><img style="width:100%" src="images/Fig48.jpg"
+      alt="Fig. 48.--Roller Screen" title="Fig. 48.--Roller Screen" /></a>
+    <span class="sc">Fig. 48.</span>&mdash;Roller Screen.
+  </div>
+
+  <p>A so-called 'transparent' sheet is made of very thin linen, and
+  intended to work with the lantern <i>behind</i> it, showing the picture
+  through the linen to the audience on the other side, but this is seldom
+  used except in the open air for religious or political meetings,
+  &amp;c.</p>
+
+  <p>An <i>opaque</i> sheet can be had in one piece up to 9 feet square;
+  larger sizes than this must have at least one seam, and most skilful
+  sewing is necessary, especially with large sheets consisting of several
+  strips sewn together.</p>
+
+  <p>Sheets such as these are usually supplied with either eyelet holes
+  round the edges or else linen tapes sewn on, and the exact method of
+  hanging must be left to circumstances. <!-- Page 81 --><span
+  class="pagenum"><a name="page81"></a>{81}</span></p>
+
+  <p>In the case of a small sheet it will be sufficient to stretch it at
+  the four corners, and this can often be done by screwing into the walls
+  or some convenient girder two screw eyes and similar eyes into the floor,
+  all four being considerably farther apart than the size of the sheet.</p>
+
+  <div class="figcenter" style="width:21%;">
+      <a href="images/Fig49.jpg"><img style="width:100%" src="images/Fig49.jpg"
+      alt="Fig. 49.--Portable Screen Stand" title="Fig. 49.--Portable Screen Stand" /></a>
+    <span class="sc">Fig. 49.</span>&mdash;Portable Screen Stand.
+  </div>
+
+  <p>A stout cord being then passed through the two upper eyes, long enough
+  for both ends to reach near the floor, one end of each can be fastened to
+  the two top corners of the sheet and the latter drawn up, the two bottom
+  corners being afterwards stretched and tied down tightly to the lower
+  eyes. In the case of large sheets this hardly suffices, and it will be
+  found necessary to fasten the sheet at intervals all round or it will
+  exhibit awkward creases, and this again is a matter where the lanternist
+  must use his own initiative according to the possibilities.</p>
+
+  <p>In some halls the erection of a sheet in the way above described is a
+  sheer impossibility, and in such cases a frame must be made by nailing
+  strips of wood together, or better by utilising a portable screen stand
+  (Fig. 49).</p>
+
+  <p>These stands are usually made of bamboo, with short brass connecting
+  tubes, and the method of using them is so obvious that a description need
+  hardly be given. The screen frames are supplied by all the leading
+  opticians, but an intending purchaser would be well advised to see one
+  erected before ordering. I have actually seen a 12-foot screen frame
+  offered for sale that was too weak to carry its own weight, let alone the
+  weight of the sheet!</p>
+
+  <p><br style="clear:both" /></p>
+<hr class="full" />
+
+<p><!-- Page 82 --><span class="pagenum"><a name="page82"></a>{82}</span></p>
+
+<h3>CHAPTER XI</h3>
+
+<p class="cenhead">THE PRACTICAL MANIPULATION OF A LANTERN</p>
+
+  <p>Having now described the optical lantern in its various forms and the
+  more important accessories, we come to the question of practical
+  manipulation. In making arrangements for an exhibition the first thing to
+  be seen to is to ensure that every accessory that will be required will
+  be there, and the best plan is to make a complete list of all sundries to
+  be provided. Such items as string (for the sheet), lime tongs if
+  limelight is used, pliers for changing carbons if the arc is to be the
+  illuminant, screw-driver, matches, the <i>key of the lantern box</i>, and
+  other similar items, are likely to be left behind unless such a list is
+  made and carefully checked. On arriving at the hall, the first thing to
+  be done as a rule is to get up the sheet, after which the professional
+  operator generally begins to feel happy again.</p>
+
+  <p>The next thing, if it has not been done first, is to determine the
+  position of the lantern, and this, as has been explained in Chapter VII,
+  is a matter of the size of picture to be shown and the focus of the
+  objective.</p>
+
+  <p>It is a mistake to show too large a picture; a little 'white' round
+  the edges is a good thing, and it is better to have a small disc well
+  illuminated than a large one less bright. Convenience, however, must also
+  be considered, and it is often justifiable to go back a few feet farther
+  than other considerations would dictate in order to place the lantern in
+  a gallery or other spot where it is out of the way.</p>
+
+  <p>Having fixed the position of the lantern, it should be got into place,
+  the cable or tubing connected or whatever else is necessary, according to
+  the illuminant to be used. It should then be lit up, the flasher of the
+  lens opened, and the light <!-- Page 83 --><span class="pagenum"><a
+  name="page83"></a>{83}</span>centred sufficiently to produce some sort of
+  disc upon the screen. (It is, of course, presumed that the lenses,
+  &amp;c., have previously been cleaned.)</p>
+
+  <p>A carrier should now be placed in the stage and a slide inserted into
+  it, and the method of doing so requires a little explanation. The slide
+  must be placed in the carrier upside down, as will be obvious to anyone
+  who has studied Chapter VII, but in addition to this it must be turned
+  the correct way, otherwise the picture will be reversed from left to
+  right. This in the case of certain subjects, such as a copy of a picture,
+  may not greatly matter; but in slides depicting buildings or landscapes
+  with which the audience may be familiar, or worse still, printing or
+  writing, is a serious blunder.</p>
+
+  <p>Slides made by a commercial firm will usually be 'spotted,' that is to
+  say, will have two white spots on the face of the slide when the latter
+  is viewed in its correct position, and at the top. The slides should be
+  turned upside down and placed in the carrier with the spots, of course,
+  now at the bottom and <i>towards the condenser</i>.</p>
+
+  <p>If a slide is not spotted it should be viewed as it is to appear on
+  the screen, and then placed in the carrier with the face that was towards
+  the operator as he viewed it turned to the condenser, and of course
+  inverted.</p>
+
+  <p>The above remarks apply only in cases where the image is thrown
+  <i>on</i> the screen; in the comparatively rare instances where it is
+  shown <i>through</i> the latter the slides must be turned round
+  laterally, but of course still inverted. The slide having been placed in
+  the stage it should be 'focussed' by racking the objective in or out, and
+  if necessary pulling out the draw tube as well until the image on the
+  screen is sharply defined. So far the light has only been roughly
+  centred, sufficiently so to enable the slide to be focussed, and to
+  complete the operation both slide and carrier should next be taken out of
+  the lantern, leaving a clear disc on the screen, and this disc may
+  resemble any of the appearances shown in Fig. 50. <!-- Page 84 --><span
+  class="pagenum"><a name="page84"></a>{84}</span></p>
+
+  <p>If it resembles <span class="scac">A</span> the light must be moved to
+  the left, if like <span class="scac">B</span> to the right, like <span
+  class="scac">C</span> it must be lowered, like <span
+  class="scac">D</span> it must be raised, always moving it to the side
+  opposite to the dark shade until this is central on the disc. If it now
+  resembles <span class="scac">E</span>, the light must be moved nearer the
+  condenser; if, on the contrary, the centre is dark, it must be drawn back
+  until finally the circle should be as nearly as possible clear and bright
+  all over, as at <span class="scac">F</span>.</p>
+
+  <div class="figcenter" style="width:55%;">
+      <a href="images/Fig50.jpg"><img style="width:100%" src="images/Fig50.jpg"
+      alt="Fig. 50.--Adjustment of the Light" title="Fig. 50.--Adjustment of the Light" /></a>
+    <span class="sc">Fig. 50.</span>&mdash;Adjustment of the Light.
+  </div>
+
+  <p>It is important to note that this adjustment <i>cannot</i> be properly
+  made while a slide is in position, and neither can it be made until the
+  lantern has been focussed, so the above procedure is the only way to get
+  a satisfactory result. With some of the larger illuminants, such as a
+  paraffin-oil lamp, there are no centering adjustments, the size of the
+  radiant rendering exact centering unnecessary, and generally speaking the
+  smaller the luminous point, the more exact must the operation be.</p>
+
+  <p>In the case of such illuminants as acetylene or limelight <!-- Page 85
+  --><span class="pagenum"><a name="page85"></a>{85}</span>care must be
+  taken that they are turned fully on before centering, otherwise turning
+  on the fuller amount afterwards will raise the position of the luminous
+  spot.</p>
+
+  <p>The centering achieved, the slide carrier may be replaced, the first
+  slides placed in position, the remainder arranged in their proper order,
+  the system of signalling with the lecturer determined, and all is
+  ready.</p>
+
+  <p>If there is still an interval before commencing, the light may be
+  switched off or turned out, or in the case say of limelight, turned down
+  very low until wanted.</p>
+
+  <p>It is of extreme importance to see that all the slides are in their
+  right order, though the duty of seeing to this usually rests with the
+  lecturer rather than with the operator. I remember hearing of one lecture
+  on the life of Queen Victoria, when the lecturer announced, 'The next
+  picture will be a photograph of the Royal Prince who for many years
+  shared the Throne with our gracious Sovereign.' At the words the operator
+  brought on the next slide, which proved to be <i>a restored specimen of a
+  prehistoric monster</i> (tableaux!). Such mistakes 'bring down the
+  house,' but in serious lectures, and especially at religious services,
+  cannot be too carefully guarded against.</p>
+
+  <p>Mention has already been made of the liability of moisture to condense
+  on the surfaces of the condensers or slides, and to avoid this, so far as
+  the condensers are concerned, it is well to light up say ten minutes
+  before the lantern is actually wanted, or alternatively to take out the
+  condensers and thoroughly warm them in front of a stove, or to place them
+  wrapped in a cloth on hot-water pipes. The slides should in the same
+  manner be warmed before using and should be finally held above the
+  lantern or placed on the top, if this is flat, the last thing before
+  being placed in the carrier. If these precautions are omitted, on a cold
+  night the first surface of the condenser will become so covered with
+  moisture as to almost obscure the slide, and this will quickly disappear
+  <!-- Page 86 --><span class="pagenum"><a
+  name="page86"></a>{86}</span>with the heat of the lantern. Next, the two
+  inner surfaces of the condensers will behave in turn in the same way, and
+  will take considerably longer to clear, especially if the ventilation of
+  the condenser is poor; then the fourth surface will take up the running,
+  and finally, when the lanternist is congratulating himself that the
+  trouble is over, each successive slide will become affected in the same
+  way. With an operator who knows his business, none of these troubles
+  should occur.</p>
+
+  <p><b>Accidents.</b>&mdash;These will occur sometimes, even in the best
+  managed exhibition; the rubber tubing feeding a limelight jet gets kinked
+  or trodden on, or a fuse melts if electric light is being used, &amp;c.,
+  and out goes the light. In such cases a loud request such as, 'Would you
+  mind turning up the light for a minute, please,' accompanied by a
+  good-humoured laugh, usually allays the fears of 'nervy' people. An
+  operator must never get 'nervy' himself. I have known of more than one
+  fiasco because some little hitch occurred, and two or three timid ladies
+  crowded round and asked anxious questions, till the lanternist lost his
+  head. In one such case the cautious superintendent at a children's
+  entertainment decided that it would be safer not to have the exhibition
+  at all, simply because a regulator was not screwed tightly enough into a
+  cylinder to prevent an escape of gas, only the operator (a somewhat
+  youthful one) had been driven to the verge of lunacy by continual
+  questions of the standard type, 'Are you sure it is safe?' 'Will it blow
+  up?' 'Are you certain you understand it?' &amp;c., &amp;c. More serious
+  accidents, such as the entire lantern getting upset, ought never to
+  occur, and it is up to the lanternist to take whatever precautions he
+  deems necessary to safeguard his instrument. With a juvenile audience,
+  for example, it is often a good thing to arrange a barricade of forms
+  round the lantern and to see that no one comes within it.</p>
+
+  <p>Finally, 'whatever is worth doing at all is worth doing well,' and
+  this is as true of lantern exhibiting as of anything <!-- Page 87
+  --><span class="pagenum"><a name="page87"></a>{87}</span>else. There are
+  a deplorable number of lantern exhibitions given with the sheet hanging
+  in creases, dirty lenses, light poorly adjusted and centred, and
+  occasionally slides shown upside down. A conscientious lanternist should
+  see to <i>every</i> detail; slipshod methods, as in everything else, mean
+  poor results.</p>
+
+  <div class="poem">
+    <div class="stanza">
+      <p>Printed by <span class="sc">Spottiswoode, Ballantyne &amp; Co. Ltd</span>.</p>
+      <p>Colchester, London &amp; Eton, England</p>
+    </div>
+  </div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of Optical Projection, by 
+Lewis Wright and Russell S. Wright
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+Project Gutenberg's Optical Projection, by Lewis Wright and Russell S. Wright
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Optical Projection
+       Part 1: Projection of Lantern Slides
+
+Author: Lewis Wright
+        Russell S. Wright
+
+Release Date: October 31, 2010 [EBook #33899]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK OPTICAL PROJECTION ***
+
+
+
+
+Produced by Chris Curnow, Keith Edkins and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+OPTICAL PROJECTION
+
+A TREATISE ON THE USE OF THE LANTERN IN
+EXHIBITION AND SCIENTIFIC DEMONSTRATION
+
+BY
+
+LEWIS WRIGHT
+
+AUTHOR OF 'LIGHT: A COURSE OF EXPERIMENTAL OPTICS'
+
+5TH EDITION
+
+RE-WRITTEN AND BROUGHT UP-TO-DATE BY
+
+RUSSELL S. WRIGHT, M.I.E.E.
+
+IN TWO PARTS
+
+PART I
+
+_THE PROJECTION OF LANTERN SLIDES_
+
+
+
+_WITH ILLUSTRATIONS_
+
+
+
+LONGMANS, GREEN, AND CO.
+39 PATERNOSTER ROW, LONDON, E.C. FOURTH AVENUE & 30TH STREET, NEW YORK
+BOMBAY, CALCUTTA, AND MADRAS
+
+1920
+
+(_All rights reserved_)
+
+       *       *       *       *       *
+
+
+{v}
+
+PREFACE TO THE FIFTH EDITION
+
+The first edition of this work was written by my father, the late Mr. Lewis
+Wright, and was published in 1890.
+
+The reception that it received testified to the fact that it met a
+long-felt want, and successive editions were published in 1895, 1901, and
+1906.
+
+My father, unfortunately, met his death in a railway accident in 1905, and
+the corrections and additions to the last edition, which had been to a
+certain extent prepared by him, were completed and written by myself, and
+the work as published then was again reprinted in 1911.
+
+As the original text is now thirty years old, it has seemed better entirely
+to re-write the whole book rather than make fresh revisions, the more so as
+the last ten years have seen great advances in the science of Lantern
+Projection, and especially in the developments of Acetylene and Electric
+Lighting.
+
+It has also seemed best at the present juncture to issue the book in two
+parts, the first dealing with the Projection of Lantern Slides only, and
+the second with the Demonstration of Opaque and Microscopic Objects,
+Scientific Phenomena and accessory apparatus, including Cinematograph
+Projection.
+
+It must of necessity be many months before this second volume can be
+produced, for the simple reason that Optical {vi} Instrument Makers have as
+yet hardly had time to turn round after the war and produce their new
+models, and therefore any such book written now could do little more than
+describe apparatus that was on the market prior to 1914.
+
+The present work, therefore, deals solely with the exhibition of Lantern
+Slides in the Optical Lantern, and as such I trust will be found of value
+to Schoolmasters, Social Workers, Lecturers, and, in fact, to all who use
+the lantern as a means of illustration.
+
+  RUSSELL S. WRIGHT.
