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diff --git a/41958-0.txt b/41958-0.txt index 53cec16..3deafe7 100644 --- a/41958-0.txt +++ b/41958-0.txt @@ -1,38 +1,4 @@ -The Project Gutenberg EBook of The Microscope, by Andrew Ross - -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: The Microscope - -Author: Andrew Ross - -Release Date: January 31, 2013 [EBook #41958] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE MICROSCOPE *** - - - - -Produced by Chris Curnow, Matthew Wheaton and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - +*** START OF THE PROJECT GUTENBERG EBOOK 41958 *** THE MICROSCOPE. @@ -1517,361 +1483,4 @@ adopted on the occasion just mentioned with perfect success. 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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: The Microscope - -Author: Andrew Ross - -Release Date: January 31, 2013 [EBook #41958] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE MICROSCOPE *** - - - - -Produced by Chris Curnow, Matthew Wheaton and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - THE MICROSCOPE. - - BEING THE ARTICLE CONTRIBUTED BY - - ANDREW ROSS - - - TO THE "PENNY CYCLOPÆDIA," PUBLISHED BY THE SOCIETY - FOR THE DIFFUSION OF USEFUL KNOWLEDGE. - - FULLY ILLUSTRATED. - - - NEW YORK: - THE INDUSTRIAL PUBLICATION COMPANY. - 1877. - - - - -THE MICROSCOPE. - - -Microscope, the name of an instrument for enabling the eye to see -distinctly objects which are placed at a very short distance from it, -or to see magnified images of small objects, and therefore to see -smaller objects than would otherwise be visible. The name is derived -from the two Greek words, expressing this property, MIKROS, _small_, -and SKOPEO, _to see_. - -So little is known of the early history of the microscope, and so -certain is it that the magnifying power of lenses must have been -discovered as soon as lenses were made, that there is no reason for -hazarding any doubtful speculations on the question of discovery. We -shall proceed therefore at once to describe the simplest forms of -microscopes, to explain their later and more important improvements, -and finally to exhibit the instrument in its present perfect state. - -In doing this we shall assume that the reader is familiar with the -information contained in the articles "Light," "Lens," "Achromatic," -"Aberration," and the other sub-divisions of the science of Optics, -which are treated of in this work. - -The use of the term _magnifying_ has led many into a misconception of -the nature of the effect produced by convex lenses. It is not always -understood that the so-called magnifying power of a lens applied to -the eye, as in a microscope, is derived from its enabling the eye to -approach more nearly to its object than would otherwise be compatible -with distinct vision. The common occurrence of walking across the -street to read a bill is in fact magnifying the bill by approach; and -the observer, at every step he takes, makes a change in the optical -arrangement of his eye, to adapt it to the lessening distance between -himself and the object of his inquiry. This power of spontaneous -adjustment is so unconsciously exerted, that unless the attention be -called to it by circumstances, we are totally unaware of its exercise. - -In the case just mentioned the bill would be read with eyes in a very -different state of adjustment from that in which it was discovered on -the opposite side of the street, but no conviction of this fact would -be impressed upon the mind. If, however, the supposed individual -should perceive on some part of the paper a small speck, which he -suspects to be a minute insect, and if he should attempt a very close -approach of his eye for the purpose of verifying his suspicion, he -would presently find that the power of natural adjustment has a limit; -for when his eye has arrived within about ten inches, he will discover -that a further approach produces only confusion. But if, as he -continues to approach, he were to place before his eye a series of -properly arranged convex lenses, he would see the object gradually and -distinctly increase in apparent size by the mere continuance of the -operation of approaching. Yet the glasses applied to the eye during -the approach from ten inches to one inch, would have done nothing more -than had been previously done by the eye itself during the approach -from fifty feet to one foot. In both cases the magnifying is effected -really by the approach, the lenses merely rendering the latter periods -of the approach compatible with distinct vision. - -A very striking proof of this statement may be obtained by the -following simple and instructive experiment. Take any minute object, a -very small insect for instance, held on a pin or gummed to a slip of -glass; then present it to a strong light, and look at it through the -finest needle-hole in a blackened card placed about an inch before it. -The insect will appear quite distinct, and about ten times larger than -its usual size. Then suddenly withdraw the card without disturbing the -object, which will instantly become indistinct and nearly invisible. -The reason is, that the naked eye cannot see at so small a distance as -one inch. But the card with the hole having enabled the eye to -approach within an inch, and to see distinctly at that distance, is -thus proved to be as decidedly a magnifying instrument as any lens or -combination of lenses. - -This description of magnifying power does not apply to such -instruments as the solar or gas microscope, by which we look not at -the object itself, but at its shadow or picture on the wall; and the -description will require some modification in treating of the compound -microscope, where, as in the telescope, an image or picture is formed -by one lens, that image or picture being viewed as an original object -by another lens. - -It is nevertheless so important to obtain a clear notion of the real -nature of the effect produced by a lens applied to the eye, that we -will adduce the instance of spectacles to render the point more -familiar. If the person who has been supposed to cross the street for -the purpose of reading a bill had been aged, the limit to the power of -adjustment would have been discovered at a greater distance, and -without so severe a test as the supposed insect. The eyes of the very -aged generally lose the power of adjustment at a distance of thirty or -forty inches instead of ten, and the spectacles worn in consequence -are as much magnifying glasses to them as the lenses employed by -younger eyes to examine the most minute objects. Spectacles are -magnifying glasses to the aged because they enable such persons to see -as closely to their objects as the young, and therefore to see the -objects larger than they could themselves otherwise see them, but not -larger than they are seen by the unassisted younger eye. - -In saying that an object appears larger at one time, or to one person, -than another, it is necessary to guard against misconception. By the -apparent size of an object we mean the angle it subtends at the eye, -or the angle formed by two lines drawn from the centre of the eye to -the extremities of the object. In Fig. 1, the lines A E and B E drawn -from the arrow to the eye form the angle A E B, which, when the angle -is small, is nearly twice as great as the angle C E D, formed by lines -drawn from a similar arrow at twice the distance. The arrow A B will -therefore appear nearly twice as long as C D, being seen under twice -the angle, and in the same proportion for any greater or lesser -difference in distance. The angle in question is called the angle of -vision, or the visual angle. - -[Illustration: Fig. 1.] - -The angle of vision must, however, not be confounded with the angle of -the pencil of light by which an object is seen, and which is explained -in Fig. 2. Here we have drawn two arrows placed in relation to the eye -as before, and from the centre of each have drawn lines exhibiting the -quantity of light which each point will send into the eye at the -respective distances. - -[Illustration: Fig. 2.] - -Now if E F represent the diameter of the pupil, the angle E A F shows -the size of the cone or pencil of light which enters the eye from the -point A, and in like manner the angle E B F is that of the pencil -emanating from B, and entering the eye. Then, since E A F is double E -B F, it is evident that A is seen by four times the quantity of light -which could be received from an equally illuminated point at B; so -that the nearer body would appear brighter if it did not appear -larger; but as its apparent area is increased four times as well as -its light, no difference in this respect is discovered. But if we -could find means to send into the eye a larger pencil of light, as for -instance that shown by the lines G A H, without increasing the -apparent size in the same proportion, it is evident that we should -obtain a benefit totally distinct from that of increased magnitude, -and one which is in some cases of even more importance than size in -developing the structure of what we wish to examine. This, it will be -hereafter shown, is sometimes done; for the present, we wish merely to -explain clearly the distinction between apparent magnitude, or the -angle under which the object is seen, and apparent brightness, or the -angle of the pencil of light by which each of its points is seen, and -with these explanations we shall continue to employ the common -expressions magnifying glass and magnifying power. - -[Illustration: Fig. 3.] - -The magnifying power of a single lens depends upon its focal length, -the object being in fact placed nearly in its principal focus, or so -that the light which diverges from each point may, after refraction by -the lens, proceed in parallel lines to the eye, or as nearly so as is -requisite for distinct vision. In Fig. 3, A B is a double convex lens, -near which is a small arrow to represent the object under examination, -and the cones drawn from its extremities are portions of the rays of -light diverging from those points and falling upon the lens. These -rays, if suffered to fall at once upon the pupil, would be too -divergent to permit their being brought to a focus upon the retina by -the optical arrangements of the eye. But being first passed through -the lens, they are bent into nearly parallel lines, or into lines -diverging from some points within the limits of distinct vision, as -from C and D. Thus altered, the eye receives them precisely as if they -emanated from a larger arrow placed at C D, which we may suppose to be -ten inches from the eye, and then the difference between the real and -the imaginary arrow is called the magnifying power of the lens in -question. - -From what has been said it will be evident that two persons whose eyes -differed as to the distance at which they obtained distinct vision, -would give different results as to the magnifying power of a lens. To -one who can see distinctly with the naked eye at a distance of five -inches, the magnifying power would seem and would indeed be only half -what we have assumed. Such instances are, however, rare; the focal -length of the eye usually ranges from six to twelve or fourteen -inches, so that the distance we first assumed of ten inches is very -near the true average, and is a convenient number, inasmuch as a -cipher added to the denominator of the fraction which expresses the -focal length of a lens gives its magnifying power. Thus a lens whose -focal length is one-sixteenth of an inch is said to magnify 160 times. - -When the focal length of a lens is very small, it is difficult to -measure accurately the distance between its centre and its object. In -such cases the best way to obtain the focal length for parallel or -nearly parallel rays is to view the image of some distant object -formed by the lens in question through another lens of one inch solar -focal length, keeping both eyes open and comparing the image presented -through the two lenses with that of the naked eye. The proportion -between the two images so seen will be the focal length required. Thus -if the image seen by the naked eye is ten times as large as that shown -by the lenses, the focal length of the lens in question is one-tenth -of an inch. The panes of glass in a window, or courses of bricks in a -wall, are convenient objects for this purpose. - -In whichever way the focal length of the lens is ascertained, the -rules given for deducing its magnifying power are not rigorously -correct, though they are sufficiently so for all practical purposes, -particularly as the whole rests on an assumption in regard to the -focal length of the eye, and as it does not in any way affect the -actual measurement of the object. To calculate with great precision -the magnifying power of a lens with a given focal length of eye, it is -necessary that the thickness of the lens be taken into the account, -and also the focal length of the eye itself. - -We have hitherto considered a magnifying lens only in reference to its -enlargement of the object, or the increase of the angle under which -the object is seen. A further and equally important consideration is -that of the number of rays or quantity of light by which every point -of the object is rendered visible. The naked eye, as shown in Fig. 2, -admits from each point of every visible object a cone of light having -the diameter of the pupil for its base, and most persons are familiar -with that beautiful provision by which in cases of excessive -brilliancy the pupil spontaneously contracts to reduce the cone of -admitted light within bearable limits. This effect is still further -produced in the experiment already described, of looking at an object -through a needle-hole in a card, which is equivalent to reducing the -pupil to the size of a needle-hole. Seen in this way the object -becomes comparatively dark or obscure; because each point