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diff --git a/33899-8.txt b/33899-8.txt new file mode 100644 index 0000000..3723904 --- /dev/null +++ b/33899-8.txt @@ -0,0 +1,3090 @@ +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. 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