+  _January 1920._
+
+       *       *       *       *       *
+
+
+{vii}
+
+CONTENTS
+
+  CHAPTER                                                     PAGE
+
+  I. INTRODUCTORY                                                1
+
+  II. THE ILLUMINANT                                             3
+
+  III. PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS   6
+
+  IV. THE ACETYLENE LIGHT                                       11
+
+  V. LIMELIGHT AND THE ACETYLENE BLAST                          16
+
+  VI. THE ELECTRIC LIGHT                                        39
+
+  VII. THE OPTICAL SYSTEM OF A LANTERN                          57
+
+  VIII. THE BODY OF THE LANTERN                                 70
+
+  IX. LANTERN BOXES, STANDS, READING LAMPS, ETC.                76
+
+  X. SCREENS AND SCREEN STANDS                                  79
+
+  XI. THE PRACTICAL MANIPULATION OF A LANTERN                   82
+
+{viii}
+
+ILLUSTRATIONS
+
+  FIG.                                                     PAGE
+
+   1. Oil Lamp                                                6
+   2. Inverted  Incandescent Lamp                             8
+   3. Methylated Spirit Burner                                9
+   4. Luna Lamp                                              10
+   5. The Moss Generator                                     12
+   6. The A.L. or 'Popular' Model                            14
+   7. Acetylene Jet                                          15
+   8. Oxygen Cylinder in hemp cover                          17
+   9. Double Lever Key                                       18
+  10. Fine Adjustment Valve                                  19
+  11. Construction of Beard's Regulator                      20
+  12. Beard's Regulator                                      21
+  13. Regulator and Gauge                                    22
+  14. Gas-bags                                               24
+  15. 'Blow-through' Nozzles                                 25
+  16. 'Blow-through' Jet                                     25
+  17. Mixed Jet                                              27
+  18. Mixed Jet, Gwyer pattern                               27
+  19. Mixing Chamber of Jet                                  28
+  20. 'Injector' Jet                                         30
+  21. 'Gridiron' Saturator                                   32
+  22. 'Pendant' Saturator                                    33
+  23. Fallot Air Blast                                       37
+  24. Fallot Air Blast, and Cylinder                         37
+  25. Lime-tongs                                             39
+  26. Universal Hand-fed Arc Lamp                            45
+  27.                                                        46
+  28. Resistance                                             49
+  29. 'Scissors' Arc Lamp                                    51
+  30. 'Right-angled' Arc Lamp                                52
+  31. 'Westminster' Arc Lamp                                 53
+  32. Arc Lamp with Induction Ring                           56
+  33. The Optical System of a Lantern            _facing p._ 57
+  33A. Optical System of Lantern                             57
+  34. Optical System without Condenser                       59
+  35. Action of Condenser                                    59
+  36. Forms of Condensers                                    60
+  37. Double Sliding Carrier                                 62
+  38. Beard's Dissolving Carrier                             63
+  39. Focussing Action of Lens                               64
+  40. Achromatic Lens                                        65
+  41. Petzval Combination                                    66
+  42. Hughes' Short-Range Lantern                            71
+  43. Long-Range Lantern                                     72
+  44. Connections for a Bi-unial Lantern                     73
+  45. Beard's Circulating Water Tank                         75
+  46. Quadruple Lantern Stand                                78
+  47. Reading Lamp                                           79
+  48. Roller Screen                                          80
+  49. Portable Screen Stand                                  81
+  50. Adjustment of the Light                                84
+
+       *       *       *       *       *
+
+
+{1}
+
+OPTICAL PROJECTION
+
+A TREATISE ON THE USE OF THE OPTICAL LANTERN
+
+CHAPTER I
+
+INTRODUCTORY
+
+Lantern Projection, as commonly understood, may be broadly subdivided into
+two branches: (A) The Projection of Lantern Slides, and (B) The Projection
+of Scientific Phenomena, Opaque Objects, Microscopic Specimens, &c.,
+usually referred to broadly under the heading of 'Scientific
+Demonstration.'
+
+To these two classes may perhaps now be added a third, viz. The Projection
+of So-called Living Pictures, or, in other words, the Cinematograph. In the
+earlier editions of this work both A and B were dealt with in the same
+volume, but, as there are thousands who require to use a lantern for the
+demonstration of lantern slides only, and who have no interest or concern
+with Science Projection, it has seemed to the writer that the work might,
+with advantage, be divided into two portions, Vol. I. dealing with slides
+only, and Vol. II. with the various adaptations of the science lantern.
+This present book therefore only deals with the exhibition of lantern
+slides, and as such it will, I trust, be found to be of real assistance to
+the ordinary user of the optical lantern, including clergymen,
+schoolmasters, army and cadet officers, and others {2} who require advice
+and instruction in the purchase or use of a lantern.
+
+The essential parts of a lantern are: (_a_) A _slide-holder_ or _carrier_
+to hold the slide; (_b_) a _lens_ to 'focus' it on the screen; (_c_) a
+_condenser_ to converge the light upon slide and lens; (_d_) a source of
+light or _radiant_ to provide the necessary illumination; and (_e_) a
+_body_ or framework to hold the whole together. All possible variations in
+choice of a suitable lantern relate to one or another of the above parts,
+and will be treated of in turn; but, fortunately, we have this
+all-important simplification that every ordinary English lantern slide is
+the same _standard size_, viz. 3-1/4 inches square. Some Continental and
+American slides differ in one dimension from the above, but not enough to
+cause any serious difficulty, and the convenient English standard is being
+gradually adopted throughout the world.
+
+The varieties of slide-holders or carriers are therefore comparatively few
+and are chiefly concerned with the question of rapidly and easily changing
+the slides. The choice of a focussing lens or objective is mainly a matter
+of the size of picture required, and the most convenient distance from the
+screen for the lantern to be placed. Variations in condensers, which are
+comparatively small, are usually only a matter of conforming these with the
+size or type of objective to be used, and should be left to the
+manufacturer's judgment. The question of a suitable radiant is partly a
+matter of the amount of illumination required, and partly that of the
+practical possibilities; for example, if electric current is available some
+form of electric light is usually the most convenient, as well as the least
+expensive, but where this is not the case, paraffin-oil, methylated spirit,
+incandescent gas, acetylene, limelight, &c., are alternatives which all
+have their uses and must be considered on their own merits.
+
+Sometimes, as for example in the case of a travelling lecturer, a lantern
+is required fitted with a range of lenses for {3} halls of different size,
+and also with a variety of illuminants, and this in most lanterns can be
+easily provided for.
+
+The body is usually a matter of taste and price only, and may range from a
+simple but efficient shell of Russian iron to an elaborate mahogany
+instrument with a brass front, screw tilting arrangements and other
+adornments; but of late years there has been a wholesome reaction against
+unnecessary finish, and a simple metal body of some description is now
+chiefly the order of the day. In the foregoing remarks the various parts of
+a lantern have been mentioned in what I should consider the correct order,
+starting from the slide and slide-holder, and so to speak building up the
+rest of the instrument round these items; but I now propose somewhat to
+vary the procedure and for convenience deal in detail first with the
+Radiant, or _Illuminant_.
+
+       *       *       *       *       *
+
+
+CHAPTER II
+
+THE ILLUMINANT
+
+The first necessity for lantern projection is a strong light, and this can
+be obtained from a variety of sources, the principal means in common use
+being approximately in order of excellence as follows: paraffin-oil,
+incandescent spirit, incandescent gas, acetylene, acetylene air blast,
+oxyhydrogen (limelight), oxyether, and electric light in its various forms.
+The ideal characteristics to be sought for are (1) great intrinsic
+brilliancy; (2) minimum _size_ of luminous spot; (3) freedom from flicker;
+(4) freedom from smell; (5) absence of any preponderating colour; (6)
+cheapness; and (7) convenience. There is no question whatever as to which
+of the available sources of light most perfectly combines all the above if
+it is available, viz. the electric arc. If a current supply is in {4} the
+building, this form of lighting easily excels all others, except possibly
+in the matter of flicker, and even in this respect there is very little
+fault to be found with it.
+
+From all other points of view it is wellnigh perfect, inasmuch as it
+provides an extremely concentrated and intensely luminous spot, of almost
+perfect whiteness (if anything slightly bluish), no smell, comparatively
+little heat, convenient and inexpensive. So great is the advantage of the
+electric arc that attempts have been made to use it from accumulators in
+places where a current supply is not available, but this cannot be
+seriously recommended, except in special cases. Where an electric supply
+is, however, available there can be no real choice, whether the lantern is
+required for use in a large hall or a small class-room. The advantages of
+using the arc are so great that no other method need be seriously
+considered.
+
+The one real objection that I have heard urged against it is due, curiously
+enough, to its very perfection, and that is, that it lends itself to such
+exceedingly sharp definition that any slight imperfection in the slide is
+too faithfully reproduced on the screen, for which reason it is sometimes
+recommended that the operator shall work with the objective the least
+fraction out of focus; but this is a matter for individual taste and
+judgment.
+
+If, however, there is no possibility of using the electric current, one of
+the other sources of illumination must perforce be adopted, and for a
+_large_ hall this can only be limelight in one of its many forms, viz.
+oxyhydrogen, oxyether, oxyacetylene, &c. As regards results on the screen,
+this light compares well even against the electric arc, but it involves the
+expense and trouble of compressed gas cylinders, or the infinitely worse
+recourse to the now obsolete method of filling gas-bags.
+
+Limelight is therefore now but little used in this country, as the majority
+of large halls are equipped with the electric {5} current, and for smaller
+buildings it is deemed unnecessary and too expensive.
+
+ACETYLENE is undoubtedly the illuminant most in favour next to electric
+light, as the light is brilliant enough to illuminate a picture 12 feet in
+diameter at a distance up to, say, 30 feet from the screen, and this
+suffices in a large majority of cases, and acetylene is comparatively
+cheap, and reasonably simple to work.
+
+INCANDESCENT-GAS is often employed for small class-rooms and is fairly
+effective for a picture not exceeding 9 or 10 feet in diameter, and the
+same can be said of the same type of burner heated by methylated spirit.
+
+PARAFFIN-OIL is the poorest of all present-day forms of lantern
+illuminants. The flame is large, impairing the definition, yellow in
+colour, uneven in illumination, liable to smoke and smell, and barely equal
+to incandescent gas in illuminating power.
+
+It is therefore going gradually out of use in this country, but in
+out-of-the-way places, especially abroad, it is sometimes the only
+practicable light, and is therefore still employed from the best of all
+reasons, necessity.
+
+It is not the intention of the author to give precise working instruction
+for all and every variety of the above illuminants as manufactured by
+different firms. For such the reader must be referred to the directions
+usually issued by the makers themselves, but a general description of the
+various types offered for choice will not be out of place, and it will be
+more convenient to begin with the poorest, viz. paraffin-oil, and finish
+with the most perfect, the electric arc.
+
+       *       *       *       *       *
+
+
+{6}
+
+CHAPTER III
+
+PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS
+
+[Illustration: FIG. 1.--Oil Lamp.]
+
+There are several varieties of oil lamps on the market, but in practically
+every case they take the same general form, a metal reservoir sliding in
+grooves in the lantern body and holding approximately a pint of oil with
+(usually) four wicks _nearly_ parallel, but slightly converging from rear
+to front, these enclosed in a flame chamber of Russian iron, with _loose_
+well-annealed ends of sheet glass and an adjustable reflector at the back,
+or sometimes the reflector itself forms the rear end of the flame chamber.
+The chimney must be tall and is now usually made adjustable, though I have
+never been able to trace any real advantage from this complication {7}
+(Fig. 1). The whole secret of obtaining the best results from these lamps
+may be summed up--_good oil and perfect cleanliness_; and it is wonderful
+what can be done when these points are properly attended to.
+
+Care should be taken in trimming the wicks to see that no charred parts
+fall down between the wick holders, but it makes little difference whether
+the trimming is done with scissors or by rubbing with the finger. Special
+lamp scissors are sold by all makers with a large flat on one side to catch
+the portions cut off.
+
+These lamps should be well rubbed over the last thing before use, as
+paraffin-oil is apt to 'creep,' and the operator does not want to be told
+that his apparatus is suggestive of a fried fish shop. In working with
+these lamps it is difficult to avoid a dark streak down the centre of the
+sheet, representing the space between the two centre wicks; to a certain
+extent this can be obviated by adjusting the reflector, and in any case is
+not very obvious when the slide is in place. Lamps constructed with either
+three or five wicks are better in this respect, but the former are usually
+considered to be too poor in illuminating power, and the latter are apt to
+crack the sheet-glass ends by excessive heat.
+
+INCANDESCENT GAS.--Incandescent gas burners do not need much description,
+as they are practically similar to those in general use for house lighting.
+They may be either of the erect or inverted forms, the latter being
+preferable owing to the light being more concentrated, and a reflector is
+provided to increase the illumination (Fig. 2).
+
+These reflectors should be _spherical_ and so adjusted that the radiant is
+in the centre of curvature, thus ensuring that the light from the reflector
+passes again through the original source. If this point is not attended to,
+we shall be dealing with essentially two sources of light instead of one,
+to the detriment of the definition.
+
+The same remark applies to every lantern illuminant {8} which is
+supplemented by a reflector, and it is extraordinary how often it is
+neglected by the manufacturer. Of course the opacity of the illuminant
+destroys much of the efficiency of the reflector, and hence in the case of
+incandescent gas mantles there is not much real gain in making use of them,
+but with these comparatively weak illuminants every fraction tells, and the
+reflector does not add much to the cost.
+
+[Illustration: FIG. 2.--Inverted Incandescent Lamp.]
+
+In light the inverted gas burner is very little superior to oil, but it is
+whiter, slightly more concentrated, and freer from smell, and therefore to
+be regarded as preferable if a supply of gas is available.
+
+METHYLATED SPIRIT BURNERS.--Incandescent mantles heated by methylated
+spirit are also largely used, and provide a light decidedly superior to gas
+and nearly equal to acetylene. Some arrangement must be made for
+volatilising the spirit and driving the vapour out under pressure, and the
+most usual contrivance is somewhat as illustrated in Fig. 3.
+
+In this apparatus the spirit is contained in a metal reservoir at the rear
+and air pressure is provided by a pair of rubber balls and valves after the
+manner of a medical spray. Sufficient {9} pressure having been obtained,
+the liquid spirit is forced into a vaporising chamber immediately behind
+the mantle, and a kind of miniature pitchfork, with its prongs wrapped in
+asbestos wool, is soaked in spirit, and pushed over the brass fitting of
+the burner in such a way that when lighted the flame heats the chamber and
+volatilises the spirit. The burner can now be lit, and although the fork
+burns out in the course of a minute or so, the heat from the mantle itself
+is thereafter sufficient to vaporise the spirit as rapidly as required.
+This lamp works exceedingly well in practice, but has one drawback, viz.
+that it is possible to obtain too much pressure and squirt _liquid_ spirit
+through the burner, when it naturally catches fire and may even run on to
+the floor.
+
+[Illustration: FIG. 3.--Methylated Spirit Burner.]
+
+An accident of this sort is rare and usually harmless even if it does
+occur, but an audience is easily frightened, and hence this burner should
+only be used by _an operator with experience_. An altogether better
+arrangement is that made by Messrs. Hughes of Kingsland and known as the
+'Luna' Lamp (Fig. 4).
+
+In this burner there is no pump and no volatilising chamber; {10} the
+spirit is contained as before in a metal reservoir and a separate burner
+underneath is used to keep this sufficiently hot to both vaporise the
+spirit and provide the necessary pressure. The heat can be regulated by
+means of an adjustable sheath to the burner, and a simple safety valve
+provides against an excess of vapour.
+
+I do not say that an accident of the sort previously referred to is
+impossible even with this burner, but I have never heard of it happening,
+and the lamp is certainly the best apparatus of its kind that I am
+acquainted with.
+
+[Illustration: FIG. 4.--Luna Lamp.]
+
+INCANDESCENT ELECTRIC LAMPS.--Incandescent electric lamps of the ordinary
+metal or carbon filament type are also frequently used in small
+class-rooms, and should be mentioned here, as they provide approximately
+the same illumination as a gas mantle, or in some cases rather better. It
+will, however, be more convenient to deal with the question of electric
+lighting as a whole in the chapter devoted to it.
+
+It will suffice here to say that lamps are made for the purpose with a
+special filament arranged to provide a concentrated light, the ordinary
+type being almost useless in this respect, and that small battery lamps,
+worked by a suitable accumulator, can also be used, but except under very
+special circumstances are hardly worth the trouble of keeping the batteries
+charged.
+
+       *       *       *       *       *
+
+
+{11}
+
+CHAPTER IV
+
+ACETYLENE
+
+There is no doubt that at present acetylene holds second place to electric
+light in popularity for optical lantern work. The light is good; not, it is
+true, _so_ good as limelight or the electric arc, but still sufficient for
+a picture up to 12 feet in diameter at a working distance from the screen
+of not more than 30 feet, and this suffices for the large majority of
+halls.
+
+It has great advantages over limelight in convenience and cheapness,
+although on both these points it must yield place to the electric arc,
+always providing that current is available, and therefore it is chiefly
+used in country districts and in gas-lit halls in large towns.
+
+Acetylene gas is formed, as is well known, by the action of water upon
+carbide of calcium, and the generators constructed for lantern work are
+essentially the same in construction as for other purposes.
+
+The alterations introduced are chiefly directed towards obtaining a light
+as _steady_ as possible from a comparatively small generator, and,
+secondly, towards the entire elimination of smell, which obviously is far
+more serious in a lecture hall than, for instance, on a motor car. The
+generators in most common use may be divided into two classes, i.e. those
+on the gasometer principle in which the carbide is gradually lowered into
+the water, and those in which the water is allowed slowly to gain access to
+the carbide. A good example of the former is perhaps that made by Messrs.
+Moss of Birmingham, though there are several others equally good, and clear
+and explicit directions for working should be supplied by the makers. The
+Moss Generator (Fig. 5) consists of a tall iron vessel A fitted with a gas
+tap at bottom, this communicating {12} with a vertical iron tube within the
+vessel. Into this container fits the inner bell or container B, divided
+internally into two concentric portions entirely separated from each other,
+but connected by the pipe P P and the tap T.
+
+A guide inside the bell encircles the iron tube in the outer tank and
+prevents rotation. Into the inner portion fits again the carbide-container
+(shown separately on the left), which is locked when in place by giving it
+a half turn, when a hook inside the bell engages with the lower edge of the
+carbide container and prevents it from falling.
+
+The carbide container is fitted with a series of shelves, and the contents
+of a 2 lb. tin of carbide should be roughly divided among them; there is no
+need to make any accurate division. The carbide used should be that known
+as 1/2 inch mesh, and should be _pure_. That described as 'chemically'
+treated is apt to give trouble by over-generation in these gasometers and
+should be scrupulously avoided.
+
+[Illustration: FIG. 5.--The Moss Generator.]
+
+The carbide having been placed in the receptacles, these should be closed
+by means of the loose flap and the whole pushed into the bell and secured.
+
+Water should be poured into the outer vessel up to a mark on the iron tube,
+and the bell placed in position. The lower tap being then turned on and the
+upper one closed, air from the outer portion of the bell can gradually
+escape by means of the iron tube and lower tap, and the bell gradually
+sinks by its own weight until it is on the bottom, but still {13} no water
+can reach the carbide, the air imprisoned in the inner portion of the bell
+effectually excluding it.