is seen by -means of a very small cone of light, and a little consideration will -suffice to explain the different effects produced by the needle-hole -and the lens. Both change the angular value of the cone of light -presented to the eye, but the lens changes the angle by bending the -extreme rays within the limits suited to distinct vision, while the -needle-hole effects the same purpose by cutting off the rays which -exceed those limits. - -It has been shown that removing a brilliant object to a greater -distance will reduce the quantity of light which each point sends into -the eye, as effectually as viewing it through a needle-hole; and -magnifying an object by a lens has been shown to be the same thing in -some respects as removing it to a greater distance. We have to see the -magnified picture by the light emanating from the small object, and it -becomes a matter of difficulty to obtain from each point a sufficient -quantity of light to bear the diffusion of a great magnifying power. -We want to perform an operation just the reverse of applying the card -with the needle-hole to the eye--we want in some cases to bring into -the eye the largest possible pencil of light from each point of the -object. - -Referring to Fig. 3, it will be observed that if the eye could see the -small arrow at the distance there shown without the intervention of -the lens, only a very small portion of the cones of light drawn from -its extremities would enter the pupil; whereas we have supposed that -after being bent by the lens the whole of this light enters the eye as -part of the cones of smaller angle whose summits are at C and D. These -cones will further explain the difference between large and small -pencils of light; those from the small arrow are large pencils; the -dotted cones from the large arrow are small pencils. - -In assuming that the whole of this light could have been suffered to -enter the eye through the lens A B, we did so for the sake of not -perplexing the reader with too many considerations at once. He must -now learn that so large a pencil of light passing through a single -lens would be so distorted by the spherical figure of the lens, and by -the chromatic dispersion of the glass, as to produce a very confused -and imperfect image. This confusion may be greatly diminished by -reducing the pencil; for instance, by applying a stop, as it is -called, to the lens, which is neither more nor less than the -needle-hole applied to the eye. A small pencil of light may be thus -transmitted through a single lens without suffering from spherical -aberration or chromatic dispersion any amount of distortion which will -materially affect the figure of the object; but this quantity of light -is insufficient to bear diffusion over the magnified picture, which is -therefore too obscure to exhibit what we most desire to see--those -beautiful and delicate markings by which one kind of organic matter is -distinguished from another. With a small aperture these markings are -not seen at all: with a large aperture and a single lens they exhibit -a faint nebulous appearance enveloped in a chromatic mist, a state -which is of course utterly valueless to the naturalist, and not even -amusing to the amateur. - -It becomes therefore a most important problem to reconcile a large -aperture with distinctness, or, as it is called, _definition_; and -this has been done in a considerable degree by effecting the required -amount of refraction through two or more lenses instead of one, thus -reducing the angles of incidence and refraction, and producing other -effects which will be shortly noticed. This was first accomplished in -a satisfactory manner by-- - - DR. WOLLASTON'S DOUBLET, - -invented by the celebrated philosopher whose name it bears; it -consists of two plano-convex lenses (Fig. 4) having their focal -lengths in the proportion of 1 to 3, or nearly so, and placed at a -distance which can be ascertained best by actual experiment. Their -plane sides are placed towards the object, and the lens of shortest -focal length next the object. - -[Illustration: Fig. 4.] - -It appears that Dr. Wollaston was led to this invention by considering -that the Achromatic Huyghenean Eye-piece, which will be hereafter -described, would, if reversed, possess similar good properties as a -simple microscope. But it will be evident when the eye-piece is -understood, that the circumstances which render it achromatic are very -imperfectly applicable to the simple microscope, and that the doublet, -without a nice adjustment of the stop, would be valueless. Dr. -Wollaston makes no allusion to a stop, nor is it certain that he -contemplated its introduction, although his illness, which terminated -fatally soon after the presentation of his paper, may account for the -omission. - -The nature of the corrections which take place in the doublet is -explained in the annexed diagram (Fig. 5), where L O L´ is the object, -P a portion of the pupil, and D D the stop, or limiting aperture. - -Now, it will be observed that each of the pencils of light from the -extremities L L´ of the object is rendered eccentrical by the stop, -and of consequence each passes through the two lenses on opposite -sides of their common axis O P; thus each becomes affected by opposite -errors, which to some extent balance and correct each other. To take -the pencil L, for instance, which enters the eye at R B, R B; it is -bent to the right at the first lens, and to the left at the second; -and as each bending alters the direction of the blue rays more than -the red, and, moreover, as the blue rays fall nearer the margin of the -second lens, where the refraction, being more powerful than near the -centre, compensates in some degree for the greater focal length of the -second lens, the blue and red rays will emerge very nearly parallel, -and of consequence colorless to the eye. At the same time the -spherical aberration has been diminished by the circumstance that the -side of the pencil which passes one lens nearest the axis passes the -other nearest the margin. - -This explanation applies only to the pencils near the extremities of -the object. The central pencil, it is obvious, would pass both lenses -symmetrically; the same portions of light occupying nearly the same -relative places on both lenses. The blue light would enter the second -lens nearer to its axis than the red, and being thus less refracted -than the red by the second lens, a small amount of compensation would -take place, quite different in principle and inferior in degree to -that which is produced in the eccentrical pencils. In the intermediate -spaces the corrections are still more imperfect and uncertain; and -this explains the cause of the aberrations which must of necessity -exist even in the best-made doublet. It is, however, infinitely -superior to a single lens, and will transmit a pencil of an angle of -from 35° to 50° without any very sensible errors. It exhibits, -therefore, many of the usual test-objects in a very beautiful manner. - -[Illustration: Fig. 5.] - -[Illustration: Fig. 6.] - -The next step in the improvement of the simple microscope bears more -analogy to the eye-piece. This improvement was made by Mr. Holland, -and it consists (as shown in Fig. 6) in substituting two lenses for -the first in the doublet, and retaining the stop between them and the -third. The first bending, being thus effected by two lenses instead of -one, is accompanied by smaller aberrations, which are therefore more -completely balanced or corrected at the second bending, in the -opposite direction, by the third lens. This combination, though called -a triplet is essentially a doublet, in which the anterior lens is -divided into two. For it must be recollected that the first pair of -lenses merely accomplishes what might have been done, though with less -precision, by one; but the two lenses of the doublet are opposed to -each other; the second diminishing the magnifying power of the first. -The first pair of lenses in the triplet concur in producing a certain -amount of magnifying power, which is diminished in quantity and -corrected as to aberration at the third lens by the change in relation -to the position of the axis which takes place in the pencil between -what is virtually the first and second lens. In this combination the -errors are still further reduced by the close approximation to the -object which causes the refractions to take place near the axis. Thus -the transmission of a still larger angular pencil, namely 65°, is -rendered compatible with distinctness, and a more intense image is -presented to the eye. - -Every increase in the number of lenses is attended with one drawback, -from the circumstance that a certain portion of light is lost by -reflection and absorption each time that the ray enters a new medium. -This loss bears no sensible proportion to the gain arising from the -increased aperture, which, being as the square of the diameter, -multiplies rapidly; or, if we estimate by the angle of the admitted -pencil, which is more easily ascertained, the intensity will be as the -square of twice the tangent of half the angle. To explain this, let D -B (Fig. 7) represent the diameter of the lens, or of that part of it -which is really employed; C A the perpendicular drawn from its -centre, and A B, A D, the extreme rays of the incident pencil of light -DAB. Then the diameter being 2 C B, the area to which the intensity of -vision is proportional will be (2 C B)², and C B is evidently the -tangent of the angle C A B, which is half the angle of the admitted -pencil D A B. Or, if _a_ be used to denote the angular aperture, the -expression for the intensity is (2 tan. ½_a_)² which increases so -rapidly with the increase of _a_ as to make the loss of light by -reflection and absorption of little consequence. - -[Illustration: Fig. 7.] - -The combination of three lenses approaches, as has been stated, very -close to the object; so close, indeed, as to prevent the use of more -than three; and this constitutes a limit to the improvement of the -simple microscope, for it is called a simple microscope, although -consisting of three lenses, and although a compound microscope may be -made of only three or even two lenses; but the different arrangement -which gives rise to the term compound will be better understood when -that instrument is explained. - -Before we proceed to describe the simple microscope and its -appendages, it will be well to explain such other points in reference -to the form and materials of lenses as are most likely to be -interesting. - -A very useful form of lens was proposed by Dr. Wollaston, and called -by him the Periscopic lens. It consisted of two hemispherical lenses, -cemented together by their plane faces, having a stop between them to -limit the aperture. A similar proposal was made Mr. Coddington, who, -however, executed the project in a better manner, by cutting a groove -in a whole sphere, and filling the groove with opaque matter. His -lens, which is the well-known Coddington lens, is shown in Fig. 8. It -gives a large field of view, which is equally good in all directions, -as it is evident that the pencils A A and B B pass through under -precisely the same circumstances. Its spherical form has the further -advantage of rendering the position in which it is held of -comparatively little consequence. It is therefore very convenient as -a hand-lens, but its definition is of course not so good as that of a -well-made doublet or achromatic lens. - -[Illustration: Fig. 8.] - -Another very useful form of doublet was proposed by Sir John Herschel, -chiefly like the Coddington lens, for the sake of a wide field, and -chiefly to be used in the hand. It is shown in Fig. 9; it consists of -a double convex or crossed lens, having the radii of curvature as 1 to -6, and of a plane concave lens whose focal length is to that of the -convex lens as 13 to 5. - -Various, indeed innumerable, other forms and combinations of lenses -have been projected, some displaying much ingenuity, but few of any -practical use. In the Catadioptric lenses the light emerges at right -angles from its entering direction, being reflected from a surface cut -at an angle of 45 degrees to the axes of the curved surfaces. - -[Illustration: Fig. 9.] - -It was at one time hoped, as the precious stones are more refractive -than glass, and as the increased refractive power is unaccompanied by -a correspondent increase in chromatic dispersion, that they would -furnish valuable materials for lenses, inasmuch as the refractions -would be accomplished by shallower curves, and consequently with -diminished spherical aberration. But these hopes were disappointed; -everything that ingenuity and perseverance could accomplish was tried -by Mr. Varley and Mr. Pritchard, under the patronage of Dr. Goring. It -appeared, however, that the great reflective power, the -doubly-refracting property, the color, and the heterogeneous structure -of the jewels which were tried, much more than counterbalanced the -benefits arising from their greater refractive power, and left no -doubt of the superiority of skillfully made glass doublets and -triplets. The idea is now, in fact, abandoned; and the same remark is -applicable to the attempts at constructing fluid lenses, and to the -projects for giving to glass other than spherical surfaces--none of -which have come into extensive use. - -By the term _simple_ microscope is meant one in which the object is -viewed directly through a lens or combination of lenses, just as we -have supposed an arrow or an insect to be viewed through a glass held -in the hand. When, however, the magnifying power of the glass is -considerable, in other words, when its focal length is very short, and -its proper distance from its object of consequence equally short, it -requires to be placed at that proper distance with great precision: it -cannot, therefore, be held with