+
+The lower tap should now be connected by means of india-rubber or flexible
+metallic tubing to the burner in the lantern (of which more anon), and the
+upper tap on the generator turned on, the tap or taps on the burner being
+likewise opened. The air from the inner portion of the bell can now escape
+by the pipe P P into the outer part, and thence through the iron tube, and
+out through tubing and jet, and as it does so water will rise in the
+interior and attack the carbide.
+
+In a few moments the burner can be lit; but the gas, being generated far in
+excess of requirements, and filling both the inner and outer portions of
+the bell faster than it can escape, lifts the latter until the carbide is
+entirely out of the water, when in a few minutes generation ceases.
+
+If the jet is left burning the bell will gradually sink again as the gas is
+used up, and should thereafter maintain an automatic balance without
+attention.
+
+It can be turned off at any moment by simply closing the taps at the jet
+or, better, the lower tap at the generator, when the bell rises
+sufficiently to take the carbide out of the water; but if it is required to
+leave the generator unlit for a considerable time, it is better to turn off
+the tap on the top first. This causes the inner portion of the bell to fill
+with gas which cannot escape, and as that in the outer part burns out, the
+bell sinks to the bottom and remains there, the gas itself imprisoned in
+the inner chamber excluding the water from the carbide. The exact
+arrangement varies in different patterns of generator, but the above may be
+taken as roughly indicating the action, and further information may always
+be obtained from the maker or dealer.
+
+_Emptying_ should always be done out of doors, as the odour of acetylene
+gas is most objectionable, and for the same reason rubber tubes, &c.,
+should be securely tied on, so that the slightest escape may be avoided.
+{14}
+
+If the exhibition has been a short one it will often be found that the
+upper cells have not been affected by the water, in which case they may be
+put back in the tin and used again, but it is not generally advisable to
+put in less than the full charge to begin with as the weight of the carbide
+plays a definite part in securing the smooth action of the apparatus. The
+sludge should be thrown away (it forms a good manure for the garden) and
+the entire generator thoroughly dried, otherwise rust will quickly appear.
+
+[Illustration: FIG. 6.--The A.L. or 'Popular' Model.]
+
+Theoretically one of these generators may be filled and left standing
+indefinitely, but in practice it is not advisable, as the damp in the
+atmosphere is apt to produce a very slow generation of gas, sufficient
+often to cause a decided smell.
+
+Of generators which act by admitting water to the carbide perhaps the best
+known is the A.L. or 'Popular' Model (Fig. 6), this being, in fact, a
+pattern designed for motor-car head-lights, but which answers well for
+lantern work.
+
+Its exact operation need hardly be described here in full detail. It will
+suffice to say that the water gains access to the carbide by 'creeping' up
+between two concentric copper cones, and in the event of over-generation
+the pressure of the gas automatically checks the flow.
+
+This generator is smaller than the gasometer pattern, and hence can be
+recommended for portability; but in my experience the light is not quite so
+steady, and the control rather less delicate, thereby causing on occasions
+a perceptible smell, especially if left standing for a considerable time.
+
+There are other types of generators, such as the 'Water {15} dropping'
+variety, in which the water drips on to the carbide, and the reverse, in
+which fine granulated carbide drops a little at a time into water; but
+these types are not very frequently met with and need hardly be described.
+
+It should never be forgotten that acetylene is an explosive gas and should
+be treated as such. Searching for a leak with a lighted match, though
+perhaps permissible when the operator knows his business, may be a
+dangerous proceeding when the contrary is the case.
+
+[Illustration: FIG. 7.--Acetylene Jet.]
+
+Acetylene burners are generally of the 'Batswing' type, and are as a rule
+four in number, mounted in a row with a reflector behind, each burner being
+separately controlled by its own tap (Fig. 7). An acetylene flame is very
+smoky, and care must be taken that the burners are not turned too high. A
+nipple cleaner, consisting of a fine wire in a short handle, can usually be
+obtained from any dealer, and is very handy.
+
+Acetylene gas can also be used for lantern illumination in quite another
+way, viz. by a blast from a blowpipe, in combination with either air or
+oxygen, on to a special 'Pastille' provided for the purpose, or an ordinary
+limelight jet can be used. These methods entail the use of acetylene _under
+pressure_, and are so analogous to limelight that I shall for convenience
+deal with them in the chapter devoted to that illuminant.
+
+       *       *       *       *       *
+
+
+{16}
+
+CHAPTER V
+
+LIMELIGHT AND THE ACETYLENE BLAST
+
+The illumination possible with this light is almost unlimited, and for
+really large halls it is, as remarked before, the _only_ substitute for the
+electric arc. It consists essentially of a blowpipe flame, composed of
+oxyhydrogen, oxyether, oxyspirit, oxy-acetylene, &c., or acetylene air
+blast, heating to incandescence a block of lime, or other refractory
+material, and the essential feature is that one at least of these gases
+must be under _pressure_. Thirty years ago this was usually achieved by
+storing the gas in rubber bags, and obtaining the requisite pressure by
+means of heavy weights; but except in a very few outlying districts this
+method has now been completely superseded by the use of compressed gas
+cylinders. The earlier editions of this work contained very full directions
+for manufacturing gas for storage in bags, but it is so exceptional now to
+find an operator who uses this method that it seems hardly necessary to
+devote much space to it, and the same may be said of automatic oxygen
+'generators.' The present work will therefore deal chiefly with compressed
+gas cylinders.
+
+Most elaborate precautions are now enforced by the Board of Trade to ensure
+the absolute safety of these, and any doubt existing from occasional
+accidents of years ago may be promptly dismissed. Humanly speaking, an
+accident nowadays _cannot_ happen, except by such wilful negligence on the
+part of the maker or filler as would almost render the culprit subject to
+criminal proceedings.
+
+Compressed gas cylinders are painted a distinctive colour, oxygen for
+example being black and coal gas or hydrogen red; the screw connections to
+the pumps, and all nozzle {17} and regulator fittings, are made with a
+totally different screw and therefore cannot be interchanged; the cylinders
+themselves are bound by law to be reannealed and retested under hydraulic
+pressure at regular intervals; the steel itself has to be of a guaranteed
+quality; and, in fact, every possible risk is guarded against.
+
+The most usual sizes of cylinders supplied for lantern exhibitions are
+those containing 6, 12, 20, or 40 cubic feet, and are usually sent out in
+wooden or hemp cases.
+
+[Illustration: FIG. 8.--Oxygen Cylinder in hemp cover.]
+
+Fig. 8 shows a 12-foot cylinder in its hemp case, the approximate size
+without case being 22 in. by 4 in. This size cylinder will supply an
+average limelight jet for just over two hours. The extra powerful jets as
+used for cinematograph work or for illuminating a very large screen take a
+good deal more, but for the usual apparatus as supplied for ordinary
+lantern purposes this is a pretty safe figure.
+
+A 12-foot cylinder is therefore the favourite size for a lantern exhibition
+lasting from an hour to one and a half hours, as it leaves a fair margin
+for gas used in adjusting the instrument, &c., and a 20-foot cylinder will
+usually suffice for _two_ such exhibitions.
+
+The price of gas per cubic foot varies with the size of the cylinder, being
+less for large cylinders than for small ones, and the cost of transit is
+also less in proportion--hence it is frequently an economy to hire a large
+cylinder and retain it for several exhibitions. On the other hand most
+suppliers charge a small rent if a cylinder is retained beyond a definite
+{18} time, so this is a question to be decided by each user on its own
+merits.
+
+Alternatively, of course, cylinders can be _purchased_, and the question of
+rent does not then come in; also gas is supplied a little cheaper in a
+customer's own cylinder than if sent on hire. If purchase is decided on it
+is frequently an economy to buy _two_, or two of each gas, if coal gas
+cylinders are required as well.
+
+[Illustration: FIG. 9.--Double Lever Key.]
+
+The whole contents of the cylinders can then be used up without waste, as
+if a cylinder should become exhausted during the course of a lecture, it is
+only a matter of a minute or two to change over to the spare one, whereas
+the compressors are required by law to empty out every cylinder returned to
+them for refilling, and any remaining gas is thereby wasted.
+
+It is extremely tantalising, to say the least of it, to find the pressure
+gauge indicating that there is, say, 8 feet of gas remaining in a cylinder,
+and to be compelled to waste this or else risk running short for the next
+exhibition, and duplicate cylinders are the only way of avoiding the loss.
+
+The cylinders are filled to a pressure of 120 atmospheres, or 1800 lb. per
+square inch, and are closed by strong screw nozzles. The keys for opening
+or closing these are of three types, viz. the 'T' pattern, 'Spanner'
+pattern, and that known as the 'Double Lever' type. This latter is so made
+that in closing the valve it shuts up to half its length and {19} opens out
+to double the leverage when being used to _open_ the cylinder (Fig. 9). The
+idea is to avoid the possibility, which has been known to occur, of the
+cylinder valve being screwed down by a powerful wrist and defying the
+efforts of the despairing lanternist to open it.
+
+[Illustration: FIG. 10.--Fine Adjustment Valve.]
+
+Cylinder nozzles are unfortunately not yet standardised, but those most
+frequently met with in this country are those adopted by the British Oxygen
+Company, both oxygen and coal gas cylinders being fitted with corresponding
+_internal_ screws 7/8 inch diameter, those for oxygen being _right-handed_,
+and those for coal gas _left-handed_, and in each case terminated at the
+bottom by a hollow metal cone.
+
+As such an internal screw cannot obviously be connected to a piece of
+rubber tubing, some type of screw connector must be employed, and this may
+take one of three forms: (1) A simple connecting nozzle, (2) a fine
+adjustment valve, or (3) a regulator. The first is seldom used in practice
+for lantern work, for the reason that the direct pressure of a full
+cylinder (120 atmospheres) cannot be checked or controlled by a tap on the
+jet, as the intervening rubber tubing would either burst or blow off, and
+must therefore be regulated at the cylinder nozzle itself, and gradually
+readjusted as the pressure diminishes.
+
+To achieve this regulation with the ordinary cylinder key is difficult,
+though possible to a careful operator, but for a slight extra expense a
+combined nozzle and _fine adjustment valve_ (Fig. 10) can be obtained, and
+regulation with this is {20} infinitely easier. The best plan of all,
+however, is to use an automatic regulator, which not only reduces the
+pressure so as to permit of the required adjustments being made at the
+jet-taps, but also maintains a practically steady supply as the cylinder
+empties, thereby obviating continual readjustments. Regulators are now so
+inexpensive that they have come into almost universal use, and are
+generally reckoned an indispensable part of a limelight lantern equipment.
+The form of regulator in most common use is that usually known as
+'Beard's,' having been originally designed and patented by Messrs. R. Beard
+& Sons, though as the patent has now expired it is open to any firm to make
+the same article if they desire.
+
+[Illustration: FIG. 11.--Construction of Beard's Regulator.]
+
+The construction of Beard's Regulator is shown in Fig. 11. The gas enters
+from below into a rubber bag, C, from which it can emerge through the
+nozzle.
+
+Any accumulation of gas raises the bellows against the pressure of a spiral
+spring pressing it down, and this brings into action an arrangement of
+so-called 'Lazy Levers,' which in turn presses down a small conical valve
+and closes the supply from the cylinder, this valve re-opening immediately
+the pressure diminishes.
+
+The outward form of this regulator is shown in Fig. 12, {21} which
+incidentally also illustrates the usual form of connection to the cylinder,
+referred to later on.
+
+In Beard's Regulator the pressure at which the gas can be delivered is
+determined by the strength of the spiral spring, and can only be altered by
+changing this spring.
+
+[Illustration: FIG. 12.--Beard's Regulator.]
+
+In practice Beard's Regulators are supplied set to a low pressure for
+ordinary mixed or 'blow-through' jets and for a higher pressure (14-16
+inches) for 'injector' jets. At this latter pressure the rubber tubing used
+must be fairly thick and strong and well tied on, and even so the taps of
+the jet should not be turned entirely off unless the gas at the cylinder is
+likewise turned off immediately afterwards. The British Oxygen Company make
+a regulator which can be set to any desired pressure, but it is not quite
+so delicate in its action as Beard's, and Messrs. Clarkson also make a
+pattern regulator which is widely used and well spoken of. The attachment
+of any of these fittings to the cylinder is a somewhat peculiar one, as
+will be seen on reference to Fig. 10 or Fig. 12. The regulator or nozzle
+ends at its lower extremity in a screw and cone, the latter being intended
+to make a gas-tight connection with the internal cone on the cylinder, and
+over this screws a loose wing piece with another outer screw, this latter
+fitting the thread in the cylinder.
+
+In making the connection care must be taken that the wing piece is not
+screwed too far down the inner screw, or the cone will not reach down and
+make a tight fit on its {22} seating; in its correct position the wing
+piece when clamped down should leave a turn or two of its thread exposed,
+in order to ensure that the cone does bed properly.
+
+[Illustration: FIG. 13.--Regulator and Gauge.]
+
+Care should be taken that the nozzle of the cylinder is free from dust
+before attaching any of these fittings: the best plan is first to blow into
+it, and finally wipe it round with the finger. Most professional operators
+_hammer_ the wing piece home with a spanner or other convenient implement a
+barbarous method and really unnecessary if the cones are in good condition,
+but, nevertheless, almost always adopted in practice.
+
+PRESSURE GAUGES.--These are useful in determining the amount of gas
+remaining in a cylinder and are of a very usual type; they may either be
+screwed on to the cylinder before commencing to work and taken off again to
+screw on the regulator, or they can be supplied fitted to the regulator
+itself, in which case they can be observed during the course of the
+exhibition (Fig. 13). As the same gauge may be used for cylinders of
+different sizes (though _never_ for those containing {23} different gases),
+they simply register in atmospheres, and knowing that a full cylinder shows
+a pressure of 120 atmospheres, the requisite calculation must be made to
+determine how many cubic feet are unused.
+
+In the case of oxygen cylinders an approximate idea of the amount of gas
+remaining can be got by _weighing_ it carefully when known to be either
+absolutely full or absolutely empty, and re-weighing it when information is
+required. Oxygen weighs approximately 1.4 oz. per cubic foot, and this is
+easily detected by an average scale. Coal gas is too light to be gauged in
+this way.
+
+GAS-BAGS AND GENERATORS.--It has already been remarked that there are two
+alternative methods of obtaining gas under pressure for limelight purposes,
+viz. gas-bags and generators (the latter for oxygen alone: there is no good
+hydrogen generator that I know of). In both these cases the oxygen is
+generated by heating a mixture of chlorate of potash and manganese black
+oxide. In the case of gas-bags the gas is prepared beforehand and passed
+through suitable purifiers into a rubber gas-bag. With a generator the
+oxygen is evolved during the exhibition itself; but this method has never
+come into very general use.
+
+Coal gas or hydrogen is very seldom home generated; a gas-bag can, if
+necessary, be filled a few miles away and brought full to the place of
+exhibition, or filled on the spot if gas is laid on; or, failing this,
+acetylene or ether, or even methylated spirit may be utilised instead.
+
+The bags in use are placed between double pressure boards (if _both_ gases
+are required under pressure) and weights sufficiently heavy placed on the
+top (Fig. 14), or with a 'blow-through' jet only the oxygen need be stored
+in a bag and the coal gas used from the supply main.
+
+Cylinders have, however, so universally superseded these appliances, that
+space is hardly warranted in fully describing them, especially as any
+operator wishing to adopt {24} the process can obtain full directions from
+any responsible dealer.
+
+LIMELIGHT JETS.--These are of three general types, viz. the 'Blow-through,'
+the 'Mixed,' and the 'Injector.'
+
+Of these the 'Blow-through' is now very little made, having been largely
+superseded by the 'Injector' pattern, but, as there are hundreds in common
+use in this country, they cannot yet be regarded as a thing of the past.
+
+[Illustration: FIG. 14.--Gas-bags.]
+
+The exact design of this jet varies considerably, but all are alike in
+this, that a jet of coal gas is burned at the orifice of a more or less
+open nozzle, and a stream of oxygen _blown_ _through_ it on to a cylinder
+of lime which it thereby renders incandescent. Fig. 15 represents the
+various designs chiefly adopted for this jet, that marked A being perhaps
+the most usual, though C is also frequently met with.
+
+In light-giving power there is not much to choose between the various
+types; probably D on the whole is the best in this respect, but so much
+depends upon the exact position of the two nozzles, and the _smoothness_ or
+otherwise of that {25} provided for the oxygen blast, that exact
+comparisons are difficult.
+
+[Illustration: FIG. 15.--'Blow-through' Nozzles.]
+
+'Blow-through' jets are the weakest form of limelight as used at the
+present day, and may be taken roughly as some 50 per cent. better than
+acetylene, or in other words, sufficient to illuminate a 12-foot picture at
+a distance of some 40 to 50 feet; but their advantage is, or was, that they
+only required one gas (oxygen) under pressure, the coal gas supply being
+obtained from the ordinary house main.
+
+[Illustration: FIG. 16.--'Blow-through' Jet.]
+
+This advantage is now shared by the more recently introduced 'Injector'
+jets, which give a far better light, and have therefore rendered the
+'Blow-through' type nearly extinct.
+
+The general construction of a 'Blow-through' jet is shown in Fig. 16, and
+it will be seen that a short vertical spindle is {26} provided to carry the
+lime cylinder, and that this can be rotated from the back by means of
+bevelled gear wheels, which at the same time screw the spindle up and down.