sufficient accuracy and steadiness by -the unassisted hand, but must be mounted in a frame having a rack or -screw to move it towards or from another frame or stage which holds -the object. It is then called a microscope, and it is furnished, -according to circumstances, with lenses and mirrors to collect and -reflect the light upon the object, and with other conveniences which -will now be described. - -One of the best forms of a stand for a simple microscope is shown in -Fig. 10, where A is a brass pillar screwed to a tripod base; B is a -broad stage for the objects, secured to the stem by screws, whose -milled heads are at C. By means of the large milled head D, a -triangular bar, having a rack, is elevated out of the stem A, carrying -the lens-holder E, which has a horizontal movement in one direction, -by means of a rack worked by the milled head F, and in the other -direction by turning on a circular pin. A concave mirror G reflects -the light upwards through the hole in the stage, and a lens may be -attached to the stage for the purpose of throwing light on an opaque -object, in the same way that the forceps H for holding such objects is -attached. This microscope is peculiarly adapted, by its broad stage -and its general steadiness, for dissecting; and it is rendered more -convenient for this purpose by placing it between two inclined planes -of mahogany, which support the arms and elevate the wrists to the -level of the stage. This apparatus is called the dissecting rest. When -dissecting is not a primary object, a joint may be made at the lower -end of the stem A, to allow the whole to take an inclined position; -and then the spring clips shown upon the stage are useful to retain -the object in its place. Numerous convenient appendages may be made to -accompany such microscopes, which it will be impossible to mention in -detail; the most useful are Mr. Varley's capillary cages for -containing animalculæ in water, and parts of aquatic plants; also his -tubes for obtaining and separating such objects, and his phial and -phial-holder for preserving and exhibiting small living specimens of -the Chara, Nitella, and other similar plants, and observing their -circulation. The phial-microscope affords facilities for observing the -operations of minute vegetable and animal life, which will probably -lead to the most interesting discoveries. The recent volumes of the -Transactions of the Society of Arts contain an immense mass of -information of this sort, and to these we refer the reader. - -[Illustration: Fig. 10.] - -The mode of illuminating objects is one on which we must give some -further information, for the manner in which an object is lighted is -second in importance only to the excellence of the glass through which -it is seen. In investigating any new or unknown specimen, it should be -viewed in turns by every description of light, direct and oblique, as -a transparent object and as an opaque object, with strong and with -faint light, with large angular pencils and with small angular pencils -thrown in all possible directions. Every change will probably develop -some new fact in reference to the structure of the object, which -should itself be varied in the mode of mounting in every possible way. -It should be seen both wet and dry, and immersed in fluids of various -qualities and densities, such as water, alcohol, oil, and Canada -balsam, for instance, which last has a refractive power nearly equal -to that of glass. If the object be delicate vegetable tissue, it will -be in some respects rendered more visible by gentle heating or -scorching by a clear fire placed between two plates of glass. In this -way the spiral vessels of asparagus and other similar vegetables may -be beautifully displayed. Dyeing the objects in tincture of iodine -will in some cases answer this purpose better. - -But the principal question in regard to illumination is the magnitude -of the illuminating pencil, particularly in reference to transparent -objects. Generally speaking the illuminating pencil should be as large -as can be received by the lens, and no larger. Any light beyond this -produces indistinctness and glare. The superfluous light from the -mirror may be cut off by a screen having various-sized apertures -placed below the stage; but the best mode of illumination is that -proposed by Dr. Wollaston, and called the Wollaston condenser. A tube -is placed below the stage of the instrument containing a lens A B -(Fig. 11), which can be elevated or depressed within certain limits at -pleasure; and at the lower end is a stop with a limited aperture C D. -A plane mirror E F receives the rays of light L L from the sky or a -white cloud, which last is the best source of light, and reflects them -upwards through the aperture in C D, so that they are refracted, and -form an image of the aperture at G, which is supposed to be nearly -the place of the object. The object is sometimes best seen when the -image of the aperture is also best seen; and sometimes it is best to -elevate the summit G of the cone A B G above the object, and at others -to depress it below: all which is done at pleasure by the power of -moving the lens A B. If artifical light (as a lamp or candle) be -employed, the flame must be placed in the principal focus of a large -detached lens on a stand, so that the rays L L may fall in parallel -lines on the mirror, or as they would fall from the cloud. This will -be found an advantage, not only when the Wollaston condenser is -employed, but also when the mirror and diaphragm are used. A good mode -of imitating artificially the light of a white cloud opposite the sun -has been proposed by Mr. Varley; he covers the surface of the mirror -under the stage with carbonate of soda or any similar material, and -then concentrates the sun's light upon its surface by a large -condensing lens. The intense white light diffused from the surface of -the soda forms an excellent substitute for the white cloud, which, -when opposite the sun, and of considerable size, is the best daylight, -as the pure sky opposite to the sun is the worst. - -[Illustration: Fig. 11.] - -_The Compound Microscope_ may, as before stated, consist of only two -lenses, while a simple microscope has been shown to contain sometimes -three. In the triplet for the simple microscope, however, it was -explained that the effect of the two first lenses was to do what might -have been accomplished, though not so well, by one; and the third -merely effected certain modifications in the light before it entered -the eye. But in the compound microscope the two lenses have totally -different functions; the first receives the rays from the object, and, -bringing them to new foci, forms an image, which the second lens -treats as an original object, and magnifies it just as the single -microscope magnified the object itself. - -[Illustration: Fig. 12.] - -The annexed figure (12) shows the course of the rays through a -compound microscope of two lenses. The rays proceeding from the object -A B are so acted upon by the lens C D, near it, and thence called the -object glass, that they are converged to foci in A´ B´, where they -form an enlarged image of the object, as would be evident if a piece -of oiled paper or ground glass were placed there to receive them. They -are not so intercepted, and therefore the image is not rendered -visible at that place; but their further progress is similar to what -it would have been had they really proceeded from an object at A´ B´. -They are at length received by the eye-lens L M, which acts upon them -as the simple microscope has been described to act on the light -proceeding from its objects. They are bent so that they may enter the -eye at E in parallel lines, or as nearly so as is requisite for -distinct vision. When we say that the rays enter the eye in nearly -parallel lines, we mean only those which proceed from one point of the -original object. Thus the two parallel rays M E have proceeded from -and are part of the cone of rays C A D, emanating from the point A of -the arrow; but they do not form two pictures in the eye, because any -number of parallel rays which the pupil can receive will be converged -to a point by the eye, and will convey the impression of one point to -the mind. In like manner the rays L E are part of the cone of rays -emanating from B, and the angle L E M is that under which the eye will -see the magnified image of the arrow, which is evidently many times -greater than the arrow could be made to occupy in the naked eye at any -distance within the limits of distinct vision. The magnifying power -depends on two circumstances: first, on the ratio between the anterior -distance A C or B D and the posterior focal length C B´ or D A´; and -secondly, on the power of the eye-lens L M. The first ratio is the -same as that between the object A B and the image A´ B´; this and the -focal length or power of the eye lens are both easily obtained, and -their product is the power of the compound instrument. - -Since the power depends on the ratio between the anterior and -posterior foci of the object-glass, it is evident that by increasing -that ratio any power may be obtained, the same eye-glass being used; -or having determined the first, any further power may be obtained by -increasing that of the eye-glass; and thus, by a pre-arrangement of -the relative proportions in which the magnifying power shall be -divided between the object-glass and the eye-glass, almost any given -distance (within certain limits) between the first and its object may -be secured. This is one valuable peculiarity of the compound -instrument; and another is the large field, or large angle of view, -which may be obtained, every part of which will be nearly equally -good; whereas with the best simple microscopes the field is small, and -is good only in the centre. The field of the compound instrument is -further increased by using two glasses at the eye-end; the first being -called, from its purpose, the field-glass, and the two constituting -what is called the eye-piece. This will be more particularly explained -in the figure of the achromatic compound microscope presently given. - -For upwards of a century the compound microscope, notwithstanding the -advantages above mentioned, was a comparatively feeble and inefficient -instrument, owing to the distance which the light had to traverse, and -the consequent increase of the chromatic and spherical aberrations. To -explain this we have drawn in Fig. 12 a second image near A´ B´, the -fact being that the object-glass would not form one image, as has been -supposed, but an infinite number of variously-colored and -various-sized images, occupying the space between the two dotted -arrows. Those nearest the object-glass would be red, and those nearest -the eye-glass would be blue. The effect of this is to produce so much -confusion, that the instrument was reduced to a mere toy, although -these errors were diminished to the utmost possible extent by limiting -the aperture of the object-glass, and thus restricting the angle of -the pencil of light from each point of the object. But this involved -the defects, already explained, of making the picture obscure, so that -on the whole the best compound instruments were inferior to the simple -microscopes of a single lens, with which, indeed, all the important -observations of the last century were made. - -Even after the improvement of the simple microscope by the use of -doublets and triplets, the long course of the rays, and the large -angular pencil required in the compound instrument, deterred the most -sanguine from anticipating the period when they should be conducted -through such a path free both from spherical and chromatic errors. -Within twenty years of the present period, philosophers of no less -eminence than M. Blot and Dr. Wollaston predicted that the compound -would never rival the simple microscope, and that the idea of -achromatizing its object-glass was hopeless. Nor can these opinions be -wondered at when we consider how many years the achromatic telescope -had existed without an attempt to apply its principles to the compound -microscope. When we consider the smallness of the pencil required by -the telescope, and the enormous increase of difficulty attending every -enlargement of the pencil--when we consider further that these -difficulties had to be contended with and removed by operations on -portions of glass so small that they are themselves almost microscopic -objects, we shall not be surprised that even a cautious philosopher -and most able manipulator like Dr. Wollaston should prescribe limits -to improvement. - -Fortunately for science, and especially for the departments of animal -and vegetable physiology, these predictions have been shown to be -unfounded. The last fifteen years have sufficed to elevate the -compound microscope from the condition we have described to that of -being the most important instrument ever bestowed by art upon the -investigator of nature. It now holds a very high rank among -philosophical implements, while the transcendant beauties of form, -color and organization, which it reveals to us in the minute works of -nature, render it subservient to the most delightful and instructive -pursuits. To these claims on our attention, it appears likely to add a -third of still higher importance. The microscopic examination of the -blood and other human organic matter will in all probability afford -more satisfactory and conclusive evidence regarding the nature and -seat of disease than any hitherto appealed to, and will of consequence -lead to similar certainty in the choice and application of remedies. - -We have thought it necessary to state thus at large the claims of the -modern achromatic microscope upon the attention of the reader, as a -justification of the length at which we shall give its recent history -and explain its construction; and we are further