+A lime cylinder of the usual pattern being placed on this spindle can be
+rotated from time to time to expose a fresh surface, as that in use
+gradually becomes 'pitted' by the blast, while the screw provides
+sufficient vertical movement to ensure that a complete rotation does not
+bring round the same position again.
+
+Some arrangement is also generally provided by which the distance between
+the lime spindle and the jet can be adjusted. The exact position of this
+does not matter within a reasonable margin, but limes vary in size, and
+'Pastilles,' and other substitutes for limes, which will be referred to
+later, vary still more, at any rate as regards this adjustment. The average
+distance which gives the best result is usually about half an inch, and
+once set need not be altered with that particular jet unless a lime of
+different size is employed; minor variations due to limes being drilled
+slightly out of centre, &c., do not seriously matter.
+
+There is no accepted rule for colouring jet-taps in accordance with the
+cylinders, and although jets are sometimes met with painted in this way,
+_i.e._ red for coal gas and black for oxygen, it is more usual to find coal
+gas taps _black_ and oxygen _bright_, or sometimes both black or both
+bright. Care must therefore be taken that the right cylinder is connected
+to the right tap on the jet, but there should be no difficulty in telling
+which is which, and fortunately any mistake, even if it be made, is quite
+harmless.
+
+THE MIXED-GAS OR DOUBLE-PRESSURE JET.--This jet is fundamentally different
+from the 'blow-through' form, inasmuch as the two gases are combined in one
+mixing chamber before combustion, and burn in their correct proportions at
+one nipple.
+
+It is usually stated that this jet necessitates both gases being under
+equal or approximately equal pressure, but this {27} is not literally
+accurate, and I have given many a lantern exhibition with one of these
+jets, using coal gas from the ordinary supply, and oxygen from a cylinder.
+To use a mixed jet in this way needs care, as a very slight excess of
+oxygen puts the light out with a 'pop' which, although not dangerous, is
+disconcerting, while the light obtained under these conditions is very
+little better than with a 'blow-through' jet, and far inferior to the
+'Injector' jets to be described next.
+
+[Illustration: FIG. 17.--Mixed Jet.]
+
+[Illustration: FIG. 18.--Mixed Jet, Gwyer pattern.]
+
+The mixed-gas jet is intended then to be used with both gases under
+pressure, and is the _only_ jet to be seriously {28} considered in cases
+where a really powerful light is required. The power of this jet is indeed
+almost unlimited, and those made with large bores, such for example as used
+for cinematograph work, provide a light amounting often to some two or
+three thousand candles, and consume an enormous amount of gas; but the
+ordinary pattern, with a nipple of one-twentieth to one-sixteenth of an
+inch bore, and using some 5 feet of each gas per hour, or perhaps slightly
+more for the coal gas, will suffice for all ordinary work.
+
+[Illustration: FIG. 19.--Mixing Chamber of Jet.]
+
+The mixed-gas jet, like the 'blow-through,' is made in many forms, but
+these may be roughly divided into two main types, viz. those with small
+mixing chambers immediately below the nipple (Fig. 17), and those with
+larger chambers in the horizontal part of the jet as in the 'Gwyer' pattern
+(Fig. 18).
+
+The construction of the mixing chamber itself varies also, but that
+advocated by my father, the original author of this work, is generally
+followed, the chamber being packed with alternate discs pierced as in Fig.
+19, which ensures a thorough mixture of the gases. A layer or two of gauze
+is often introduced as well by way of further improvement. The distance
+between the lime and nipple is much less than with the 'blow-through' jet,
+and the adjustment has to be more exactly made. About 1/8 inch is
+approximately correct for a jet of moderate power, and rather more for a
+bigger bore; also care must be taken to turn the lime frequently, as the
+latter 'pits' pretty quickly with these jets, and if it is neglected the
+jet may spurt back out of the hole, which is gradually formed, and crack
+the condenser. {29}
+
+There is still an erroneous opinion extant that these jets are dangerous,
+and if the operator is working with the now obsolete gas-bags it is
+certainly a fact that an accident in careless hands is _possible_; but with
+cylinders there is, so far as I know, no possibility even of an accident
+under ordinary conditions.
+
+It is true that if too much oxygen is turned on the jet may suddenly go out
+with a loud snap or pop, and this is in reality a miniature explosion in
+the mixing chamber; but it can in any case hardly be serious enough to
+matter, though I have found after such a snap that the gauze packing,
+inside the chamber above referred to, has been pierced right through, and,
+when first lit afterwards, the jet has for a few minutes burnt with a
+characteristic green flame, denoting the presence in the gas of fine copper
+or brass particles.
+
+To obtain a good light with these jets, and in fact with _all_ jets, great
+care must be taken that the nipple is absolutely smooth, otherwise the
+flame is bound to hiss. The simplest plan is to slightly roughen a suitable
+sized needle with emery paper and to burnish the inside of the nipple from
+time to time with this. Especially if there has been one of the 'snaps'
+referred to is it desirable to see that the inside of the nipple is
+thoroughly smooth and polished.
+
+MANIPULATION OF THE MIXED-GAS JET.--On this point there is not much to be
+said. A good hard stone lime must be used--'soft' limes are useless for
+this jet--and the coal gas flame should be lit first, and the lime
+thoroughly heated with this before the oxygen is slowly turned on. As the
+oxygen increases the flame will gradually disappear and the light increase,
+until it is at a maximum for that particular amount of coal gas. This
+latter can then be turned on a little more, and more oxygen passed to
+balance it until the jet begins to 'roar,' when we are getting the maximum
+light for that particular sized nipple. When the two gases are, however, in
+the proper proportion to give the best light, there will always be a slight
+excess of coal gas flame visible playing about the lime. {30}
+
+THE INJECTOR JET.--This is essentially a mixed jet, and in outward
+appearance differs but little from one of the ordinary type (Fig. 20), but
+is so constructed that the pressure of oxygen 'sucks' coal gas into the
+mixing chamber, and so obviates all necessity for the latter being under
+pressure.
+
+With this jet there is little or no danger of the jet 'snapping' out
+through a surplus of oxygen, as the greater the flow of this gas, the
+greater the suction on the coal gas side.
+
+The light is not quite equal to a good mixed jet, but very nearly so, and
+therefore this jet is deservedly gaining in favour every day.
+
+[Illustration: FIG. 20.--'Injector' Jet.]
+
+One point must be noted: the oxygen itself must be under greater pressure
+than with the ordinary mixed jet if the best light is to be obtained, and
+therefore a special regulator must be used, or one of ordinary type
+modified (which can easily be done by the maker), and rubber connections
+must be securely tied both on to jet and regulator, as the pressure
+required to work this jet to advantage, while not enough to burst a rubber
+tube, is enough to blow it off an easy fitting connection.
+
+THE OXYETHER LIGHT.--This is practically similar to the oxyhydrogen, except
+that ether vapour is used in place of the hydrogen or coal gas. The method
+adopted consists essentially of passing a stream of oxygen through a vessel
+packed with some porous material (such as cotton wool or cotton gauze)
+which is saturated with ether. The oxygen {31} becomes saturated with ether
+vapour, and the mixture is then used in place of the coal gas supply in a
+double-pressure jet, an additional supply of free oxygen being still
+required through the ordinary oxygen tap.
+
+The arrangement is cheap, as it dispenses with the necessity for a coal gas
+cylinder, and effective, as the light is little, if at all inferior to the
+oxyhydrogen, but differs from the latter in this, that with careless
+handling an accident is _possible_.
+
+In competent hands there is no danger, and I have used ether saturators
+myself scores of times without one single contretemps; but it should _not_
+be entrusted to any chance amateur.
+
+The use of the ether light has a curious history. In the earlier days
+before the proper construction of ether saturators was understood, and
+gas-bags were still in vogue, it was largely condemned on the score of
+danger. Modern improvements in apparatus rendered it perfectly safe against
+anything but gross carelessness or bungling, and the London County Council
+and other similar bodies immediately supplied it broadcast to elementary
+schools (in disregard of warnings offered by myself and others), where it
+was often entrusted to incompetent operators or even senior boys. So far as
+I know no serious accident ever resulted, a pretty conclusive proof that
+the light is really safe, but in time the London County Council realised
+that the universal adoption of this illuminant was not advisable, and I
+believe _now_ prohibit it altogether in halls licensed by them for
+entertainments.
+
+In time, no doubt, they will learn to adopt a sane policy between the two
+extremes, but at present the official attitude in many localities has
+placed ether saturators out of the running, and before purchasing one the
+would-be operator should ascertain that he will be allowed to use it.
+
+Ether saturators as made at the present day may be divided into two
+principal patterns, viz. those in which saturator and jet are combined in
+one piece of apparatus {32} which fits bodily into the lantern, and
+saturators which are used outside and connected by means of tubing to any
+ordinary oxyhydrogen double-pressure jet.
+
+Both forms have their advantages and disadvantages; the first pattern tends
+to become too warm from its position in the lantern and generates ether
+vapour too quickly, while the second has the fault of becoming too cold
+(owing to evaporation of the ether) and therefore not vaporising quickly
+_enough_.
+
+[Illustration: FIG. 21.--'Gridiron' Saturator.]
+
+Writing at the present date, when manufacturers are slowly beginning to
+resume their normal occupations after the stress of war work, it is
+impossible to say exactly what models will or will not be made, but I will
+mention one typical example of each pattern as made in pre-war days.
+
+The first of these is the 'Gridiron' (Fig. 21), adopted largely by the
+London County Council in the days I have referred to, and certainly one of
+the best designed saturators ever put on the market.
+
+In the 'Gridiron' saturator there are three taps: two at the {33} rear and
+one in front, between the saturator and the mixing chamber. Between the
+rear taps is the inlet for the oxygen, which divides into two channels,
+that on the left passing upwards through the U tube shown in the
+illustration (the corresponding tube on the right is merely a dummy), and
+thence through the saturator and out through the horizontal tube and tap
+into the mixing chamber, whence the saturated stream of oxygen finally
+passes to the nipple, and the combination burns with a whitish flame
+closely resembling that produced by coal gas.
+
+[Illustration: FIG. 22.--'Pendant' Saturator.]
+
+The other channel for the oxygen is to the right, down the vertical tube
+shown there (the lower vertical tube on the left is also a dummy),
+underneath the saturator, and finally coming up into the mixing chamber
+from below, transforming the white flame into an intensely hot blowpipe
+exactly as it does with a coal gas jet. The front tap controls the supply
+of saturated ether to the mixing chamber, and whereas at first a good
+stream of oxygen is needed to pick up enough ether, by degrees as the
+instrument warms in the lantern, the oxygen passing through the saturator
+can be cut off entirely, and even then the front tap must be gradually
+closed down to prevent the hot ether coming off too fast.
+
+There is a disagreeable feeling of 'sitting on the safety-valve' in doing
+this, but in reality the pressure is never likely to become great enough to
+cause danger.
+
+Of saturators for use outside the lantern the best-known is probably the
+'Pendant' (Fig. 22). With this instrument the oxygen supply is connected to
+the inlet marked A; B goes {34} direct to the oxygen tap of any ordinary
+mixed-gas jet; while C, from whence issues the saturated stream, is
+connected to the coal gas tap of the jet. Whichever pattern is used, the
+essential thing is to keep a good supply of oxygen well saturated. If the
+lime becomes incandescent without any free oxygen, or it is found that this
+requires gradually turning off, it indicates that the saturation is
+becoming defective, and to continue is to risk the jet snapping out. In the
+case of an outside saturator such as the 'Pendant,' this may even blow off
+the connecting tubes with a loud report, though no worse accident is likely
+to happen, and for this reason an outside saturator should be placed _as
+close_ to the jet as possible, so that the rubber tube may be kept short,
+and incidentally this keeps the saturator warm and accelerates
+vaporisation.
+
+As ether vapour usually contains a certain amount of moisture which does
+not vaporise to any great extent, this gradually accumulates and the
+capacity of the instrument becomes reduced. It is therefore usually
+necessary to return a saturator to the makers every now and again for
+repacking.
+
+The only real danger with a modern saturator is not in using but in
+_filling_. This should be done if possible in the open air, and at any rate
+never near a light. Ordinary sulphuric ether of specific gravity 720-730 is
+usually considered the best, and a quarter of a pint will keep an ordinary
+small-bore jet going for nearly two hours.
+
+More precise directions are usually sent out by the makers, and as the
+various patterns of saturator in use are pretty numerous, it would be
+useless here to attempt more detailed instructions for working.
+
+OXY-ACETYLENE JETS.--Any good mixed gas jet may be used with acetylene
+instead of coal gas, provided that it is under pressure more or less
+corresponding to that from an oxygen cylinder, and at the present day there
+is no difficulty in obtaining this, in civilised countries at all events,
+by {35} means of compressed or, to speak more correctly, 'dissolved'
+acetylene cylinders, referred to later on.
+
+With an 'Injector' jet there is no need for the acetylene gas to be under
+pressure at all, and a simple generator such as described on page 12 will
+answer perfectly, though in practice very seldom used. With such a
+generator the pressure is so low that in many cases the jet will not even
+burn until _some_ oxygen is turned on; but this introduces no real
+difficulty, as with a good 'Injector' a snap is practically impossible,
+provided the generator is large enough to evolve sufficient acetylene. It
+is far better in every way, however, to use the acetylene from a cylinder,
+just as with coal gas. Only in this case the cylinder is completely filled
+with a porous material, and this again filled with liquid acetone or other
+suitable fluid, in which the acetylene is dissolved as rapidly as it is
+pumped into the cylinder.
+
+To compress acetylene in the ordinary way is neither safe nor practicable;
+but these 'dissolved' cylinders are now used extensively for both
+oxy-acetylene welding and motor car lighting, and may be entirely relied
+upon.
+
+The D.A. (Dissolved Acetylene) Company were the pioneers in this country of
+the industry, and their methods of business are peculiar and ingenious. The
+user is requested in the first place to purchase a cylinder, and he then
+becomes the owner of _a_ cylinder, but not of one _particular_ cylinder. A
+list is supplied to him of various depots in the country where the
+Company's cylinders are stored, and when empty he can, on payment of a
+fixed sum, exchange his empty cylinder for a full one, which then becomes
+_his_ cylinder _pro tem_.
+
+This saves the delay and expense of returning a cylinder to London, and
+incidentally clears the customer of any question of deterioration, this
+being obviously covered by degrees with each individual exchange. The
+system was first introduced in connection with the lighting of cars and
+only applies to the standard size for this purpose, viz. 20 cubic feet
+capacity, {36} but as this is, on the whole, the most convenient size for
+lantern work also, the limitation is not a disadvantage. The arrangement is
+also in vogue to a less extent with cylinders of 6 feet capacity (a size
+sometimes used for motor _cycles_), but the depots of exchange are at
+present far fewer for this size.
+
+The oxy-acetylene blast is much _hotter_ than the ordinary oxyhydrogen, and
+therefore produces a more intense light. I have therefore used it with
+success on occasions when even the ordinary limelight would fail, and the
+choice has lain between an oxyhydrogen jet of enormous bore (and, of
+course, corresponding consumption of gas), and the oxy-acetylene.
+
+For this very reason great care must be taken only to use the hardest
+limes, and even then to use the lime-turning movement frequently, or the
+lime will pit or crack and a broken condenser follow.
+
+THE FALLOT ACETYLENE LIGHT.--This light consists of a jet of acetylene
+under pressure, without oxygen, but producing its own _air blast_ from the
+atmosphere by suction, much as the 'Injector' jet does, but the reverse way
+round.
+
+The light is better than with an ordinary acetylene jet, though not quite
+so good as with a 'blow-through' jet; but as it only requires a cylinder of
+dissolved acetylene, or even a 'Pressure' generator, it is fast coming into
+favour.
+
+The peculiarity of the Fallot apparatus is that, instead of providing a
+direct beam of light in the direction of the screen, it projects the beam
+_backwards_ on to a concave mirror, and it is the reflected light from this
+that is used (Fig. 23).
+
+Instead of a lime is used a spherical 'Pastille' of peculiar composition,
+and before use each pastille must be burnt off exactly like an incandescent
+gas mantle, after which it is extremely fragile and difficult to handle.
+
+To use this illuminant one lens of the condenser must be removed, the
+curvature of the mirror taking its place, and it will be seen at once that
+the pastille itself will get in its own {37} light and throw a shadow,
+which actually happens, but it is hardly perceptible unless specially
+looked for.
+
+[Illustration: FIG. 23.--Fallot Air Blast.]
+
+A complete Fallot Air Blast Outfit, with cylinder, fine adjustment valve,
+pressure gauge and burner, with two spare pastilles, is shown in Fig. 24,
+but if preferred a regulator, such as previously described for oxygen, can
+be used instead of the fine adjustment valve.
+
+[Illustration: FIG. 24.--Fallot Air Blast, and Cylinder.]
+
+{38}
+
+FALLOT OXY-ACETYLENE BLAST.--This is similar to the foregoing, utilising
+oxygen from a cylinder instead of air, and the light is equal to a powerful
+limelight, and may be considered as an efficient substitute, though for
+_long range_ work the shadow before alluded to becomes more noticeable (for
+optical reasons which need not be here discussed). The Fallot Company also
+make a special 'Pressure Generator' which can be used instead of a D.A.
+Cylinder; but my experience of this so far is that, although perfectly
+safe, the blast from it is a little unsteady as compared with a cylinder.
+
+LIMES AND ACCESSORIES.--Limes for Optical Lantern work are usually supplied
+in the form of cylinders, the 'ordinary' size being 7/8 inch in diameter
+and about 1-1/2 inches long, with a hole drilled longitudinally to take the
+lime pin. Extra large limes up to 2 inches in diameter are supplied for
+more powerful jets.
+
+So-called 'soft' limes used to be recommended for 'blow-through' jets as
+giving a better light than 'hard' limes, but the advantage, if any, is very
+little, and these limes are now very seldom heard of, possibly because
+'blow-through' jets themselves are becoming less and less used, and 'soft'
+limes will not stand the heat of a mixed or 'Injector' jet for long.
+
+'Hard' limes are turned out of the hardest stone lime, and must be kept in
+sealed tins until used, as they rapidly disintegrate when exposed to the
+air. There are one or two quarries known to provide the best lime for
+lantern purposes, and the various good brands on the market practically
+have the same origin as regards raw material, though called by different
+trade names; and the 'Hardazion' (hard as iron) limes, placed on the market
+some years ago by a well-known wholesale firm, to be countered shortly
+after by the 'Hardastil' (harder still) brand, are, I take it, legitimate
+though amusing instances of phonetic advertisement.
+
+Even the best of limes is liable to crack under the heat {39} of a powerful
+jet, and so a pair of lime-tongs should always be provided, and there is
+nothing better than the simple form shown in Fig. 25, and which is, or
+should be, sold by all dealers.
+
+[Illustration: FIG. 25.--Lime-tongs.]
+
+SUBSTITUTES FOR LIMES.--A good substitute for lime, that will give the same
+light, stand heat equally as well, and _not_ deteriorate if exposed to the
+atmosphere, has long been sought for, and some of the more recently
+discovered refractory materials are more or less satisfactory. 'Mabor'
+limes, for example, belong to this class, and so do some of the
+'pastilles,' which before the war came chiefly from France and to a less
+extent from Germany.
+
+       *       *       *       *       *
+
+
+CHAPTER VI
+
+THE ELECTRIC LIGHT
+
+The electric current provides _the_ light for an optical lantern, though it
+may take various forms, such as the incandescent glow-lamp in some shape or
+other, the comparatively new Ediswan 'Pointolite' lamp, the enclosed arc,
+and the open arc. This little book is not a treatise on electricity, but a
+few elementary notes may not be out of place, and may be of assistance to
+the non-technical lanternist.
+
+The first point then to be considered in adopting the electric light for
+the purpose of lantern projection is the character of the supply, and the
+information required may be summed up thus: (1) _E.M.F._, _voltage_, or
+_tension_ (these three expressions having exactly the same meaning); (2)
+_amperage_ or amount of current available; (3) whether current is (_a_)
+_continuous_, _constant_, or _direct_ (again three words meaning {40} the
+same thing), or (_b_) _alternating_. The E.M.F. or tension corresponds to
+_pressure_, to use the mechanical analogy of a water pipe, and the
+_amperage_ to volume, and the voltage of the supply currents in this
+country are usually between 100 and 250 volts. Private lighting sets are
+frequently as low as 50, and current derived from accumulators may be
+anything from a few volts and upwards. _Power_ currents, such as commonly
+employed for tramways, &c., are usually about 500 volts, but the use of
+these currents for lighting purposes, though practicable, is not to be
+advocated.
+
+Amperes and volts are convertible terms in a sense; that is to say, a
+current of 10 amperes at 100 volts requires the same horse-power to
+generate it as one of 5 amperes at 200 volts, or 20 amperes at 50 volts,
+but they are by no means convertible as regards their _efficient_ use for
+our purpose. The amperes used multiplied by the number of volts give the
+total power consumed in _watts_, and 1000 watts used for one hour represent
+1 _unit_ as charged for on our dreaded lighting bills. The current
+available from a public supply may be said to be unlimited so far as our
+purpose is concerned, and the amount actually used depends only on the
+total electrical resistance of our circuit, and this is measured in _ohms_,
+the three factors, viz. volts, amperes, and resistance, being connected by
+the well-known and simple equation C = E / R, C representing the current in
+amperes, E the tension or E.M.F. (electro-motive force) in volts, and R the
+resistance in ohms. The total current we _can_ use, however, is limited by
+the size of the cable laid on in the building, and this is automatically
+safeguarded (or should be) by the _fuses_, which consist, as is generally
+known, of thin wires or strips of tin or lead fixed on a fuse board in an
+easily accessible place, and which melt directly the current exceeds a safe
+amount in amperes. Whatever method of lighting we use therefore, _enough_
+resistance must always be kept in the circuit to ensure {41} that no more
+current can pass than has been provided for, and in the case of an arc lamp
+this usually means a _resistance_ or rheostat being retained in the circuit
+in addition to the arc itself, through which the current is passed and
+absolutely wasted, though fortunately the waste in money is negligible, and
+for reasons to be discussed later such a resistance is necessary with an
+optical lantern arc lamp in any case.
+
+In the case of a glow-lamp, the entire resistance is provided by the
+filament of the lamp itself, and that is why an ordinary metal or carbon
+filament lamp, for say 200 volts, has to be manufactured with an extremely
+long and slender, and therefore fragile, filament, while with an ordinary
+pocket-torch, which is usually supplied with current from a dry battery of
+some 3 or 4 volts only, the filament can be short and thick.
+
+Speaking generally, glow-lamps on a low voltage current can be made more
+efficient than on a high one, and are also longer lived for very obvious
+reasons; but, on the other hand, the transmission of current over long
+distances is cheaper the higher the tension, as for a given number of watts
+the amperage is less, and therefore smaller cables can be employed. On the
+whole, then, currents of 200 to 250 volts have during recent years become
+more common than 100, in spite of greater difficulties in making the lamps;
+but occasionally one finds a hall where two or more lamps are wired in
+_series_, two 100-volt lamps for example being wired together in series on
+a 200-volt circuit. If we are using current for our lantern from an
+ordinary lamp socket, this is a possibility that must be borne in mind.
+
+The same considerations, viz. the economy of transmitting power at high
+tension and of _using_ it at a lower one, have been mainly responsible for
+the rapidly increasing number of alternating current circuits now met with,
+especially in sparsely populated districts. An alternating current main is
+one in which the current reverses its direction, usually in this country
+50, but sometimes 60, 80, 90, or even 100 times {42} per second (there
+being unfortunately in Great Britain no standard 'Periodicity' or number of
+cycles per second), and for technical reasons which need not be entered
+upon here, with these alternating currents the tension and amperage can be
+mutually converted by means of _transformers_, so that current can be
+transmitted at so high a tension, for instance, as 10,000 volts, and used
+at a voltage of 50 or 100 or whatever is required, the amperage available
+being increased in inverse ratio as the tension is decreased. The same
+ready power of transformation unfortunately does not apply to the
+continuous current, or alternating currents would probably never have been
+heard of, but as it is they are very common. For glow-lamps it is
+immaterial which current is available, but for arc lamps the continuous is
+much to be preferred, though both can be utilised.
+
+With these initial remarks, I will now take in order of illumination the
+various methods of utilising the electric current for optical lantern work.
+
+THE ELECTRIC GLOW-LAMP.--The ordinary metal filament lamp is not very
+suitable for lantern work, the light not being sufficiently concentrated,
+but from what has already been said, it will be evident that this method of
+lighting is more suitable where currents of low voltage are available.
+
+An extremely good and intense light can be obtained from a comparatively
+small battery of accumulators, which can easily be carried in the hand, and
+a short and thick metal filament lamp, similar to those supplied with a
+powerful electric torch; and this arrangement is actually used to some
+extent by travelling lecturers, but the mess and trouble of keeping the
+accumulators in order have prevented the method being generally adopted.
+
+When _alternating_ current is available such a lamp will work well with a
+transformer to step down the voltage to the required degree, and the
+arrangement is simple, cheap, and efficient, and produces a light at least
+equal to that from {43} acetylene. In comparatively small halls, where the
+current is alternating, this is undoubtedly the best method of working, as
+it is simpler than the arc and amply brilliant enough for all practical
+purposes.
+
+With the continuous current the problem is not so simple, as transformation
+of voltage is not an easy matter, and a glow-lamp on; say, a 200-volt
+circuit involves a long and fragile filament, which it is difficult to
+arrange in a small space.
+
+Many years ago the Ediswan Company produced a 'Focus' lamp for the purpose,
+with the filament arranged in the form of a square grid, and this lamp gave
+a light of about 100 candles, and was fairly successful for a small room.
+More recently the Osram Company introduced a similar lamp with a metal
+filament arranged somewhat in the form of a cone, and this lamp also
+sufficed for a small class-room. It was, I believe, made in Germany and was
+practically unobtainable during the war. I understand the Osram Company are
+at present arranging to manufacture it in this country, but up to the time
+of writing it has not made its appearance.
+
+None of these lamps worked direct on a public lighting circuit can be
+regarded as really satisfactory, as it has been found impossible so far to
+get a _concentrated_ light; the 100-volt lamps have, of course, been
+superior to those made for 200 or 250, but they are all for lantern
+purposes far behind low voltage lamps, which are really good when a
+suitable current can be obtained.
+
+THE POINTOLITE LAMP.--This lamp produced by the Ediswan Company is in
+reality a miniature arc with tungsten electrodes in a highly exhausted
+vacuum bulb. To attempt a technical description would be beyond the scope
+of this book; it will suffice to say that the action depends upon the same
+principle as the various wireless vacuum valves or the Coolidge X-ray tube.
+
+This lamp requires a peculiar starting device which is supplied with it,
+and gives a good, intense, and concentrated {44} light, not equal to the
+ordinary arc or to limelight, but comparing well with any other form of
+illuminant. It can only be used with the continuous current.
+
+THE NERNST LAMP.--This lamp at the present moment is practically
+non-existent in this country, having been made exclusively in Germany. Also
+as recent improvements in metal filament lamps have rendered it almost
+obsolete for ordinary lighting purposes, it is, I think, very doubtful
+whether it is still manufactured even in that country, and hence I do not
+propose to waste space in an extensive description.
+
+It will suffice to say that the lamp consists of one or more straight rods
+or filaments of a refractory material, which are semi-conducting to the
+electric current when hot, but non-conducting when cold. To commence with
+the filament must be heated, and in the lamps as supplied for lantern work
+this is usually done by means of a spirit lamp, which can be removed
+immediately the current begins to pass, as the filament is thereafter
+maintained at a white heat automatically.
+
+A three-filament Nernst lamp gives as much as 1000 candles, but it is
+extremely hot, and the light rather diffuse. The filaments are also very
+fragile, so on the whole the lamp was never very much in favour; but on the
+other hand it consumed very little current, and could be worked from any
+ordinary house lighting main, points which led to its adoption in certain
+cases.
+
+THE ELECTRIC ARC.--We now come to _the_ light for optical lantern work, the
+brightest, the most concentrated, the cheapest, the easiest to work, in
+fact, the illuminant which combines all the virtues and but few drawbacks,
+but of course requires one indispensable condition, viz. electric current
+laid on. This current may be of any voltage from 70-250, or even higher; it
+may be continuous or alternating, though the former is to be preferred; and
+it requires a cable for _at least_ 5 amperes, and for a large hall 10 or 12
+amperes.
+
+The simplest form of arc lamp for lantern purposes is the {45} hand-fed
+type as illustrated in Fig. 26. The essential feature is the pair of carbon
+rods, the remainder of the apparatus consisting of mechanical adjustments
+to 'feed' these as they burn away, and to accurately maintain them in their
+proper positions and in the optical centre of the lantern. Just because the
+electric arc provides so small and concentrated a light, it is of extreme
+importance that the centring should be exact; and hence mechanical
+movements are usually provided for this which are unnecessary with other
+illuminants.
+
+[Illustration: FIG. 26.--Hand-fed Arc Lamp.]
+
+The whole question of optical adjustments has, however, been left over for
+a future chapter, as it more or less applies to whatever illuminant is
+used.
+
+The illustration shows a lamp arranged for continuous current, the upper
+carbon, which must be connected to the _positive_ wire, being larger than
+the lower (the negative), and very slightly behind it. The light from a
+continuous current arc lamp comes chiefly from this upper or positive
+carbon, {46} which 'craters' as it is used, and this arrangement has the
+effect of radiating the light in the direction required (Fig. 27).
+
+The positive carbon is usually of the 'cored' type, that is provided with a
+core of softer carbon, as this assists the 'cratering' action, while the
+negative is generally used 'solid,' that is homogeneous right through.
+
+The arc has to be 'struck' in the first place by touching the carbons
+together for a moment by the mechanical means provided, and then separating
+them to the working distance, which is approximately 1/8 inch. They must
+then be maintained at that distance by 'feeding' as they slowly burn away,
+and this 'feeding' in arc lamps for lantern work is usually done by hand,
+as in the lamp illustrated in Fig. 26, but may be done by an automatic
+arrangement, as will be described later.
+
+[Illustration: FIG. 27.]
+
+The current is really carried across the arc by _convection_, or in other
+words conducted by a bridge of white hot carbon particles, which
+continually stream across from the positive carbon to the negative, and
+this bridge, while conducting the current, interposes a very considerable
+_resistance_ (otherwise it would not of course become hot).
+
+A certain potential or tension is therefore necessary if a given current is
+to be maintained, and this potential has to be greater the longer the arc
+and also (though not in direct proportion) the smaller the carbons.
+
+When, however, everything is in the best proportion, _i.e._ length of arc,
+size of carbons, and current passing, the potential at the arc lamp
+terminals required is approximately 45 volts, and this may be taken as a
+fixed figure for any current.
+
+The length of arc to give the best results may also be taken {47} as
+approximately fixed at 1/8 inch, and the _variable_ factor for different
+currents as required is provided by altering the sizes of carbons employed.
+
+The error must not be made, however, of assuming that an E.M.F. of 45 volts
+is sufficient to work an arc lamp, as the minimum in practice is at least
+65 volts, and 100 or even 200 volts are advantageous.
+
+I have come across more than one private generating installation where the
+innocent owner has put in a dynamo for 45 or 50 volts, depending upon some
+carelessly written statement that this is sufficient.
+
+_Why_ a higher E.M.F. is required can be simply explained.
+
+Take for instance an average hand-fed arc lamp as used for lantern work and
+consuming, say, 10 amperes.
+
+Take also, as a fact, the statement given above that the necessary E.M.F.
+at the actual terminals of the arc lamp may be accepted as a constant at 45
+volts, and reverting to the equation given on page 40, C = E / R, and
+substituting these figures we get--
+
+  Current (10 amperes) = E (45 volts) / R (Resistance of Arc).
+
+It is therefore obvious that under these exact conditions the resistance or
+back E.M.F. of the arc, as it is termed, must equal 4.5 ohms.
+
+Now suppose the lamp left for a few seconds unattended, while the carbons
+are burning away and the arc is lengthening; in a very few moments the
+resistance will have increased, owing to the greater distance between the
+carbons, and we will suppose it to have become 5 ohms instead of 4.5.
+
+The current passing will now be 45 / 5 = 9 amperes only.
+
+In other words, a very slight lengthening of the arc has reduced the
+current, and therefore the light, by 10 per cent.
+
+Not only so, but 45 volts being needed to maintain an arc of {48} normal
+length, it is insufficient to maintain a longer one, and in practice the
+effect of leaving an arc under these conditions to itself for even a few
+seconds is that it _goes out_, to the annoyance of the lecturer and the
+confusion of the operator.
+
+It is just _possible_ to work an arc lamp with a total E.M.F. of 45 volts
+by giving one's whole attention to it and never taking the hand off the
+feeding handle; but in practice no one with any experience would attempt
+it. The arc would almost certainly go out several times during the
+exhibition.
+
+Now, take an example of a similar arc lamp consuming 10 amperes but worked
+from a supply of 200 volts.
+
+Our equation C = E / R must then obviously become
+
+  C (10 amperes) = E (200 volts) / Total Resistance (20 ohms).
+
+The resistance of the arc itself being the same as before, viz. 4.5 ohms,
+it is obviously necessary to put an _extra_ fixed resistance equal to 15.5
+ohms in series with it in order to make up the total of 20 ohms.
+
+_Now_ leave the arc unattended until the resistance of 4.5 ohms has again
+become 5 ohms; the only effect is that our current, instead of remaining at
+10 amperes, has become 200 / 20.5 or 9.8 nearly, a difference which is
+imperceptible.
+
+This is not all, for it is an elementary rule in electrical science that
+the total E.M.F. of any circuit distributes itself along that circuit in
+proportion to the distribution of resistance.
+
+In other words, our original E.M.F. of 200 volts will so distribute itself
+as to reserve, so to speak, an E.M.F. of 45 volts for the arc, while the
+resistance of this remains at 4.5 ohms, but directly this resistance
+increases, the E.M.F. at the arc lamp terminals automatically rises, and
+therefore the actual diminution in current is even less than the figures
+above quoted. {49}
+
+Should the arc tend to 'break' or go out, the resistance across it
+automatically becomes infinite and the _whole_ 200 volts is at that moment
+available to prevent the occurrence.
+
+Under these conditions, therefore, the operator can safely leave the arc
+for many minutes at a time. In carrying out experimental work I have often
+left the lantern, walked up to the screen, discussed results with a friend,
+and walked back, and the arc has shown no signs of misbehaviour whatever.
+
+[Illustration: Fig. 28.--Resistance.]
+
+In practice any current from 100 volts to 250 volts may be considered as
+satisfactory for lantern work with a suitable resistance. Less than this
+involves feeding the arc rather frequently, and more may give a nasty
+shock, should the operator inadvertently touch a live wire, though I have
+worked an arc lamp on a current of as much as 500 volts.