induced to this -course by the consideration that the subject is entirely new ground, -and that there are at this time not more than two or three makers of -achromatic microscopes in England. - -Soon after the year 1820 a series of experiments was begun in France -by M. Selligues, which were followed up by Frauenhofer at Munich, by -Amici at Modena, by M. Chevalier at Paris, and by the late Mr. Tulley -in London. In 1824 the last-named excellent artist, without knowing -what had been done on the Continent, made the attempt to construct an -achromatic object-glass for a compound microscope, and produced one of -nine-tenths of an inch focal length, composed of three lenses, and -transmitting a pencil of eighteen degrees. This was the first that had -been made in England; and it is due to Mr. Tulley to say, that as -regards accurate correction throughout the field, that glass has not -been excelled by any subsequent combination of three lenses. Such an -angular pencil, and such a focal length, would bear an eye-piece -adapted to produce a gross magnifying power of one hundred and twenty. -Mr. Tulley afterwards made a combination to be placed in front of the -first mentioned, which increased the angle of the transmitted pencil -to thirty-eight degrees, and bore a power of three hundred. - -While these practical investigations were in progress, the subject of -achromatism engaged the attention of some of the most profound -mathematicians in England. Sir John Herschel, Professor Airy, -Professor Barlow, Mr. Coddington, and others, contributed largely to -the theoretical examination of the subject; and though the results of -their labors were not immediately applicable to the microscope, they -essentially promoted its improvement. - -For some time prior to 1829 the subject had occupied the mind of a -gentleman, who, not entirely practical, like the first, nor purely -mathematical, like the last-mentioned class of inquirers, was led to -the discovery of certain properties in achromatic combinations which -had been before unobserved. These were afterwards experimentally -verified; and in the year 1829 a paper on the subject, by the -discoverer, Mr. Joseph Jackson Lister, was read and published by the -Royal Society. The principles and results thus obtained enabled Mr. -Lister to form a combination of lenses which transmitted a pencil of -fifty degrees, with a large field correct in every part; as this paper -was the foundation of the recent improvements in achromatic -microscopes, and as its results are indispensable to all who would -make or understand the instrument, we shall give the more important -parts of it in detail, and in Mr. Lister's own words. - -"I would premise that the plano-concave form for the correcting flint -lens has in that quality a strong recommendation, particularly as it -obviates the danger of error which otherwise exists in centering the -two curves, and thereby admits of correct workmanship for a shorter -focus. To cement together also the two surfaces of the glass -diminishes by very nearly half the loss of light from reflection, -which is considerable at the numerous surfaces of a combination. I -have thought the clearness of the field and brightness of the picture -evidently increased by doing this; it prevents any dewiness or -vegetation from forming on the inner surfaces; and I see no -disadvantage to be anticipated from it if they are of identical -curves, and pressed closely together, and the cementing medium -permanently homogeneous. - -"These two conditions then, that the flint lens shall be -plano-concave, and that it shall be joined by some cement to the -convex, seem desirable to be taken as a basis for the microscopic -object-glass, provided they can be reconciled with the destruction of -the spherical and chromatic aberrations of a large pencil. - -"Now in every such glass that has been tried by me which has had its -correcting lens of either Swiss or English glass, with a double convex -of plate, and has been made achromatic by the form given to the outer -curve of the convex, the proportion has been such between the -refractive and dispersive powers of its lenses, that its figure has -been correct for rays issuing from some point in its axis not far from -its principal focus on its plane side, and either tending to a -conjugate focus within the tube of a microscope, or emerging nearly -parallel. - -"Let A B (Fig. 13) be supposed such an object-glass, and let it be -roughly considered as a plano-convex lens, with a curve A C B running -through it, at which the spherical and chromatic errors are corrected -which are generated at the two outer surfaces; and let the glass be -thus free from aberration for rays F D E G issuing from the radiant -point F, H E being a perpendicular to the convex surface, and I D to -the plane one. Under these circumstances, the angle of emergence G E H -much exceeds that of incidence F D I, being probably nearly three -times as great. - -"If the radiant is now made to approach the glass, so that the course -of the ray F D E G shall be more divergent from the axis, as the -angles of incidence and emergence become more nearly equal to each -other, the spherical aberration produced by the two will be found to -bear a less proportion to the opposing error of the single correcting -curve A C B; for such a focus therefore the rays will be -over-corrected. - -[Illustration: Fig. 13.] - -"But if F still approaches the glass, the angle of incidence -continues to increase with the increasing divergence of the ray, till -it will exceed that of emergence, which has in the meanwhile been -diminishing, and at length the spherical error produced by them will -recover its original proportion to the opposite error of the curve of -correction. When F has reached this point F´´ (at which the angle of -incidence does not exceed that of emergence so much as it had at first -come short of it), the rays again pass the glass free from spherical -aberration. - -"If F be carried from hence towards the glass, or outwards from its -original place, the angle of incidence in the former case, or of -emergence in the latter, becomes disproportionately effective, and -either way the aberration exceeds the correction. - -"These facts have been established by careful experiment: they accord -with every appearance in such combinations of the plano-convex glasses -as have come under my notice, and may, I believe, be extended to this -rule, that in general an achromatic object-glass, of which the inner -surfaces are in contact, or nearly so, will have on one side of it two -foci in its axis, for the rays proceeding from which it will be truly -corrected at a moderate aperture; that for the space between these two -points its spherical aberration will be over-corrected, and beyond -them either way under-corrected. - -"The longer aplanatic focus may be found, when one of the plano-convex -object-glasses is placed in a microscope, by shortening the tube, if -the glass shows over-correction; if under-correction, by lengthening -it, or by bringing the rays together, should they be parallel or -divergent, by a very small good telescope. The shorter focus is got at -by sliding the glass before another of sufficient length and large -aperture that is finely corrected, and bringing it forwards till it -gives the reflection of a bright point from a globule of quicksilver, -sharp and free from mist, when the distance can be taken between the -glass and the object. - -"The longer focus is the place at which to ascertain the utmost -aperture that may be given to the glass, and where, in the absence of -spherical error, its exact state of correction as to color is seen -most distinctly. - -"The correction of the chromatic aberration, like that of the -spherical, tends to excess in the marginal rays; so that if a glass -which is achromatic, with a moderate aperture, has its cell opened -wider, the circle of rays thus added to the pencil will be rather -over-corrected as to color. - -"The same tendency to over-correction is produced, if, without varying -the aperture, the divergence of the incident rays is much augmented, -as in an object-glass placed in front of another; but generally in -this position a part only of its aperture comes into use; so that the -two properties mentioned neutralize each other, and its chromatic -state remains unaltered. If, for example, the outstanding colors were -observed at the longer focus to be green and claret, which show that -the nearest practicable approach is made to the union of the spectrum, -they usually continue nearly the same for the whole space between the -foci, and for some distance beyond them either way. - -"The places of these two foci and their proportions to each other -depend on a variety of circumstances. In several object-glasses that I -have had made for trial, plano-convex, with their inner surfaces -cemented, their diameters the radius of the flint lens, and their -color pretty well corrected, those composed of dense flint and light -plate have had the rays from the longer focus emerging nearly -parallel; and this focus has been not quite three times the distance -of the shorter from the glass: with English flint the rays have had -more convergence, and the shorter focus has borne a rather less -proportion to the longer. - -"If the surfaces are not cemented, a striking effect is produced by -minute differences in their curves. It may give some idea of this, -that in a glass of which nearly the whole disk was covered with color -from contact of the lenses, the addition of a film of varnish, so thin -that this color was not destroyed by it, caused a sensible change in -the spherical correction. - -"I have found that whatever extended the longer aplanatic focus, and -increased the convergence of its rays, diminished the relative length -of the shorter. Thus by turning to the concave lens the flatter -instead of the deeper side of a convex lens, whose radii were to each -other as 31 to 35, the pencil of the longer aplanatic focus, from -being greatly divergent, was brought to converge at a very small -distance behind the glass; and the length of the shorter focus, which -had been one-half that of the longer, became but one-sixth of it. - -"The direction of the aplanatic pencils appears to be scarcely -affected by the differences in the thickness of glasses, if their -state as to color is the same. - -"One other property of the double object-glass remains to be -mentioned, which is, that when the longer aplanatic focus is used, the -marginal rays of a pencil not coincident with the axis of the glass -are distorted, so that a coma is thrown outwards; while the contrary -effect of a coma directed towards the centre of the field is produced -by the rays from the shorter focus. These peculiarities of the coma -seem inseparable attendants on the two foci, and are as conspicuous in -the achromatic meniscus as in the plano-convex object-glass. - -[Illustration: Fig. 14.] - -"Of several purposes to which the particulars just given seem -applicable, I must at present confine myself to the most obvious one. -They furnish the means of destroying with the utmost ease both -aberrations in a large focal pencil, and of thus surmounting what has -hitherto been the chief obstacle to the perfection of the microscope. -And when it is considered that the curves of its diminutive -object-glasses have required to be at least as exactly proportioned as -those of a large telescope to give the image of a bright point equally -sharp and colorless, and that any change made to correct one -aberration was liable to disturb the other, some idea may be formed of -what the amount of that obstacle must have been. It will, however, be -evident that if any object-glass is but made achromatic, with its -lenses truly worked and cemented, so that their axes coincide, it may -with certainty be connected with another possessing the same -requisites and of suitable focus, so that the combination shall be -free from spherical error also in the centre of its field. For this -the rays have only to be received by the front glass B (Fig. 14) from -its shorter aplanatic focus F´´, and transmitted in the direction of -the longer correct pencil F A of the other glass A. It is desirable -that the latter pencil should neither converge to a very short focus -nor be more than very slightly if at all divergent; and a little -attention at first to the kind of glass used will keep it within this -range, the denser flint being suited to the glasses of shorter focus -and larger angle of aperture. - -"The adjustment of the microscope is then perfected, if necessary, by -slightly varying the distance between the object-glasses; and after -that is done, the length of the tube which carries the eye-pieces may -be altered greatly without disturbing the correction, opposite errors -which balance each other being produced by the change. - -"If the two glasses which in the diagram are drawn at some distance -apart are brought nearer together (if the place of A, for instance, is -carried to the dotted figure), the rays transmitted by B in the -direction of the longer aplanatic pencil of A will plainly be derived -from some point Z more distant than F´´, and lying between the -aplanatic foci of B; therefore (according to what has been stated) -this glass, and consequently the combination, will then be spherically -over-corrected. If, on the other hand, the distance between A and B is -increased, the opposite effects are of course produced. - -"In combining several glasses together it is often convenient to -transmit an under-corrected pencil from the front glass, and to -counteract its error by over-correction in the middle one. - -"Slight errors in color may in the same manner be destroyed by -opposite ones; and on the principles described we not only acquire -fine correction for the central ray, but by the opposite effects at -the two foci on the transverse pencil, all