+
+The _resistance_ usually consists of a suitable length of wire of high
+resistance (Iron, German Silver, or those alloys known as Platinoid,
+Eureka, Manganin, Beacon, &c., are most commonly used) wound in spirals on
+a frame, and is generally supplied adjustable (Fig. 28), so that more or
+less current may be used as desired. These resistances get pretty hot in
+use, and care must be taken that they are placed where they cannot scorch
+woodwork, &c., and in cases where the lantern is a fixture it is a good
+plan to have the resistance bolted up against a wall once and for all. The
+resistance may be placed anywhere in the circuit, so long as the current
+passes through it, then through the arc lamp (or _vice versa_), and back to
+the other {50} pole of the supply main; it does not matter in the least
+whereabouts it comes.
+
+In cases, however, where one pole of the supply main is _earthed_, it is a
+good thing to place the resistance in the 'live' side, as this keeps the
+arc lamp within 45 volts of earth potential while it is working, to the
+comfort of the operator should he touch a terminal or wire, though with an
+ordinary lighting main there is no real fear of a dangerous shock in any
+case.
+
+The _amount of current required_ depends of course on the size of the
+sheet, length of the hall, and density or otherwise of the slides; but it
+is usually accepted in practice that the efficient light from a continuous
+current arc lamp equals 100 candles per ampere, and therefore a 10-ampere
+arc will give 1000 candles. This is sufficient for all ordinary halls and
+slides, but where these latter are very dense, as for example with the
+Lumiere three-colour process, as much as 20 or 25 amperes may be required.
+
+In these cases some special precautions must be taken for keeping the
+slides cool, or the result may be disastrous, but this is a question that
+will be referred to in a later chapter. A current of 10 amperes is pretty
+safe for all ordinary slides, and may be taken as the normal current used
+in large halls, though in arranging for the wiring it is as well to
+stipulate for at least 12 or even 15 amperes, especially as there must
+necessarily be a momentary increase of current at the instant the arc is
+'struck.'
+
+VARIETIES OF HAND-FED ARC LAMPS.--The pattern of hand-fed arc lamp
+illustrated in Fig. 26 is only typical of many of the same general design,
+and there are others in which the design itself is fundamentally different.
+Of these the 'Scissors' arc lamp made by several firms deserves mention on
+account of its simplicity and cheapness. As its name implies, the mechanism
+resembles a pair of scissors, the carbons being attached to the ends of a
+pair of levers hinged together {51} (Fig. 29). In this lamp centring
+movements are usually dispensed with, the arc being clamped on to a tray
+pin as in the case of a limelight jet. This is not, of course, so
+convenient, and a further disadvantage of this pattern arc lamp is that the
+feeding process gradually alters the position and angle of the carbons. In
+fact, the one great merit of the lamp is cheapness, and where expense is an
+object, it should certainly be considered.
+
+[Illustration: FIG. 29.--'Scissors' Arc Lamp.]
+
+Yet another arc lamp deserving of mention is the 'Parallel,' a name again
+very aptly chosen, as the two carbons are either exactly parallel to each
+other or very slightly inclined. In the former case the arc has to be
+'struck' by touching the ends of the carbon rods with a piece of metal or
+carbon. Of the actual manipulation of this lamp I have had very little
+practical experience, but I have heard it well spoken of, though I believe
+it has so far only been made for currents of 5 amperes or so.
+
+Yet another type which must not be ignored is the 'Right-angled' pattern
+(Fig. 30), a name again self-descriptive. The horizontal carbon is the
+positive, and the vertical the {52} negative, and this lamp again is made
+by several manufacturers in slightly different forms.
+
+This pattern lamp is in my experience the best of all for _small_ currents,
+say, of 5 amperes or so, but inferior to Fig. 26 for currents of 10 amperes
+or more. This last remark perhaps hardly applies to _alternating_ currents,
+which, however, I have not yet discussed. I cannot conclude this brief
+category of arc lamps without referring to the _enclosed_ pattern, of which
+the 'Westminster' is perhaps the best-known and most popular (Fig. 31).
+
+[Illustration: FIG. 30.--'Right-angled' Arc Lamp.]
+
+This is a lamp of the right-angled type, but the arc burns in a cylindrical
+glass chamber, not air-tight, but partially so. After burning a few minutes
+the oxygen in this chamber becomes used up and its place is taken by
+carbonic-acid gas and other products of combustion, after which the carbons
+burn away very much more slowly, and therefore require feeding at much
+greater intervals.
+
+This lamp again is chiefly made for small currents not exceeding 5 amperes
+(and can therefore be used from any ordinary lamp socket), and for a
+moderate-sized hall is on the {53} whole as cheap, efficient and simple a
+lamp as any I am acquainted with. It can be supplied with or without
+mechanical centring movements as required, and is usually sent out with its
+own resistance for the particular current on which it is to be used, so
+that it only requires connecting up to the nearest lamp socket, and is
+ready for use.
+
+[Illustration: FIG. 31.--'Westminster' Arc Lamp.]
+
+It is _not_ sufficient for anything larger than a 12-foot sheet or for
+working at a greater distance than, say, 40 feet, but within these limits
+the lamp, and in fact _any_ good 5-ampere arc lamp, will be found quite
+satisfactory and saves the expense of putting in a special cable.
+
+AUTOMATIC ARC LAMPS.--Arc lamps for lantern work in which the feeding is
+done automatically are also made. Like hand-fed lamps, they vary in exact
+design, but all, or practically all, are so designed that the carbons are
+brought together by means of springs or weights, and some form of 'brake'
+controlled by a system of electro-magnets checks the {54} movement. As the
+carbons burn away the arc lengthens, the current weakens, the
+electro-magnets lose their grip, and the carbons move together until the
+increasing current puts on the brake again. Some of these lamps are
+'semi-automatic' only, that is to say, the arc has to be struck by hand,
+while others perform this operation automatically as well, usually by an
+additional magnet which draws back the carbons by the correct amount after
+the arc is struck.
+
+My frank advice to intending lanternists is to leave these lamps alone.
+Some of them are satisfactory up to a point, but they are all apt to be
+'jumpy,' and on the whole the hand-fed type is in my opinion to be
+preferred.
+
+ARC LAMPS ON ALTERNATING CURRENTS.--The alternating current is not so good
+as the continuous for lantern work with arc lamps: the light per ampere is
+not so great, the light has an irritating habit of travelling round the
+carbons and there is always a slight 'hum.'
+
+The sum total of these drawbacks is nothing very serious, provided that
+proper arrangements are adopted, and I have frequently manipulated arc
+lamps on alternating circuits with such good results that professional
+lecturers have at first refused to believe that the circuit really _was_
+alternating.
+
+As it is frequently stated that to obtain a steady light with an
+alternating current is impossible, I can understand their surprise, and I
+can also understand the statement in question, as the problem is usually
+tackled on entirely wrong lines.
+
+It is almost always stated that arc lamps for alternating currents should
+be arranged with the carbons _vertical_, and many makers actually so
+construct their lamps as to allow of this.
+
+To obtain a steady light under these conditions _is_ impossible and I pity
+anyone who attempts it; but the statement that this is the best method of
+working has been repeated so often that it seems to have been taken for
+granted.
+
+The best arrangement (in my hands at any rate) is to {55} slant the carbons
+as for the continuous current, and also to have the upper carbon cored and
+the lower one solid, but to use a rather larger lower carbon than would be
+correct if the main were continuous.
+
+Also the upper carbon should not be _quite_ so far back as with D.C.; to
+have the front edges of the two carbons practically in line is about
+correct, but the _exact_ position should be carefully adjusted to obtain
+the steadiest light, and it will be found that a slight alteration makes a
+considerable difference.
+
+It is also a great help to have a weak electro-magnet, or its equivalent,
+so arranged that it tends by its influence to keep the arc to the front. On
+some lamps this is provided for, as even with a continuous current it is
+quite harmless and, if anything, beneficial; but, if not, any competent
+mechanic can easily fit an 'Induction Ring,' consisting of a single turn of
+stout copper wire, which has sufficient magnetic influence to do all that
+is required (Fig. 32).
+
+This ring must be wired in series with the arc itself, and as the current
+passing in it automatically reverses in synchronism with the arc, its
+effect is _always_ to deflect the arc in the same direction, and care must
+of course be taken that it is so wired that the deflection is forward and
+not backward. This is the exact arrangement I have myself adopted, and I
+never experience any difficulty on the score of the arc wandering.
+
+Right-angled arc lamps, as described on pages 52 and 53, are also very
+efficient on A.C. mains, and frequently these lamps are already equipped
+with electro-magnets for the purpose required. The 'hum' of an alternating
+current cannot be altogether eliminated, but can be reduced to a minimum
+_by reducing the voltage as far as possible_.
+
+As has been already said, the A.C. lends itself readily to transformation
+of voltage, and I find in practice 90-100 to {56} be ideal. More than this
+is inclined to be noisy, and less is apt to result in an unsteady arc.
+
+The arrangement, therefore, which I recommend from long experience is to
+employ a transformer to reduce the E.M.F. to 100 volts or thereabouts, and
+then work with a resistance in the usual way (if the original current is
+100 volts, of course _no_ transformer is required) with a properly
+constructed arc lamp fitted with an induction ring or electro-magnet. No
+difficulty should then be experienced in obtaining a good, steady, and
+fairly quiet light.
+
+[Illustration: FIG. 32.--Arc Lamp with Induction Ring.]
+
+Any little 'hum' remaining can be silenced to a very considerable extent by
+placing the entire lantern on a thick block of saddlers' felt, but in
+practice I have never found this necessary with ordinary currents, though a
+few abnormal circuits where the 'periodicity' is very high are noisier than
+others.
+
+[Illustration: FIG. 33.--THE OPTICAL SYSTEM OF A LANTERN.]
+
+{57}
+
+The following table gives the sizes and particulars of carbons for various
+currents that I have found best in actual practice:
+
+                    CONTINUOUS CURRENT
+
+  Amperes.        + Carbon _Cored_.     - Carbon _Solid_.
+
+    7-10                   12 mm.                   7 mm.
+   10-15                   13 ,,                    8 ,,
+   15-20                   16 ,,                   10 ,,
+
+                    ALTERNATING CURRENT
+
+  Amperes.    Upper Carbon _Cored_.    Lower Carbon _Solid_.
+
+    7-10                   12 mm.                  10 mm.
+   10-15                   13 ,,                   11 ,,
+   15-20                   16 ,,                   13 ,,
+
+       *       *       *       *       *
+
+
+CHAPTER VII
+
+THE OPTICAL SYSTEM OF A LANTERN
+
+[Illustration: FIG. 33A.--Optical System of Lantern.]
+
+As previously noted, the essential parts of an Optical Lantern are, in
+order from rear to front: (1) The illuminant; (2) the condenser; (3) the
+slide and slide stage; (4) the objective, to which must be added, (5) the
+body or framework which holds the whole together. Fig. 33 is a diagrammatic
+representation of the entire optical system and Fig. 33A shows all the
+various parts _in situ_: A being the illuminant, shown in Fig. 33 {58} as
+an arc lamp, B the condenser, C the slide stage, and D the objective. The
+foundation, so to speak, of the whole instrument is of course the slide,
+which, as made in this country, consists of a square of glass 3-1/4 inches
+diameter, the slide itself being somewhat less than this on account of the
+binding, &c.; in making calculations it is usually taken as a 3-inch
+circle. Slides are usually made by binding together with strips of paper or
+cloth two such squares, on one of which is the photographic film or
+painting forming the picture, the other being simply a plain cover glass
+placed over the slide surface to protect it, and between the two being
+placed a paper mask with an aperture of whatever size or shape is required,
+that of the aforesaid 3-inch circle being usually taken as the standard or
+normal dimension for this aperture.
+
+The slide being illuminated by one of the various methods discussed in the
+previous chapters, is focussed on the screen by the objective, which must
+be selected according to the size of picture required and the distance
+between lantern and screen.
+
+These points will be gone into later, and also details as to various types
+of objectives and their respective advantages; but it may be said here that
+a lantern objective consists usually of a combination of lenses of 2 inches
+or 2-1/2 inches diameter mounted in a rackwork focussing system at a
+distance from the slide of 6 inches to 18 inches, according to the length
+of its 'focus.' As our slide is from 3 to 3-1/4 inches diameter, it is
+evident that all the light radiating from this cannot possibly get through
+the objective unless it is _converged_ upon it, and to do this is the
+function of the condenser. The following two diagrams, Figs. 34 and 35,
+will make the matter clear.
+
+S represents our glass slide of 3 inches clear diameter, R the radiant or
+illuminant, and L our objective, shown here for the sake of simplicity as a
+single lens.
+
+The slide is well illuminated by the light emanating from {59} R, but it is
+obvious that the bulk of this light will never pass through the lens, and,
+in fact, only the very centre of the slide will under these circumstances
+appear upon the screen at all.
+
+[Illustration: FIG. 34.--Optical System without Condenser.]
+
+[Illustration: FIG. 35.--Action of Condenser.]
+
+What is evidently wanted is to _converge_ these outer rays, or in other
+words to bend them in so that they also pass through the objective, and
+this is the function of the condenser as illustrated in Fig. 35. The
+condenser is here represented also by a single lens, but in practice it
+also is invariably constructed of two or even three lenses, for both
+optical and mechanical reasons. It is evident from the above diagrams that
+the condenser must be somewhat larger in diameter than the slide itself,
+and condensers for ordinary lantern work are usually 4 inches to 4-1/2
+inches diameter. The former size {60} will suffice if the condenser is
+placed very close to the slide, but it is often advisable to leave a little
+intervening space, especially if the illuminant is a powerful one, in order
+to allow any condensation of moisture readily to evaporate and escape.
+Hence lanterns for long range work (which involve, of course, good
+illumination) are usually made with condensers of 4-1/2 inches diameter.
+Lantern condensers of to-day usually take one of the two forms shown in
+Fig. 36, but the exact curve must be left to the manufacturer, as the focus
+of the condenser must have a definite relation to that of the objective.
+Taking, however, the design of E, the most common of all, the two lenses
+should not be exactly similar unless the objective is pretty short in
+focus, or, in other words, unless the distance of the illuminant on the one
+hand and that of the objective on the other are approximately equal. If the
+lantern is intended for long range work, that is equipped with a long focus
+objective, the front component of the condenser should also be constructed
+longer in focus (that is to say, with a shallower curve) than the rear one,
+and it is amazing how careless manufacturers are in this respect. If, as is
+often the case, the lantern is fitted with several objectives of different
+foci, it is usually necessary to supply alternative condensers also, or at
+least to supply an interchangeable front component.
+
+[Illustration: FIG. 36.--Forms of Condensers.]
+
+If the entire condenser is too long in focus, light is lost; if too short,
+it is impossible to obtain an even disc, as there is invariably a dark
+patch either in the centre or round the edges.
+
+The mounting of the condenser also varies with different makers; but it
+must be remembered in any case that it gets {61} extremely hot, especially
+the back component, and hence the glass must be mounted _loose_ in its
+cell, otherwise there is great danger of it cracking. Also the space
+between the components should be well ventilated, in order to provide for
+the escape of moisture which usually at the start of a lantern exhibition
+is deposited upon the glass, and should be got rid of before the actual
+lecture commences.
+
+Even with all care, the back component of a condenser will sometimes crack,
+though such an accident should be a rare occurrence; and hence a
+professional operator will usually provide himself with a spare lens, and
+the condenser should be so constructed that it can readily be changed, and
+with as little delay as possible.
+
+Condenser lenses as made in this country are usually ground from the glass
+known as 'English Crown,' and comparatively rarely crack; but they are very
+slightly green in colour. French condensers, on the other hand, are whiter,
+but the glass is more brittle, and a fracture a more common occurrence. The
+French variety are (or were before the war) cheaper and generally met with
+in cheaper instruments. More expensive lanterns are usually fitted with
+English condensers, as the tinge of green is almost imperceptible, and the
+advantage as regards greater security pretty considerable.
+
+THE SLIDE CARRIER AND SLIDE STAGE.--Taking still the optical system of the
+lantern in order from back to front, we now come to the slide, slide
+carrier, and slide stage. The slide itself has already been described, and
+the carrier is simply a mechanical contrivance, usually of wood, designed
+for the purpose of readily changing the pictures and which in its turn fits
+into the stage of the lantern. It may be asked why, if slides are now
+always made to a standard size, the slide carrier should not itself be
+built into the lantern and form the stage; but the answer is, in the first
+place, that slides of a different size, _i.e._ American or Continental,
+_may_ be met with, {62} and also that there are various mechanical slides
+on the market--for example, chromotropes or scientific models, such for
+instance as are made to illustrate the movements of the planetary
+bodies--and these slides are permanently mounted in wooden frames which
+could not be put into a carrier. The commonest form of carrier is that
+known as the 'Double Sliding' pattern (Fig. 37), which consists of a frame
+with two apertures for the slide, and an outer frame through which this
+itself slides and which fits the stage of the lantern.
+
+[Illustration: FIG. 37.--Double Sliding Carrier.]
+
+This carrier, as will be seen, allows the next picture to be placed in
+position in the second aperture while the former one is being projected,
+and at a signal from the lecturer, the inner frame slides smoothly through
+the outer, and the slides are thereby changed. This carrier is simple,
+cheap, and quiet in its action; its one disadvantage is that each alternate
+slide has to be inserted from opposite sides of the lantern, and unless the
+operator is fairly tall this almost necessitates an assistant.
+Nevertheless, the carrier is the most popular of any, its other advantages,
+especially as regards price, being so great. It is usually constructed in
+such a way that the slide, as it moves out from the central position,
+automatically rises in its groove in order to facilitate quick removal.
+
+Another pattern deservedly popular is that known as {63} 'Beard's
+Dissolving Carrier' and is shown in Fig. 38. In this ingenious carrier all
+the slides are inserted from the same side, the oncoming slide being pushed
+_in front_ of its predecessor, and being therefore somewhat out of focus it
+produces a blur on the screen.
+
+The movement is performed by pushing in a projecting handle, and on
+withdrawing this the slide which is finished with comes with it, and the
+finish of the movement presses the new slide back until it is in its proper
+position and in focus.
+
+[Illustration: FIG. 38.--Beard's Dissolving Carrier.]
+
+The entire action is simpler than it sounds, and the temporary blurring of
+the image on the screen during the process of changing is supposed to give
+somewhat the effect of 'Dissolving Views,' and hence the name 'Dissolving
+Carrier.'
+
+This appliance is three times the price of the 'Double Sliding' pattern,
+but the fact that it is worked from one side only is a decided advantage,
+though on the other hand it is not (unless great care is used) quite so
+silent in its action as the 'Double Sliding' type.
+
+A further modification of this carrier adapts it to take any of the
+recognised 'foreign' sizes of slides, so that if a few American ones, for
+instance, are met with among a collection of English manufacture, there is
+no need to change the carrier. {64}
+
+There are other varieties of carriers on the market which there is no need
+particularly to describe, such as, for example, carriers fitted with roller
+curtains to give the effect of a curtain rolling up, magazine carriers to
+hold twenty-four or more slides and exhibit them in rotation, and other
+patterns too numerous to mention. Of these the reader must be left to judge
+for himself, but, generally speaking, _simplicity_ in a carrier is the most
+important point to be looked for, and complications, however ingenious,
+should be avoided.
+
+[Illustration: FIG. 39.--Focussing Action of Lens.]
+
+The lantern stage must also receive consideration, but it will be better to
+discuss it as part of the mechanical construction of the lantern.
+
+THE OBJECTIVE is really the most vital part of a lantern, as the definition
+of the picture almost entirely depends upon the excellence or otherwise of
+this lens. This will be obvious at once when it is realised that the
+objective has to project on to the distant screen a greatly magnified image
+of the comparatively small lantern slide, and the intending purchaser is
+strongly advised to economise almost anywhere rather than on this item.
+
+The action of a lens in focussing the image is perhaps best explained by a
+simple diagram (Fig. 39), from which it will be seen that all the rays
+proceeding from any one point on the object are re-converged (when the lens
+is in focus) to a definite point on the image, and the perfection of the
+picture depends upon the lens performing this function accurately. {65}
+
+The imperfections are chiefly two, viz. those known as chromatic and
+spherical aberration respectively. Chromatic aberration simply means that
+all the colours composing the original beam of, say, white light are not
+equally refracted or converged, and therefore do not meet again at the same
+spot (the well-known prism or lustre effect), and reveals itself by
+coloured fringes round the edges of the various details in the picture.
+
+[Illustration: FIG. 40.--Achromatic Lens.]
+
+By spherical aberration we mean that the light falling upon the centre of a
+lens is not brought to a focus at exactly the same spot as the marginal
+rays, and a general want of definition is the result, usually accompanied
+also by a want of 'flatness' in the image, that is to say the edges of the
+picture do not focus at the same time as the centre.
+
+Chromatic aberration is easily cured by using an achromatic or compound
+lens made by cementing together two lenses of crown and flint glass
+respectively, as in Fig. 40.
+
+It will be seen that the flint glass component by itself is a _concave_
+lens and therefore neutralises in part, or in whole, the convex crown lens.
+Flint glass has both greater dispersive power and also greater refractive
+power than crown glass, but fortunately not to the same _degree_; hence a
+compound lens made in this way and with curves carefully worked out may
+have its chromatic effect entirely neutralised while retaining very
+considerable refractive or 'focussing' power, and simple achromatic
+objectives of this type are quite inexpensive.
+
+In lanterns intended for Science demonstration, as distinct from the mere
+projection of slides, lenses of this pattern are very frequently used, as
+they will project the latter when required reasonably well, and for the
+demonstration of {66} experiments or of apparatus on the screen have
+advantages that need not be discussed here.
+
+For very long focus lenses also they are sometimes employed, as the trouble
+from spherical aberration is much less apparent with lenses of long focus
+than with short, and the difference in expense is much more in the former
+case than in the latter. For short focus lenses, however, as used in
+moderate-sized halls, they are not good enough, and the type of lens almost
+universally employed is that known as the 'Petzval' combination (Fig. 41).
+
+[Illustration: FIG. 41.--Petzval Combination.]
+
+This lens really consists of two achromatic combinations, the pair at the
+front being cemented together, and that at the rear having an air space
+between. The combination is so designed that the spherical aberration of
+the one pair neutralises that of the other, and the result is or should be
+a lens corrected both for chromatic and spherical aberration.
+
+These lenses, however, vary very much in the perfection of their results,
+and as they are at present usually imported in bulk from France, the
+customer does well to insist upon a demonstration of his own particular
+lantern before acceptance.
+
+The magnifying power of a lens depends upon its 'focus' multiplied by its
+distance from the screen, and the focus in the case of a simple lens is
+easily determined by the familiar 'burning-glass' experiment, that is by
+focussing an image of the sun upon a piece of paper and measuring
+accurately {67} the distance the lens must be away to produce the most
+concentrated spot.
+
+In practice it is sufficiently accurate to focus a distant window, or other
+luminous object, upon the paper, any error obtained by this method being
+for ordinary purposes a negligible one.
+
+With a compound lens, such as a 'Petzval' combination, this method does not
+hold good, as the optical centre of such a lens is not necessarily midway
+between its two components.
+
+The actual focus can be got pretty approximately by focussing a window or
+other object as before and measuring the distance from one definite point
+(say the front edge of one of the lens cells) to the paper, then turning it
+round and taking a second measurement from the _same_ point, the mean
+between the two measurements giving the actual focus.
+
+In practice the 'simple equivalent focus,' as it is termed, of a lantern
+lens is usually determined by measuring the magnification of the image
+thrown upon the screen, when, by knowing the original size of the slide (a
+'standard' slide of 3 inches diameter is usually taken) and the distance
+between lantern and screen, we get the focus from the following very simple
+equation:
+
+  Diameter of picture on screen    Distance between lens and screen
+           (in feet)                        (in feet)
+  ----------------------------- = ---------------------------------
+  Diameter of slide (in inches)      Focus of lens (in inches)
+
+or perhaps more simply still:
+
+  Distance between lens  x Diameter of slide
+  and screen (in feet)      (in inches)
+  ------------------------------------------  = Focus of lens in inches;
+          Diameter of picture (in feet)
+
+or, if we know the focus of the lens but want to know how far from the
+screen we must go to produce a given-sized picture, the formula will be:
+{68}
+
+  Diameter of picture  x Focus of lens
+        (in feet)         (in inches)
+  ------------------------------------ = Distance required (in feet).
+      Diameter of slide (in inches)
+
+It is handy for the lanternist to remember that, dealing with a standard
+3-inch slide, a 6-inch lens will _always_ give a picture whose diameter is
+_one-half_ the distance from lens to screen, a 12-inch lens half this again
+or _one-quarter_, and a 9-inch lens half-way between the two.
+
+Bearing these simple figures in mind, the approximate distance can usually
+be _guessed_ sufficiently near for the first trial, and then the lantern
+shifted a little nearer or the reverse as required.
+
+The following table may, however, be useful, as showing readily the
+magnification produced at different distances by lenses of given foci:
+
+  +------+---------+--------+--------+--------+--------+--------+--------+
+  | Disc |  Focus  |  Focus |  Focus |  Focus |  Focus |  Focus |  Focus |
+  |wanted|4-1/2 in.|  6 in. |  8 in. |  10 in.|  12 in.|  15 in.|  18 in.|
+  +------+---------+--------+--------+--------+--------+--------+--------+
+  | feet.| ft. in. | ft. in.| ft. in.| ft. in.| ft. in.| ft. in.| ft. in.|
+  |   9  | 13   6  | 18   0 | 24   0 | 30   0 | 36   0 | 45   0 | 54   0 |
+  |  12  | 18   0  | 24   0 | 32   0 | 40   0 | 48   0 | 60   0 | 72   0 |
+  |  15  | 22   6  | 30   0 | 40   0 | 50   0 | 60   0 | 75   0 | 90   0 |
+  |  18  | 27   0  | 36   0 | 48   0 | 60   0 | 72   0 | 90   0 |108   0 |
+  |  20  | 30   0  | 40   0 | 53   4 | 66   8 | 80   0 |100   0 |120   0 |
+  |  25  | 37   6  | 50   0 | 66   8 | 83   4 |100   0 |125   0 |150   0 |
+  |  30  | 45   0  | 60   0 | 80   0 |100   0 |120   0 |150   0 |180   0 |
+  +------+---------+--------+--------+--------+--------+--------+--------+
+
+THE DIAMETER OF THE OBJECTIVE.--The diameter of the objective must depend
+to a certain extent upon its focus in the case of a double combination such
+as a Petzval. These lenses consist, as has already been said, of two
+achromatic components some distance apart, and for technical
+considerations, which need not be discussed here, the _distance_ between
+these components is usually about two-thirds of the focal length. This is
+not a universal rule, as the lenses of different makers vary a good deal;
+but it is generally a fact {69} that the longer the focus of the lens the
+greater is usually the separation between the two lens systems.
+
+The entire lens therefore mounted in its tube resembles a _tunnel_ of
+varying length according to its focus, and through this tunnel a _cone_ of
+light rays have to be passed. It is plain, therefore, that a lens of long
+focus, which in practice means a long tube length, must be made also of
+large diameter, or a portion of the cone will be cut off, with a consequent
+loss of light.
+
+In practice lenses up to 6 inches focus are usually made of 2 inches
+diameter, and there is no advantage in a larger size. With a lens of 8
+inches focus there is a slight gain in increasing the diameter to 2-3/8
+(the next 'standard' size), and lenses of longer focus than this should
+certainly be 2-3/8 inches up to, say, 12 inches focus, when a lens of 3
+inches diameter is preferable. These large lenses are, however, very
+expensive, both in themselves and also on account of the fact that their
+weight entails heavy and expensive brass mounting, and hence lenses up to
+14 or 15 inches focus are often supplied in the 2-3/8 size for reasons of
+economy.
+
+To sum up, _short-range_ lanterns, as they are called, are usually fitted
+with lenses of 2 inches diameter, and _long-range_ instruments either with
+3-inch lenses or the intermediate size of 2-3/8 inches. If a lantern is
+purchased for either long or short-range work, it is usually fitted with a
+brass front for a large lens, and so arranged that a shorter focus lens of
+2 inches diameter can easily be interchanged, utilising the same brass
+mounting.
+
+Lenses of _variable_ focus have also been designed, in which an additional
+lens can be added or subtracted to increase or decrease the focal length;
+but nothing very practical has yet been achieved in this direction, and
+therefore these 'Omnifocal' lenses have never come into general favour.
+
+Objectives like condensers want cleaning at times, and care must be taken
+not to scratch the glass, as the concave lens of each component is of flint
+glass, and very soft. A {70} clean chamois leather is the best thing to
+use, but a soft cloth, or even a handkerchief, may be employed with care.
+It is very important that a lens be reassembled, after cleaning, the
+correct way, as a single lens reversed would utterly spoil the definition.
+The front component is usually balsamed together, and therefore all that is
+needed is to see that the whole combination is not reversed. In the Petzval
+system this lens should have its convex constituent towards the screen
+(Fig. 41). The back combination is usually loose, and the two lenses are
+sometimes separated by a thin brass ring. In the Petzval lens the concave
+element should be inside, with its concave surface outwards, the deep curve
+of the other lens should fit into this concavity, and the flatter curve
+face towards the condenser. One or two makers, however, have introduced a
+modification of the Petzval system in which the whole of this back
+combination is reversed, and the exact arrangement should therefore be
+noted very carefully when taking the lens to pieces.
+
+       *       *       *       *       *
+
+
+CHAPTER VIII
+
+THE BODY OF THE LANTERN
+
+We now come to the mechanical construction of the optical lantern, and a
+great variety of design presents itself, according to price, type (_i.e._
+short range or long range), and the individual ideas of the various makers.
+
+Lantern bodies as a rule are now made of metal, although up till quite
+recently the better class instruments were more usually made of polished
+mahogany lined internally with iron; but there has of late been a consensus
+of opinion in favour of metal only.
+
+In the cheaper lanterns this metal body is usually made either of Russian
+iron or of sheet-iron tinned and japanned, {71} there being little to
+choose either in price or quality between the two varieties, and in all but
+the very cheapest instruments the front is usually of brass.
+
+In better lanterns the body is more often made of enamelled steel, the
+front as before being of brass; but brass, copper, or aluminium are also
+used occasionally for the body of the lantern.
+
+[Illustration: FIG. 42.--Hughes' Short-Range Lantern.]
+
+In deciding upon the type of body to be purchased the main considerations
+to be borne in mind are: (1) The type or types of illuminant to be used, a
+powerful arc lamp for example requiring a larger body than is necessary for
+a weaker radiant; (2) the size and position of the lens to be carried, a
+Petzval objective of say 3 inches diameter which has to be supported at the
+end of a long brass mount for long-range work obviously demanding a body of
+greater strength and rigidity than is required with a 6-inch focus lens of
+2 inches diameter; (3) price.
+
+Fig. 42 shows an extremely good lantern body for short-range work made by
+Messrs. Hughes, the illustration depicting the instrument complete with a
+'Luna' methylated spirit lamp, though, of course, any other illuminant
+suitable for a small lantern could be used instead. {72}
+
+This lantern illustrates well one point that has already been emphasised as
+important, viz. the ventilation of the condenser. It will be noticed that
+this is placed _outside_ the body of the instrument instead of inside as is
+usual with larger bodies, and that wide slots are cut in the condenser
+mount to allow free escape of steam.
+
+Other points of this excellent design are the screw adjustment to the slide
+stage (facilitating the use of special slides, such, for example, as those
+illustrating the movements of the planetary bodies which sometimes involve
+the use of extra thick frames) and a simple but efficient tilting
+arrangement to the base.
+
+[Illustration: FIG. 43.--Long-Range Lantern.]
+
+Such a lantern is hardly suitable for a powerful arc lamp or limelight jet,
+or for heavy long-range lenses, but is a very good typical instrument for
+use in moderate-sized halls, and a lantern of this general type is usually
+found in lantern catalogues, though, of course, the exact designs vary
+according to the ideas of the manufacturer. Of lanterns for long-range work
+a good example is perhaps Messrs. Newton & Co.'s 'Intermediate' pattern
+(Fig. 43).
+
+This again is only typical of many others by the various makers, but the
+principal points are common to all. These are: (1) The large and
+well-ventilated body; (2) the long {73} baseboard; (3) the strong and
+massive brass front necessary to carry the large long-range lenses; (4) the
+velvet curtain at the back to close in any stray light from a powerful arc
+lamp.
+
+[Illustration: FIG. 44.--Connections for a Bi-unial Lantern.]
+
+The two foregoing designs are perhaps sufficiently typical of lantern
+bodies in general to make further detailed description of individual
+designs unnecessary; but reference should be made to features which special
+requirements may render advisable.
+
+Under this heading mention must be made of _Bi-unials_ or Double Lanterns,
+as used for the once famous 'Dissolving Views.'
+
+A bi-unial lantern consists essentially of two different instruments, each
+complete with its limelight jet or other illuminant--front, condensers,
+objective, &c., usually mounted on one body--and with some arrangement for
+'dissolving' or turning the light in each lantern gradually on and off.
+
+Fig. 44 shows the back view of such a lantern with two limelight jets and
+dissolving tap, this piece of mechanism (shown below in the illustration)
+being so arranged that when the lever is horizontal _both_ lanterns are on
+full, but moving the lever either way cuts off the gas supply to one
+lantern. In the case of limelight the tap should always operate by cutting
+off the oxygen supply in advance of the coal gas (in order to avoid a
+'snap'), and the latter should never be cut off entirely, but a small bead
+of flame left to keep the jet alight, until the lantern is required for the
+next slide. {74}
+
+This is usually arranged for by means of a bye-pass, and a bye-pass is
+sometimes provided on the oxygen side as well, but is usually discarded in
+practice.
+
+A bi-unial lantern can be worked in the same way with acetylene gas, but
+with the electric arc it is impossible to turn the light on and off
+gradually, and in practice dissolving must be done by keeping both lanterns
+fully alight, and using a dissolving shutter, that is a movable shutter
+that covers each objective alternately. The same arrangement must be used
+with other illuminants, such as oil, only in this case the lanterns must be
+mounted side by side, on account of the tall chimneys. With oil lamps the
+arrangement answers fairly well, the dissolving fan, as it is termed, being
+made with serrated edges which give the _gradual_ obliteration required;
+but with the electric arc the extremely sharp definition becomes a serious
+difficulty, and a good dissolver for this illuminant has never yet been
+found, though, in view of the fact that dissolving views are more or less a
+thing of the past, the matter cannot be regarded as important.