coma can be destroyed, and -the whole field rendered beautifully flat and distinct." - -Mr. Lister's paper enters into further particulars, which are not -essential to the comprehension of the subject. It is sufficient to say -that his investigations and results proved to be of the highest value -to the practical optician, and the progress of improvement was in -consequence extremely rapid. The new principles were applied and -exhibited by Mr. Hugh Powell and Mr. Andrew Ross with a degree of -success which had never been anticipated; so perfect indeed were the -corrections given to the achromatic object-glass--so completely were -the errors of sphericity and dispersion balanced or destroyed--that -the circumstance of covering the object with a plate of the thinnest -glass or talc disturbed the corrections, if they had been adapted to -an uncovered object, and rendered an object-glass which was perfect -under one condition sensibly defective under the other. - -This defect, if that should be called a defect which arose out of -improvement, was first discovered by Mr. Ross, who immediately -suggested the means of correcting it, and presented to the Society of -Arts, in 1837, a paper on the subject, which was published in the 51st -volume of their Transactions, and which, as it is, like Mr. Lister's -essential to a full understanding of the ultimate refinements of the -instrument, we shall extract nearly in full: - -"In the course of a practical investigation (says Mr. Ross) with the -view of constructing a combination of lenses for the object-glass of a -compound microscope, which should be free from the effects of -aberration, both for central and oblique pencils of great angle, I -combined the condition of the greatest possible distance between the -object and object-glass; for in object-glasses of short focal length -their closeness to the object has been an obstacle in many cases to -the use of high magnifying powers, and is a constant source of -inconvenience. - -"In the improved combination, the diameter is only sufficient to admit -the proper pencil; the convex lenses are wrought to an edge, and the -concave have only sufficient thickness to support their figure; -consequently the combination is the thinnest possible, and it follows -that there will be the greatest distance between the object and the -object-glass. The focal length is one-eighth of an inch, having an -angular aperture of 60°, with a distance of 1-25th of an inch, and a -magnifying power of 970 times linear, with perfect definition on the -most difficult Podura scales. I have made object-glasses 1-16th of an -inch focal length; but as the angular aperture cannot be -advantageously increased, if the greatest distance between the object -and object-glass is preserved, their use will be very limited. - -"The quality of the definition produced by an achromatic compound -microscope will depend upon the accuracy with which the aberrations, -both chromatic and spherical, are balanced, together with the general -perfection of the workmanship. Now, in Wollaston's doublets, and -Holland's triplets, there are no means of producing a balance of the -aberrations, as they are composed of convex lenses only; therefore the -best that can be done is to make the aberrations a minimum; the -remaining positive aberration in these forms produces its peculiar -effect upon objects (particularly the detail of the thin transparent -class), which may lead to misapprehension of their true structure; but -with the achromatic object-glass, where the aberrations are correctly -balanced, the most minute parts of an object are accurately displayed, -so that a satisfactory judgment of their character may be formed. - -[Illustration: Fig. 15.] - -[Illustration: Fig. 16.] - -"It will be seen by Fig. 15, that when a certain angular pencil A O A´ -proceeds from the object O, and is incident on the plane side of the -first lens, if the combination is removed from the object, as in Fig. -16, the extreme rays of the pencil impinge on the more marginal parts -of the glass, and as the refractions are greater here, the aberrations -will be greater also. Now, if two compound object-glasses have their -aberrations balanced, one being situated as in Fig. 15, and the other -as in Fig. 16, and the same disturbing power applied to both, that in -which the angles of incidence and the aberrations are small will not -be so much disturbed as where the angles are great, and where -consequently the aberrations increase rapidly. - -"When an object-glass has its aberrations balanced for viewing an -opaque object, and it is required to examine that object by -transmitted light, the correction will remain; but if it is necessary -to immerse the object in a fluid, or to cover it with glass or talc, -an aberration will arise from these circumstances, which will disturb -the previous correction, and consequently deteriorate the definition; -and this effect will be more obvious with the increase of the distance -between the object and the object-glass. - -[Illustration: Fig. 17.] - -"The aberration produced with diverging rays by a piece of flat and -parallel glass, such as would be used for covering an object, is -represented at Fig. 17, where G G G G is the refracting medium, or -piece of glass covering the object O; O P, the axis of the pencil, -perpendicular to the flat surfaces; O T, a ray near the axis; and O -T´, the extreme ray of the pencil incident on the under surface of the -glass; then T R, T´ R´, will be the directions of the rays in the -medium, and R E, R´ E´, those of the emergent rays. Now if the course -of these rays is continued, as by the dotted lines, they will be found -to intersect the axis at different distances, X and Y, from the -surface of the glass; and the distance X Y is the aberration produced -by the medium which, as before stated, interferes with the previously -balanced aberrations of the several lenses composing the -object-glass. There are many cases of this, but the one here selected -serves best to illustrate the principle. I need not encumber the -description with the theoretical determination of this quantity, as it -varies with exceedingly minute circumstances which we cannot -accurately control; such as the distance of the object from the under -side of the glass, and the slightest difference in the thickness of -the glass itself; and if these data could be readily obtained, the -knowledge would be of no utility in making the correction, that being -wholly of a practical nature. - -"If an object-glass is constructed as represented in Fig. 16, where -the posterior combination P and the middle M have together an excess -of negative aberration, and if this be corrected by the anterior -combination A, having an excess of positive aberration, then this -latter combination can be made to act more or less powerfully upon P -and M, by making it approach to or recede from them; for when the -three are in close contact, the distance of the object from the -object-glass is greatest; and consequently the rays from the object -are diverging from a point at a greater distance than when the -combinations are separated; and as a lens bends the rays more, or acts -with greater effect, the more distant the object is from which the -rays diverge, the effect of the anterior combination A upon the other -two, P and M, will vary with its distance from thence. When therefore -the correction of the whole is effected for an opaque object with a -certain distance between the anterior and middle combination, if they -are then put in contact, the distance between the object and -object-glass will be increased; consequently the anterior combination -will act more powerfully, and the whole will have an excess of -positive aberration. Now the effect of the aberration produced by a -piece of flat and parallel glass being of the negative character, it -is obvious that the above considerations suggest the means of -correction by moving the lenses nearer together, till the positive -aberration thereby produced balances the negative aberration caused by -the medium. - -"The preceding refers only to the spherical aberration, but the effect -of the chromatic is also seen when an object is covered with a piece -of glass; for, in the course of my experiments, I observed that it -produced a chromatic thickening of the outline of the Podura and -other delicate scales; and if diverging rays near the axis and at the -margin are projected through a piece of flat parallel glass, with the -various indices of refraction for the different colors, it will be -seen that each ray will emerge separated into a beam consisting of the -component colors of the ray, and that each beam is widely different in -form. This difference, being magnified by the power of the microscope, -readily accounts for the chromatic thickening of the outline just -mentioned. Therefore to obtain the finest definition of extremely -delicate and minute objects, they should be viewed without a covering; -if it be desirable to immerse them in a fluid, they should be covered -with the thinnest possible film of talc, as, from the character of the -chromatic aberration, it will be seen that varying the distances of -the combinations will not sensibly affect the correction; though -object-lenses may be made to include a given fluid or solid medium in -their correction for color. - -[Illustration: Fig. 18.] - -"The mechanism for applying these principles to the correction of an -object-glass under the various circumstances, is represented in Fig. -18, where the anterior lens is set in the end of a tube A A, which -slides on the cylinder B containing the remainder of the combination; -the tube A A, holding the lens nearest the object, may then be moved -upon the cylinder B, for the purpose of varying the distance according -to the thickness of the glass covering the object, by turning the -screwed ring C C, or more simply by sliding the one on the other, and -clamping them together when adjusted. An aperture is made in the tube -A, within which is seen a mark engraved on the cylinder, and on the -edge of which are two marks, a longer and a shorter, engraved upon the -tube. When the mark on the cylinder coincides with the longer mark on -the tube, the adjustment is perfect for an uncovered object; and when -the coincidence is with the short mark, the proper distance is -obtained to balance the aberrations produced by glass one-hundredth of -an inch thick, and such glass can be readily supplied. - -"It is hardly necessary to observe, that the necessity for this -correction is wholly independent of any particular construction of the -object-glass; as in all cases where the object-glass is corrected for -an object uncovered, any covering of glass will create a different -value of aberration to the first lens, which previously balanced the -aberration resulting from the rest of the lenses; and as this -disturbance is effected at the first refraction, it is independent of -the other part of the combination. The visibility of the effect -depends on the distance of the object from the object-glass, the angle -of the pencil transmitted, the focal length of the combination, the -thickness of the glass covering the object, and the general perfection -of the corrections for chromatism and the oblique pencils. - -"With this adjusting object-glass, therefore, we can have the -requisites of the greatest possible distance between the object and -object-glass, an intense and sharply defined image throughout the -field from the large pencil transmitted, and the accurate correction -of the aberrations; also, by the adjustment, the means of preserving -that correction under all the varied circumstances in which it may be -necessary to place an object for the purpose of observation." - -In the annexed engraving, Fig. 19, we have shown the triple achromatic -object-glass in connection with the eye-piece consisting of the -field-glass F F, and the eye-glass E E, forming together the modern -achromatic microscope. The course of the light is shown by drawing -three rays from the centre and three from each end of the object O. -These rays would, if left to themselves, form an image of the object -at A A, but being bent and converged by the field-glass F F, they form -the image at B B, where a stop is placed to intercept all light except -what is required for the formation of the image. From B B therefore -the rays proceed to the eye-glass exactly as has been described in -reference to the simple microscope and to the compound of two glasses. - -[Illustration: Fig. 19.] - -If we stopped here we should convey a very imperfect idea of the -beautiful series of corrections effected by the eye-piece, and which -were first pointed out in detail in a paper on the subject published -by Mr. Varley in the 51st volume of the Transactions of the Society of -Arts. The eye-piece in question was invented by Huyghens for -telescopes, with no other view than that of diminishing the spherical -aberration by producing the refractions at two glasses instead of one, -and of increasing the field of view. It was reserved for Boscovich to -point out that Huyghens had by this arrangement accidentally corrected -a great part of the chromatic aberration, and this subject is further -investigated with much skill in two papers by Professor Airy in the -_Cambridge Philosophical Transactions_, to which we refer the -mathematical reader. These investigations apply chiefly to the -telescope, where the small pencils of light and great distance of the -object exclude considerations which become important in the -microscope, and which are well pointed out in Mr. Varley's paper -before mentioned. - -[Illustration: Fig. 20.] - -Let Fig. 20 represent the Huyghenean eye-piece of a microscope; F F -and E E being the field-glass and eye-glass, and L M N the two extreme -rays of each of the three pencils, emanating from the centre and ends -of the object, of which, but for the field-glass, a series of colored -images would be formed from R R to B B; those near R R being red, -those near B B blue, and the intermediate ones green, yellow, and so -on, corresponding with the colors of the prismatic spectrum. This -order of colors, it will be observed, is the reverse of that -described in treating of the common compound microscope (Fig. 12), in -which the single object-glass projected the red image beyond the blue. -The effect just described, of projecting the blue image beyond the -red, is purposely produced for reasons presently to be given, and is -called over-correcting the object-glass as to color. It is to be -observed also that the images B B and R R are curved in the wrong -direction to be distinctly seen by a convex eye-lens, and this is a -further defect of the compound microscope of two lenses. But the -field-glass, at the same time that it bends the rays and converges -them to foci at B´ B´ and R´ R´, also reverses the curvature of the -images as there shown, and gives them the form best adapted for -distinct vision by the eye-glass E E. The field-glass has at the same -time brought the blue and red images closer together, so that they are -adapted to pass uncolored through the eye-glass. To render this -important point more intelligible, let it be supposed that the -object-glass had not been over-corrected, that it had been perfectly -achromatic; the rays would then have become colored as soon as they -had passed the field-glass; the blue rays, to take the central pencil, -for example, would converge at _b_ and the red rays at _r_, which is -just the reverse of what the eye-lens requires; for as its blue focus -is also shorter than its red, it would demand rather that the blue -image should be at _r_ and the red at _b_. This effect we have shown -to be produced by the over-correction of the object-glass, which -protrudes the blue foci B B as much beyond the red foci R R as the sum -of the distances between the red and blue foci of the field-lens and -eye-lens; so that the separation B R is exactly taken up in passing -through those two lenses, and the whole of the colors coincide as to -focal distance as soon as the rays have passed the eye-lens. But while -they coincide as to distance, they differ in another respect; the blue -images are rendered smaller than the red by the superior refractive -power of the field-glass upon the blue rays. In tracing the pencil L, -for instance, it will be noticed that after passing the field-glass, -two sets of lines are drawn, one whole, and one dotted, the former -representing the red, and the latter the blue rays. This is the -accidental effect in the Huyghenean eye-piece pointed out by -Boscovich. This separation into colors at the field-glass is like the -over-correction of the object-glass; it leads to a subsequent complete -correction. For if the differently colored rays were kept together -till they reached the eye-glass, they would then become colored, and -present colored images to the eye; but fortunately, and most -beautifully, the separation effected by the field-glass causes the -blue rays to fall so much nearer the centre of the eye-glass, where, -owing to the spherical figure, the refractive power is less than at -the margin, that the spherical error of the eye-lens constitutes a -nearly perfect balance to the chromatic dispersion of the field-lens, -and the red and blue rays L´ and L´´ emerge sensibly parallel, -presenting, in consequence, the perfect definition of a single point -to the eye. The same reasoning is true of the intermediate colors and -of the other pencils. - -From what has been stated it is obvious that we mean by an achromatic -object-glass one in which the usual order of dispersion is so far -reversed that the light, after undergoing the singularly beautiful -series of changes effected by the eye-piece, shall come uncolored to -the eye. We can give no specific rules for producing these results. -Close study of the formulæ for achromatism given by the celebrated -mathematicians we have quoted will do much, but the principles must be -brought to the test of repeated experiment. Nor will the experiments -be worth anything, unless the curves be most accurately measured and -worked, and the lenses centered and adjusted with a degree of -precision which, to those who are familiar only with telescopes, will -be quite unprecedented. - -The Huyghenean eye-piece which we have described is the best for -merely optical purposes, but when it is required to measure the -magnified image, we use the eye-piece invented by Mr. Ramsden, and -called, from its purpose, the micrometer eye-piece. When it is stated -that we sometimes require to measure portions of animal or vegetable -matter a hundred times smaller than any divisions that can be -artificially made on any measuring instrument, the advantage of -applying the scale to the magnified image will be obvious, as compared -with the application of engraved or mechanical micrometers to the -stage of the instrument. - -The arrangement is shown in Fig. 21, where E E and F F are the eye and -field glass, the latter having now its plane face towards the object. -The rays from the object are here made to converge at A A, immediately -in front of the field-glass, and here also is placed a plane glass on -which are engraved divisions of a hundredth of an inch or less. The -markings of these divisions come into focus therefore at the same time -as the image of the object, and both are distinctly seen together. -Thus the measure of the magnified image is given by mere inspection, -and the value of such measures in reference to the real object may be -obtained thus, which, when once obtained, is constant for the same -object-glass. Place on the stage of the instrument a divided scale the -value of which is known, and viewing this scale as the microscopic -object, observe how many of the divisions on the scale attached to the -eye-piece correspond with one of those in the magnified image. If, for -instance, ten of those in the eye-piece correspond with one of those in -the image, and if the divisions are known to be equal, then the image -is ten times larger than the object, and the dimensions of the object -are ten times less than indicated by the micrometer. If the divisions -on the micrometer and on the magnified scale were not equal, it -becomes a mere rule-of-three sum, but in general this trouble is taken -by the maker of the instrument, who furnishes a table showing the -value of each division of the micrometer for every object-glass with -which it may be used. - -[Illustration: Fig. 21.] - -While on the subject of measuring it may be well to explain the mode -of ascertaining the magnifying power of the compound microscope, which -is generally taken on the assumption before mentioned, that the naked -eye sees most distinctly at the distance of ten inches. - -Place on the stage of the instrument, as before, a known divided -scale, and when it is distinctly seen, hold a rule at ten inches -distance from the disengaged eye, so that it may be seen by that eye, -overlapping or lying by side of the magnified picture of the other -scale. Then move the rule till one or more of its known divisions -correspond with a number of those in the magnified scale, and a -comparison of the two gives the magnifying power. - -Having now explained the optical principles of the achromatic compound -microscope, it remains only to describe the mechanical arrangements -for giving those principles their full effect. The mechanism of a -microscope is of much more importance than might be imagined by those -who have not studied the subject. In the first place, steadiness, or -freedom from vibration, and most particularly freedom from any -vibrations which are not equally communicated to the object under -examination, and to the lenses by which it is viewed, is a point of -the utmost consequence. When, for instance, the body containing the -lenses is screwed by its lower extremity to a horizontal arm, we have -one of the most vibratory forms conceivable; it is precisely the form -of the inverted pendulum, which is expressly contrived to indicate -otherwise insensible vibrations. The tremor necessarily attendant on -such an arrangement is magnified by the whole power of the instrument; -and as the object on the stage partakes of this tremor in a -comparatively insensible degree, the image is seen to oscillate so -rapidly, as in some cases to be wholly undistinguishable. Such -microscopes cannot possibly be used with high powers in ordinary -houses abutting on any paved streets through which carriages are -passing, nor indeed are they adapted to be used in houses in which the -ordinary internal sources of shaking exist. - -One of the best modes of mounting a compound microscope is shown in -the annexed view (Fig. 22), which, though too minute to exhibit all -the details, will serve to explain the chief features of the -arrangement. - -A massy pillar A is screwed into a solid tripod B, and is surmounted -by a strong joint at C, on which the whole instrument turns, so as to -enable it to take a perfectly horizontal or vertical position, or any -intermediate angle, such, for instance, as that shown in the -engraving. - -This movable portion of the instrument consists of one solid casting D -E F G; from F to G being a thick pierced plate carrying the stage and -its appendages. The compound body H is attached to the bar D E, and -moves up and down upon it by a rack and pinion worked by either of the -milled heads K. The piece D E F G is attached to the pillar by the -joint C, which being the source of the required movement in the -instrument, is obviously its weakest part, and about which no doubt -considerable vibration takes place. But inasmuch as the piece D E F G -of necessity transmits such vibrations equally to the body of the -microscope and to the objects on the stage, they hold always the same -relative position, and no _visible_ vibration is caused, how much -soever may really exist. To the under side of the stage is attached a -circular stem L, on which slides the mirror M, plane on one side and -concave on the other, to reflect the light through the aperture in the -stage. Beneath the stage is a circular revolving plate containing -three apertures of various sizes, to limit the angle of the pencil of -light which shall be allowed to fall on the object under examination. -Besides these conveniences the stage has a double movement produced by -two racks at right angles to each other, and worked by milled heads -beneath. It has also the usual appendages of forceps to hold minute -objects, and a lens to condense the light upon them, all of which are -well understood, and if not, will be rendered more intelligible by a -few minutes' examination of a microscope than by the most lengthened -description. One other point remains to be noticed. The movement -produced by the milled head K is not sufficiently delicate to adjust -the focus of very powerful lenses, nor indeed is any rack movement. -Only the finest screws are adapted to this purpose; and even these are -improved by means for reducing the rapidity of the screw's movement. -For this purpose the lower end of the compound body H, which carries -the object-glass, consists of a piece of smaller tube sliding in -parallel guides in the main body, and kept constantly pressed upwards -by a spiral spring but it can be drawn downward by a lever crossing -the body, and acted on by an extremely fine screw whose milled head -is seen at N, and the fineness of which is tripled by means of the -lever through which it acts on the object-glass. The instrument is of -course roughly adjusted by the rack movement, and finished by the -screw, or by such other means as are chosen for the purpose. One very -ingenious contrivance, but applied to the stage, instead of the body -of the microscope, invented by Mr. Powell, will be found described in -the 50th volume of the Transactions of the Society of Arts. - -[Illustration: Fig. 22.] - -The greater part of the directions for viewing and illuminating -objects given in reference to the simple microscope are applicable to -the compound. An argand lamp placed in the focus of a large detached -lens so as to throw parallel rays upon the mirror, is the best -artificial light; and for opaque objects the light so thrown up may be -reflected by metallic specula (called, from their inventor, -Lieberkhuns) attached to the object-glasses. - -It has been recently proposed by Sir David Brewster and by M. Dujardin -to render the Wollaston condenser achromatic, and they have -accordingly been made with three pairs of achromatic lenses instead of -the single lens before described, with very excellent effect. The -last-mentioned gentleman has also projected an ingenious apparatus, -called the Hyptioscope, attached to the eye-piece for the purpose of -erecting the magnified picture. - -The erector commonly applied to the compound microscope consists of a -pair of lenses acting like the erecting eye-piece of the telescope. -But this, though it is convenient for the purpose of dissection, very -much impairs the optical performance of the instrument. - -[Illustration: Fig. 23.] - -For drawing the images presented by the microscope the best apparatus -consists of a mirror M (Fig. 23), composed of a thin piece of rather -dark-colored glass cemented on to a piece of plate-glass inclined at -an angle of 45° in front of the eye-glass E. The light escaping from -the eye-glass is assisted in its reflection upwards to the eye by the -dark glass, which effects the further useful purpose of rendering the -paper less brilliant, and thus enabling the eye better to see the -reflected image. The lens L below the reflector is to cause the light -from the paper and pencil to diverge from the same distance as that -received from the eye-glass; in other words, to cause it to reach the -eye in parallel