+
+The advantages claimed for a double lantern are two: first, a 'Dissolving'
+effect by which one picture fades gradually into the next, and which is
+supposed to be more pleasing than the movement of a carrier; and second,
+'Dissolving Effects' can be shown, such as exhibiting a landscape by day
+and changing it into a moonlight scene, or bringing on the appearance of a
+snowstorm, which can easily be done by means of a roller slide, with minute
+perforations shown in motion by the second lantern while the landscape
+remains on the screen from the first. In the days when dissolving views
+were all the vogue, a third or even a fourth lantern has been added for
+more complicated effects, and at the famous Polytechnic demonstrations of
+years ago, I believe that as many as six were sometimes employed.
+
+In these days of the cinematograph it is doubtful how far interest in such
+effects could be revived, and a lantern has {75} gradually come to be
+looked on more as an instrument for showing illustrations as required by
+the lecturer rather than as a pleasing exhibition in itself, and as
+dissolving views have lost their attraction, the double or triple lantern
+has been relegated to the limbo of antiquity.
+
+[Illustration: FIG. 45.--Beard's Circulating Water Tank.]
+
+Among other 'special' lanterns should be mentioned models made with
+water-cooled stages, for use with very delicate slides. This elaboration is
+not necessary with ordinary slides and illuminants of moderate power, but
+where very delicate slides, such as specimens of natural colour
+photography, have to be shown, it is an advisable precaution to pass the
+beam of light first through a tank of water in order to absorb the heat.
+Lanterns intended for this work are usually constructed with a kind of
+double stage, a glass trough of water fitting into the rear aperture and
+the slide-carrier into the front one. Such an arrangement answers quite
+well for most purposes, but for extreme cases lanterns are equipped with a
+trough connected to a large outside tank and complete circulatory system,
+after the manner of the cooling tank of a gas engine.
+
+Such a lantern, constructed by Messrs. Beard, is illustrated in Fig. 45,
+and it will be seen that in this instrument the water trough is placed
+between the lenses of the condenser. {76} This is a very good position, as
+the beam of light at this point is, or should be, parallel, whereas between
+the condenser and the slide it is convergent, and therefore a condenser of
+a larger diameter than the slide must be employed in the latter case if the
+trough is of considerable width.
+
+While dealing with 'Special' lantern bodies, we should perhaps just mention
+here the numerous pattern lanterns made for the demonstration both of
+lantern slides and of Scientific Phenomena, such as the projection of
+insect life or other microscopic objects, polarised light experiments,
+electrical apparatus, opaque objects, &c. A detailed description of these
+lanterns and how to use them belongs to the second part of this work, as
+also does the popular cinematograph; but educational institutes, and even
+boys' clubs, when considering the purchase of a lantern, might well reflect
+whether it would be advisable to spend a little more money in the
+acquisition of an instrument which can be utilised for a variety of
+purposes.
+
+       *       *       *       *       *
+
+
+CHAPTER IX
+
+LANTERN BOXES, STANDS, READING LAMPS, ETC.
+
+Having now discussed all the essential parts of a lantern, the next points
+to be considered are those of lantern boxes and stands. It is best to take
+these together, as more often than not a lantern is arranged to stand upon
+its box during use, and the plan is both convenient and simple. The whole
+question is one to be settled upon its own merits in each individual case.
+Sometimes neither box nor stand is wanted at all. The lantern is put away
+into a locked-up cupboard or other safe place, and used upon a permanent
+support or (as is often the case in a church) from a gallery at the back.
+{77}
+
+In most cases, however, a box of some sort is desirable, and the two main
+considerations are strength and simplicity.
+
+All patent arrangements, such, for example, as those in which the sides of
+the box fall down and provide trays for the slides, are beautiful in
+theory, but cannot be recommended in practice. A good, simple and
+substantial box is what is required, preferably painted black, and provided
+with strong handles.
+
+One addition may be permitted, viz. a tilting top. Some means for tilting
+the lantern is always advisable, as it is seldom convenient to raise the
+instrument to the level of the centre of the screen, and a slight upward
+elevation does not appreciably distort the image. This arrangement for
+tilting may be either embodied in the lantern itself, as for instance in
+the instrument shown in Fig. 42, or may be provided for on the box or on
+the stand, if a stand is used.
+
+It is, perhaps, an elaboration that may be regarded as not strictly
+necessary, as a book or two or other article may be placed under the
+lantern base as required; but a tilting arrangement is so convenient that
+it can be strongly recommended, and the addition is not expensive.
+
+For large, long-range lanterns a strong deal box, on which the lantern can
+stand, is usually all that it is desirable to purchase in the way of a
+support. A good solid table can usually be found, which will do all the
+rest, as it must be remembered that a slight tilt at a long range means a
+good deal of total elevation.
+
+Where this is not procurable a stand must be provided, and this for a large
+lantern should be strong and rigid. Anything in the way of a collapsible
+tripod should be avoided, but such an arrangement as Fig. 46 is quite good
+and rigid enough for all practical purposes.
+
+For a _small_ lantern a tripod stand is quite suitable, though care must be
+taken that one of the legs does not get kicked, either by accident or
+design, or the result may be a catastrophe. {78}
+
+_Slide Boxes._--On this subject not much need be said. The variety of
+patterns on the market is endless, some being designed from the point of
+view of safe transit by post, others for convenience of storage and
+classification. It is essentially a case where each individual user must
+use his or her taste, and in any case the question of the box is one for
+the owner of the slides rather than for the lanternist.
+
+[Illustration: FIG. 46.--Quadruple Lantern Stand.]
+
+READING-DESKS, LAMPS, AND SIGNALS.--Some form of reading lamp for the
+lecturer is generally considered to be part of a lanternist's equipment,
+and the most usual pattern is fitted with a candle, after the manner of a
+carriage lamp, or else constructed to burn colza or other vegetable oil,
+such as supplied for cycle lamps. Oil gives the brighter light, but is apt
+to get spilled in transit, hence a candle lamp is the more {79} convenient
+for a travelling lecturer, while oil is to be preferred if transport is not
+a factor to be considered.
+
+These lamps are usually constructed with a red flashing signal at the rear,
+actuated by a simple lever, by which the lecturer can communicate his wish
+for a change of slide, &c., to the lanternist (Fig. 47).
+
+[Illustration: FIG. 47.--Reading Lamp.]
+
+There are various other devices used for the same purpose, such as a
+castanet, to be held in the lecturer's hand and clicked when necessary, an
+electric bell to ring in the lantern box, &c. If this latter is used it is
+usual to remove the gong, the buzz of the hammer being sufficiently loud
+without it. Some lecturers again prefer to use no such apparatus at all,
+but simply to say 'Next slide' as required, or to tap on the floor with a
+pointer, and the choice of a suitable means of communication between
+lecturer and lanternist must be largely a matter of individual selection.
+More elaborate _reading-desks_ are also supplied by most makers, but here
+again judgment must largely come into play in what is hardly a technical
+matter.
+
+       *       *       *       *       *
+
+
+CHAPTER X
+
+SCREENS AND SCREEN STANDS
+
+The best of all screens for lantern purposes is undoubtedly a smooth
+whitewashed wall, and this is now provided in many halls where lantern
+exhibitions are usual. In places where this is not practicable the next
+best substitute is a canvas {80} screen, which rolls up and down (Fig. 48).
+This can be obtained from any good maker, but again can only really be used
+as a _fixture_ in the hall where the lantern is to be used. It can,
+however, be fitted into a wooden box which can be painted or varnished to
+suit the other architecture, and the provision of such a screen is to be
+strongly recommended whenever possible. If portability is required, a linen
+or calico sheet that can be folded up is necessary, but this can never be
+hung absolutely flat, and also loses a considerable amount of light by
+transmission.
+
+[Illustration: FIG. 48.--Roller Screen.]
+
+A so-called 'transparent' sheet is made of very thin linen, and intended to
+work with the lantern _behind_ it, showing the picture through the linen to
+the audience on the other side, but this is seldom used except in the open
+air for religious or political meetings, &c.
+
+An _opaque_ sheet can be had in one piece up to 9 feet square; larger sizes
+than this must have at least one seam, and most skilful sewing is
+necessary, especially with large sheets consisting of several strips sewn
+together.
+
+Sheets such as these are usually supplied with either eyelet holes round
+the edges or else linen tapes sewn on, and the exact method of hanging must
+be left to circumstances. {81}
+
+In the case of a small sheet it will be sufficient to stretch it at the
+four corners, and this can often be done by screwing into the walls or some
+convenient girder two screw eyes and similar eyes into the floor, all four
+being considerably farther apart than the size of the sheet.
+
+[Illustration: FIG. 49.--Portable Screen Stand.]
+
+A stout cord being then passed through the two upper eyes, long enough for
+both ends to reach near the floor, one end of each can be fastened to the
+two top corners of the sheet and the latter drawn up, the two bottom
+corners being afterwards stretched and tied down tightly to the lower eyes.
+In the case of large sheets this hardly suffices, and it will be found
+necessary to fasten the sheet at intervals all round or it will exhibit
+awkward creases, and this again is a matter where the lanternist must use
+his own initiative according to the possibilities.
+
+In some halls the erection of a sheet in the way above described is a sheer
+impossibility, and in such cases a frame must be made by nailing strips of
+wood together, or better by utilising a portable screen stand (Fig. 49).
+
+These stands are usually made of bamboo, with short brass connecting tubes,
+and the method of using them is so obvious that a description need hardly
+be given. The screen frames are supplied by all the leading opticians, but
+an intending purchaser would be well advised to see one erected before
+ordering. I have actually seen a 12-foot screen frame offered for sale that
+was too weak to carry its own weight, let alone the weight of the sheet!
+
+       *       *       *       *       *
+
+
+{82}
+
+CHAPTER XI
+
+THE PRACTICAL MANIPULATION OF A LANTERN
+
+Having now described the optical lantern in its various forms and the more
+important accessories, we come to the question of practical manipulation.
+In making arrangements for an exhibition the first thing to be seen to is
+to ensure that every accessory that will be required will be there, and the
+best plan is to make a complete list of all sundries to be provided. Such
+items as string (for the sheet), lime tongs if limelight is used, pliers
+for changing carbons if the arc is to be the illuminant, screw-driver,
+matches, the _key of the lantern box_, and other similar items, are likely
+to be left behind unless such a list is made and carefully checked. On
+arriving at the hall, the first thing to be done as a rule is to get up the
+sheet, after which the professional operator generally begins to feel happy
+again.
+
+The next thing, if it has not been done first, is to determine the position
+of the lantern, and this, as has been explained in Chapter VII, is a matter
+of the size of picture to be shown and the focus of the objective.
+
+It is a mistake to show too large a picture; a little 'white' round the
+edges is a good thing, and it is better to have a small disc well
+illuminated than a large one less bright. Convenience, however, must also
+be considered, and it is often justifiable to go back a few feet farther
+than other considerations would dictate in order to place the lantern in a
+gallery or other spot where it is out of the way.
+
+Having fixed the position of the lantern, it should be got into place, the
+cable or tubing connected or whatever else is necessary, according to the
+illuminant to be used. It should then be lit up, the flasher of the lens
+opened, and the light {83} centred sufficiently to produce some sort of
+disc upon the screen. (It is, of course, presumed that the lenses, &c.,
+have previously been cleaned.)
+
+A carrier should now be placed in the stage and a slide inserted into it,
+and the method of doing so requires a little explanation. The slide must be
+placed in the carrier upside down, as will be obvious to anyone who has
+studied Chapter VII, but in addition to this it must be turned the correct
+way, otherwise the picture will be reversed from left to right. This in the
+case of certain subjects, such as a copy of a picture, may not greatly
+matter; but in slides depicting buildings or landscapes with which the
+audience may be familiar, or worse still, printing or writing, is a serious
+blunder.
+
+Slides made by a commercial firm will usually be 'spotted,' that is to say,
+will have two white spots on the face of the slide when the latter is
+viewed in its correct position, and at the top. The slides should be turned
+upside down and placed in the carrier with the spots, of course, now at the
+bottom and _towards the condenser_.
+
+If a slide is not spotted it should be viewed as it is to appear on the
+screen, and then placed in the carrier with the face that was towards the
+operator as he viewed it turned to the condenser, and of course inverted.
+
+The above remarks apply only in cases where the image is thrown _on_ the
+screen; in the comparatively rare instances where it is shown _through_ the
+latter the slides must be turned round laterally, but of course still
+inverted. The slide having been placed in the stage it should be 'focussed'
+by racking the objective in or out, and if necessary pulling out the draw
+tube as well until the image on the screen is sharply defined. So far the
+light has only been roughly centred, sufficiently so to enable the slide to
+be focussed, and to complete the operation both slide and carrier should
+next be taken out of the lantern, leaving a clear disc on the screen, and
+this disc may resemble any of the appearances shown in Fig. 50. {84}
+
+If it resembles A the light must be moved to the left, if like B to the
+right, like C it must be lowered, like D it must be raised, always moving
+it to the side opposite to the dark shade until this is central on the
+disc. If it now resembles E, the light must be moved nearer the condenser;
+if, on the contrary, the centre is dark, it must be drawn back until
+finally the circle should be as nearly as possible clear and bright all
+over, as at F.
+
+[Illustration: FIG. 50.--Adjustment of the Light.]
+
+It is important to note that this adjustment _cannot_ be properly made
+while a slide is in position, and neither can it be made until the lantern
+has been focussed, so the above procedure is the only way to get a
+satisfactory result. With some of the larger illuminants, such as a
+paraffin-oil lamp, there are no centering adjustments, the size of the
+radiant rendering exact centering unnecessary, and generally speaking the
+smaller the luminous point, the more exact must the operation be.
+
+In the case of such illuminants as acetylene or limelight {85} care must be
+taken that they are turned fully on before centering, otherwise turning on
+the fuller amount afterwards will raise the position of the luminous spot.
+
+The centering achieved, the slide carrier may be replaced, the first slides
+placed in position, the remainder arranged in their proper order, the
+system of signalling with the lecturer determined, and all is ready.
+
+If there is still an interval before commencing, the light may be switched
+off or turned out, or in the case say of limelight, turned down very low
+until wanted.
+
+It is of extreme importance to see that all the slides are in their right
+order, though the duty of seeing to this usually rests with the lecturer
+rather than with the operator. I remember hearing of one lecture on the
+life of Queen Victoria, when the lecturer announced, 'The next picture will
+be a photograph of the Royal Prince who for many years shared the Throne
+with our gracious Sovereign.' At the words the operator brought on the next
+slide, which proved to be _a restored specimen of a prehistoric monster_
+(tableaux!). Such mistakes 'bring down the house,' but in serious lectures,
+and especially at religious services, cannot be too carefully guarded
+against.
+
+Mention has already been made of the liability of moisture to condense on
+the surfaces of the condensers or slides, and to avoid this, so far as the
+condensers are concerned, it is well to light up say ten minutes before the
+lantern is actually wanted, or alternatively to take out the condensers and
+thoroughly warm them in front of a stove, or to place them wrapped in a
+cloth on hot-water pipes. The slides should in the same manner be warmed
+before using and should be finally held above the lantern or placed on the
+top, if this is flat, the last thing before being placed in the carrier. If
+these precautions are omitted, on a cold night the first surface of the
+condenser will become so covered with moisture as to almost obscure the
+slide, and this will quickly disappear {86} with the heat of the lantern.
+Next, the two inner surfaces of the condensers will behave in turn in the
+same way, and will take considerably longer to clear, especially if the
+ventilation of the condenser is poor; then the fourth surface will take up
+the running, and finally, when the lanternist is congratulating himself
+that the trouble is over, each successive slide will become affected in the
+same way. With an operator who knows his business, none of these troubles
+should occur.
+
+ACCIDENTS.--These will occur sometimes, even in the best managed
+exhibition; the rubber tubing feeding a limelight jet gets kinked or
+trodden on, or a fuse melts if electric light is being used, &c., and out
+goes the light. In such cases a loud request such as, 'Would you mind
+turning up the light for a minute, please,' accompanied by a good-humoured
+laugh, usually allays the fears of 'nervy' people. An operator must never
+get 'nervy' himself. I have known of more than one fiasco because some
+little hitch occurred, and two or three timid ladies crowded round and
+asked anxious questions, till the lanternist lost his head. In one such
+case the cautious superintendent at a children's entertainment decided that
+it would be safer not to have the exhibition at all, simply because a
+regulator was not screwed tightly enough into a cylinder to prevent an
+escape of gas, only the operator (a somewhat youthful one) had been driven
+to the verge of lunacy by continual questions of the standard type, 'Are
+you sure it is safe?' 'Will it blow up?' 'Are you certain you understand
+it?' &c., &c. More serious accidents, such as the entire lantern getting
+upset, ought never to occur, and it is up to the lanternist to take
+whatever precautions he deems necessary to safeguard his instrument. With a
+juvenile audience, for example, it is often a good thing to arrange a
+barricade of forms round the lantern and to see that no one comes within
+it.
+
+Finally, 'whatever is worth doing at all is worth doing well,' and this is
+as true of lantern exhibiting as of anything {87} else. There are a
+deplorable number of lantern exhibitions given with the sheet hanging in
+creases, dirty lenses, light poorly adjusted and centred, and occasionally
+slides shown upside down. A conscientious lanternist should see to _every_
+detail; slipshod methods, as in everything else, mean poor results.
+
+  Printed by SPOTTISWOODE, BALLANTYNE & CO. LTD.
+  Colchester, London & Eton, England
+
+
+
+
+
+
+End of the Project Gutenberg EBook of Optical Projection, by 
+Lewis Wright and Russell S. Wright
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