lines. - -[Illustration: Fig. 24.] - -Dr. Wollaston's camera lucida, as shown in Fig. 24, is sometimes -attached to the eye-piece of the microscope for the same purpose. In -this instrument the rays suffer two internal reflections within the -glass prism, as will be seen explained in the article "Camera Lucida." -In this minute figure we have omitted to trace the reflected rays, -merely to avoid confusion. - -[Illustration: Fig. 25.] - -[Illustration: Fig. 26.] - -[Illustration: Fig. 27.] - -Annexed are four engravings of microscopic objects, the true character -of which it is, however, impossible to give in wood, and is difficult -indeed to accomplish by any description of engraving. - -[Illustration: Fig. 28.] - -Fig. 25 shows a scale of the small insect called Podura Plumbea, the -common Skiptail, magnified about five hundred times. To define the -markings on this scale clearly is the highest test of a deep -achromatic object-glass; and this drawing is given rather to explain -what the observer should look for, than as a very correct -representation. Fig. 26 is a scale or feather of the Menelaus -Butterfly; Fig. 27 is the hair of a singular insect, the Dermestes; -and Fig. 28 is a longitudinal cutting of fir, showing the circular -glands on the vessels which distinguish coniferous woods. These latter -objects may be seen by half-inch or quarter-inch achromatic glasses. -Opaque objects are generally better exhibited by inch and two-inch -glasses, when a general view of them is required, and by higher powers -when we wish to examine their minute structure. In the latter case the -light must be obtained by condensing lenses instead of the metallic -specula. - -Although the reflecting microscope is now very little used, it may be -expected that we should mention it. In this instrument, at Fig. 29, -the object O is reflected by the inclined face of the mirror M, and -the rays are again reflected and converged by the ellipsoidal -reflector R R, which effects the same purpose as the object-glass of -the compound microscope. It forms an image which is not susceptible of -the over-correction as to color before described, and which therefore -becomes colored in passing through the eye-piece. This fact, and the -loss of light by reflection, will probably always render the -reflecting microscope inferior to the achromatic refracting. - -[Illustration: Fig. 29.] - -The solar microscope has been so nearly superseded by the -oxy-hydrogen, that a brief description of the latter must suffice, -particularly as their optical principles are similar. - -The primary object in both is to throw an intense light upon the -object, which is sometimes done by mirrors, and sometimes by lenses. -In Fig. 30, L represents the cylinder of burning lime, R R the -reflector, which concentrates the light upon the object O O; the rays -from which, passing through the two plano-convex lenses, are brought -to foci upon a screen placed at a great distance, and upon which is -formed the magnified image. - -[Illustration: Fig. 30.] - -Fig. 31 shows a combination of lenses to condense the light upon the -object. In either case the optical arrangements by which the image is -formed admit of the same perfection as those which have been described -for the compound microscopes. A few achromatic glasses for -oxy-hydrogen microscopes have been made, and they will ultimately -become valuable instruments for illustrating lectures on natural -history and physiology. One made by Mr. Ross was exhibited a few -months since at the Society of Arts to illustrate a lecture on the -physiology of woods. It should be observed, however, that the -oxy-hydrogen or solar microscope requires either a spherical screen, -or that the objects should be mounted between spherical glasses, in -order to bring the whole into focus at one time. This latter plan was -adopted on the occasion just mentioned with perfect success. - -[Illustration: Fig. 31.] - - - - - -End of the Project Gutenberg EBook of The Microscope, by Andrew Ross - -*** END OF THIS PROJECT GUTENBERG EBOOK THE MICROSCOPE *** - -***** This file should be named 41958-8.txt or 41958-8.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/4/1/9/5/41958/ - -Produced by Chris Curnow, Matthew Wheaton and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - -Updated editions will replace the previous one--the old editions -will be renamed. - -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. 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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: The Microscope - -Author: Andrew Ross - -Release Date: January 31, 2013 [EBook #41958] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE MICROSCOPE *** - - - - -Produced by Chris Curnow, Matthew Wheaton and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - -</pre> - +<div>*** START OF THE PROJECT GUTENBERG EBOOK 41958 ***</div> <div class="figcenter"> <img src="images/cover.jpg" width="363" height="600" alt="" /> @@ -1788,382 +1750,6 @@ adopted on the occasion just mentioned with perfect success.</p> <p class="caption">Fig. 31.</p> </div> - - - - - - - -<pre> - - - - - -End of the Project Gutenberg EBook of The Microscope, by Andrew Ross - -*** END OF THIS PROJECT GUTENBERG EBOOK THE MICROSCOPE *** - -***** This file should be named 41958-h.htm or 41958-h.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/4/1/9/5/41958/ - -Produced by Chris Curnow, Matthew Wheaton and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - -Updated editions will replace the previous one--the old editions -will be renamed. - -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. 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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: The Microscope - -Author: Andrew Ross - -Release Date: January 31, 2013 [EBook #41958] - -Language: English - -Character set encoding: ASCII - -*** START OF THIS PROJECT GUTENBERG EBOOK THE MICROSCOPE *** - - - - -Produced by Chris Curnow, Matthew Wheaton and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - THE MICROSCOPE. - - BEING THE ARTICLE CONTRIBUTED BY - - ANDREW ROSS - - - TO THE "PENNY CYCLOPAEDIA," PUBLISHED BY THE SOCIETY - FOR THE DIFFUSION OF USEFUL KNOWLEDGE. - - FULLY ILLUSTRATED. - - - NEW YORK: - THE INDUSTRIAL PUBLICATION COMPANY. - 1877. - - - - -THE MICROSCOPE. - - -Microscope, the name of an instrument for enabling the eye to see -distinctly objects which are placed at a very short distance from it, -or to see magnified images of small objects, and therefore to see -smaller objects than would otherwise be visible. The name is derived -from the two Greek words, expressing this property, MIKROS, _small_, -and SKOPEO, _to see_. - -So little is known of the early history of the microscope, and so -certain is it that the magnifying power of lenses must have been -discovered as soon as lenses were made, that there is no reason for -hazarding any doubtful speculations on the question of discovery. We -shall proceed therefore at once to describe the simplest forms of -microscopes, to explain their later and more important improvements, -and finally to exhibit the instrument in its present perfect state. - -In doing this we shall assume that the reader is familiar with the -information contained in the articles "Light," "Lens," "Achromatic," -"Aberration," and the other sub-divisions of the science of Optics, -which are treated of in this work. - -The use of the term _magnifying_ has led many into a misconception of -the nature of the effect produced by convex lenses. It is not always -understood that the so-called magnifying power of a lens applied to -the eye, as in a microscope, is derived from its enabling the eye to -approach more nearly to its object than would otherwise be compatible -with distinct vision. The common occurrence of walking across the -street to read a bill is in fact magnifying the bill by approach; and -the observer, at every step he takes, makes a change in the optical -arrangement of his eye, to adapt it to the lessening distance between -himself and the object of his inquiry. This power of spontaneous -adjustment is so unconsciously exerted, that unless the attention be -called to it by circumstances, we are totally unaware of its exercise. - -In the case just mentioned the bill would be read with eyes in a very -different state of adjustment from that in which it was discovered on -the opposite side of the street, but no conviction of this fact would -be impressed upon the mind. If, however, the supposed individual -should perceive on some part of the paper a small speck, which he -suspects to be a minute insect, and if he should attempt a very close -approach of his eye for the purpose of verifying his suspicion, he -would presently find that the power of natural adjustment has a limit; -for when his eye has arrived within about ten inches, he will discover -that a further approach produces only confusion. But if, as he -continues to approach, he were to place before his eye a series of -properly arranged convex lenses, he would see the object gradually and -distinctly increase in apparent size by the mere continuance of the -operation of approaching. Yet the glasses applied to the eye during -the approach from ten inches to one inch, would have done nothing more -than had been previously done by the eye itself during the approach -from fifty feet to one foot. In both cases the magnifying is effected -really by the approach, the lenses merely rendering the latter periods -of the approach compatible with distinct vision. - -A very striking proof of this statement may be obtained by the -following simple and instructive experiment. Take any minute object, a -very small insect for instance, held on a pin or gummed to a slip of -glass; then present it to a strong light, and look at it through the -finest needle-hole in a blackened card placed about an inch before it. -The insect will appear quite distinct, and about ten times larger than -its usual size. Then suddenly withdraw the card without disturbing the -object, which will instantly become indistinct and nearly invisible. -The reason is, that the naked eye cannot see at so small a distance as -one inch. But the card with the hole having enabled the eye to -approach within an inch, and to see distinctly at that distance, is -thus proved to be as decidedly a magnifying instrument as any lens or -combination of lenses. - -This description of magnifying power does not apply to such -instruments as the solar or gas microscope, by which we look not at -the object itself, but at its shadow or picture on the wall; and the -description will require some modification in treating of the compound -microscope, where, as in the telescope, an image or picture is formed -by one lens, that image or picture being viewed as an original object -by another lens. - -It is nevertheless so important to obtain a clear notion of the real -nature of the effect produced by a lens applied to the eye, that we -will adduce the instance of spectacles to render the point more -familiar. If the person who has been supposed to cross the street for -the purpose of reading a bill had been aged, the limit to the power of -adjustment would have been discovered at a greater distance, and -without so severe a test as the supposed insect. The eyes of the very -aged generally lose the power of adjustment at a distance of thirty or -forty inches instead of ten, and the spectacles worn in consequence -are as much magnifying glasses to them as the lenses employed by -younger eyes to examine the most minute objects. Spectacles are -magnifying glasses to the aged because they enable such persons to see -as closely to their objects as the young, and therefore to see the -objects larger than they could themselves otherwise see them, but not -larger than they are seen by the unassisted younger eye. - -In saying that an object appears larger at one time, or to one person, -than another, it is necessary to guard against misconception. By the -apparent size of an object we mean the angle it subtends at the eye, -or the angle formed by two lines drawn from the centre of the eye to -the extremities of the object. In Fig. 1, the lines A E and B E drawn -from the arrow to the eye form the angle A E B, which, when the angle -is small, is nearly twice as great as the angle C E D, formed by lines -drawn from a similar arrow at twice the distance. The arrow A B will -therefore appear nearly twice as long as C D, being seen under twice -the angle, and in the same proportion for any greater or lesser -difference in distance. The angle in question is called the angle of -vision, or the visual angle. - -[Illustration: Fig. 1.] - -The angle of vision must, however, not be confounded with the angle of -the pencil of light by which an object is seen, and which is explained -in Fig. 2. Here we have drawn two arrows placed in relation to the eye -as before, and from the centre of each have drawn lines exhibiting the -quantity of light which each point will send into the eye at the -respective distances. - -[Illustration: Fig. 2.] - -Now if E F represent the diameter of the pupil, the angle E A F shows -the size of the cone or pencil of light which enters the eye from the -point A, and in like manner the angle E B F is that of the pencil -emanating from B, and entering the eye. Then, since E A F is double E -B F, it is evident that A is seen by four times the quantity of light -which could be received from an equally illuminated point at B; so -that the nearer body would appear brighter if it did not appear -larger; but as its apparent area is increased four times as well as -its light, no difference in this respect is discovered. But if we -could find means to send into the eye a larger pencil of light, as for -instance that shown by the lines G A H, without increasing the -apparent size in the same proportion, it is evident that we should -obtain a benefit totally distinct from that of increased magnitude, -and one which is in some cases of even more importance than size in -developing the structure of what we wish to examine. This, it will be -hereafter shown, is sometimes done; for the present, we wish merely to -explain clearly the distinction between apparent magnitude, or the -angle under which the object is seen, and apparent brightness, or the -angle of the pencil of light by which each of its points is seen, and -with these explanations we shall continue to employ the common -expressions magnifying glass and magnifying power. - -[Illustration: Fig. 3.] - -The magnifying power of a single lens depends upon its focal length, -the object being in fact placed nearly in its principal focus, or so -that the light which diverges from each point may, after refraction by -the lens, proceed in parallel lines to the eye, or as nearly so as is -requisite for distinct vision. In Fig. 3, A B is a double convex lens, -near which is a small arrow to represent the object under examination, -and the cones drawn from its extremities are portions of the rays of -light diverging from those points and falling upon the lens. These -rays, if suffered to fall at once upon the pupil, would be too -divergent to permit their being brought to a focus upon the retina by -the optical arrangements of the eye. But being first passed through -the lens, they are bent into nearly parallel lines, or into lines -diverging from some points within the limits of distinct vision, as -from C and D. Thus altered, the eye receives them precisely as if they -emanated from a larger arrow placed at C D, which we may suppose to be -ten inches from the eye, and then the difference between the real and -the imaginary arrow is called the magnifying power of the lens in -question. - -From what has been said it will be evident that two persons whose eyes -differed as to the distance at which they obtained distinct vision, -would give different results as to the magnifying power of a lens. To -one who can see distinctly with the naked eye at a distance of five -inches, the magnifying power would seem and would indeed be only half -what we have assumed. Such instances are, however, rare; the focal -length of the eye usually ranges from six to twelve or fourteen -inches, so that the distance we first assumed of ten inches is very -near the true average, and is a convenient number, inasmuch as a -cipher added to the denominator of the fraction which expresses the -focal length of a lens gives its magnifying power. Thus a lens whose -focal length is one-sixteenth of an inch is said to magnify 160 times. - -When the focal length of a lens is very small, it is difficult to -measure accurately the distance between its centre and its object. In -such cases the best way to obtain the focal length for parallel or -nearly parallel rays is to view the image of some distant object -formed by the lens in question through another lens of one inch solar -focal length, keeping both eyes open and comparing the image presented -through the two lenses with that of the naked eye. The proportion -between the two images so seen will be the focal length required. Thus -if the image seen by the naked eye is ten times as large as that shown -by the lenses, the focal length of the lens in question is one-tenth -of an inch. The panes of glass in a window, or courses of bricks in a -wall, are convenient objects for this purpose. - -In whichever way the focal length of the lens is ascertained, the -rules given for deducing its magnifying power are not rigorously -correct, though they are sufficiently so for all practical purposes, -particularly as the whole rests on an assumption in regard to the -focal length of the eye, and as it does not in any way affect the -actual measurement of the object. To calculate with great precision -the magnifying power of a lens with a given focal length of eye, it is -necessary that the thickness of the lens be taken into the account, -and also the focal length of the eye itself. - -We have hitherto considered a magnifying lens only in reference to its -enlargement of the object, or the increase of the angle under which -the object is seen. A further and equally important consideration is -that of the number of rays or quantity of light by which every point -of the object is rendered visible. The naked eye, as shown in Fig. 2, -admits from each point of every visible object a cone of light having -the diameter of the pupil for its base, and most persons are familiar -with that beautiful provision by which in cases of excessive -brilliancy the pupil spontaneously contracts to reduce the cone of -admitted light within bearable limits. This effect is still further -produced in the experiment already described, of looking at an object -through a needle-hole in a card, which is equivalent to reducing the -pupil to the size of a needle-hole. Seen in this way the object -becomes comparatively dark or obscure; because each point is seen by -means of a very small cone of light, and a little consideration will -suffice to explain the different effects produced by the needle-hole -and the lens. Both change the angular value of the cone of light -presented to the eye, but the lens changes the angle by bending the -extreme rays within the limits suited to distinct vision, while the -needle-hole effects the same purpose by cutting off the rays which -exceed those limits. - -It has been shown that removing a brilliant object to a greater -distance will reduce the quantity of light which each point sends into -the eye, as effectually as viewing it through a needle-hole; and -magnifying an object by a lens has been shown to be the same thing in -some respects as removing it to a greater distance. We have to see the -magnified picture by the light emanating from the small object, and it -becomes a matter of difficulty to obtain from each point a sufficient -quantity of light to bear the diffusion of a great magnifying power. -We want to perform an operation just the reverse of applying the card -with the needle-hole to the eye--we want in some cases to bring into -the eye the largest possible pencil of light from each point of the -object. - -Referring to Fig. 3, it will be observed that if the eye could see the -small arrow at the distance there shown without the intervention of -the lens, only a very small portion of the cones of light drawn from -its extremities would enter the pupil; whereas we have supposed that -after being bent by the lens the whole of this light enters the eye as -part of the cones of smaller angle whose summits are at C and D. These -cones will further explain the difference between large and small -pencils of light; those from the small arrow are large pencils; the -dotted cones from the large arrow are small pencils. - -In assuming that the whole of this light could have been suffered to -enter the eye through the lens A B, we did so for the sake of not -perplexing the reader with too many considerations at once. He must -now learn that so large a pencil of light passing through a single -lens would be so distorted by the spherical figure of the lens, and by -the chromatic dispersion of the glass, as to produce a very confused -and imperfect image. This confusion may be greatly diminished by -reducing the pencil; for instance, by applying a stop, as it is -called, to the lens, which is neither more nor less than the -needle-hole applied to the eye. A small pencil of light may be thus -transmitted through a single lens without suffering from spherical -aberration or chromatic dispersion any amount of distortion which will -materially affect the figure of the object; but this quantity of light -is insufficient to bear diffusion over the magnified picture, which is -therefore too obscure to exhibit what we most desire to see--those -beautiful and delicate markings by which one kind of organic matter is -distinguished from another. With a small aperture these markings are -not seen at all: with a large aperture and a single lens they exhibit -a faint nebulous appearance enveloped in a chromatic mist, a state -which is of course utterly valueless to the naturalist, and not even -amusing to the amateur. - -It becomes therefore a most important problem to reconcile a large -aperture with distinctness, or, as it is called, _definition_; and -this has been done in a considerable degree by effecting the required -amount of refraction through two or more lenses instead of one, thus -reducing the angles of incidence and refraction, and producing other -effects which will be shortly noticed. This was first accomplished in -a satisfactory manner by-- - - DR. WOLLASTON'S DOUBLET, - -invented by the celebrated philosopher whose name it bears; it -consists of two plano-convex lenses (Fig. 4) having their focal -lengths in the proportion of 1 to 3, or nearly so, and placed at a -distance which can be ascertained best by actual experiment. Their -plane sides are placed towards the object, and the lens of shortest -focal length next the object. - -[Illustration: Fig. 4.] - -It appears that Dr. Wollaston was led to this invention by considering -that the Achromatic Huyghenean Eye-piece, which will be hereafter -described, would, if reversed, possess similar good properties as a -simple microscope. But it will be evident when the eye-piece is -understood, that the circumstances which render it achromatic are very -imperfectly applicable to the simple microscope, and that the doublet, -without a nice adjustment of the stop, would be valueless. Dr. -Wollaston makes no allusion to a stop, nor is it certain that he -contemplated its introduction, although his illness, which terminated -fatally soon after the presentation of his paper, may account for the -omission. - -The nature of the corrections which take place in the doublet is -explained in the annexed diagram (Fig. 5), where L O L' is the object, -P a portion of the pupil, and D D the stop, or limiting aperture. - -Now, it will be observed that each of the pencils of light from the -extremities L L' of the object is rendered eccentrical by the stop, -and of consequence each passes through the two lenses on opposite -sides of their common axis O P; thus each becomes affected by opposite -errors, which to some extent balance and correct each other. To take -the pencil L, for instance, which enters the eye at R B, R B; it is -bent to the right at the first lens, and to the left at the second; -and as each bending alters the direction of the blue rays more than -the red, and, moreover, as the blue rays fall nearer the margin of the -second lens, where the refraction, being more powerful than near the -centre, compensates in some degree for the greater focal length of the -second lens, the blue and red rays will emerge very nearly parallel, -and of consequence colorless to the eye. At the same time the -spherical aberration has been diminished by the circumstance that the -side of the pencil which passes one lens nearest the axis passes the -other nearest the margin. - -This explanation applies only to the pencils near the extremities of -the object. The central pencil, it is obvious, would pass both lenses -symmetrically; the same portions of light occupying nearly the same -relative places on both lenses. The blue light would enter the second -lens nearer to its axis than the red, and being thus less refracted -than the red by the second lens, a small amount of compensation would -take place, quite different in principle and inferior in degree to -that which is produced in the eccentrical pencils. In the intermediate -spaces the corrections are still more imperfect and uncertain; and -this explains the cause of the aberrations which must of necessity -exist even in the best-made doublet. It is, however, infinitely -superior to a single lens, and will transmit a pencil of an angle of -from 35 deg. to 50 deg. without any very sensible errors. It exhibits, -therefore, many of the usual test-objects in a very beautiful manner. - -[Illustration: Fig. 5.] - -[Illustration: Fig. 6.] - -The next step in the improvement of the simple microscope bears more -analogy to the eye-piece. This improvement was made by Mr. Holland, -and it consists (as shown in Fig. 6) in substituting two lenses for -the first in the doublet, and retaining the stop between them and the -third. The first bending, being thus effected by two lenses instead of -one, is accompanied by smaller aberrations, which are therefore more -completely balanced or corrected at the second bending, in the -opposite direction, by the third lens. This combination, though called -a triplet is essentially a doublet, in which the anterior lens is -divided into two. For it must be recollected that the first pair of -lenses merely accomplishes what might have been done, though with less -precision, by one; but the two lenses of the doublet are opposed to -each other; the second diminishing the magnifying power of the first. -The first pair of lenses in the triplet concur in producing a certain -amount of magnifying power, which is diminished in quantity and -corrected as to aberration at the third lens by the change in relation -to the position of the axis which takes place in the pencil between -what is virtually the first and second lens. In this combination the -errors are still further reduced by the close approximation to the -object which causes the refractions to take place near the axis. Thus -the transmission of a still larger angular pencil, namely 65 deg., is -rendered compatible with distinctness, and a more intense image is -presented to the eye. - -Every increase in the number of lenses is attended with one drawback, -from the circumstance that a certain portion of light is lost by -reflection and absorption each time that the ray enters a new medium. -This loss bears no sensible proportion to the gain arising from the -increased aperture, which, being as the square of the diameter, -multiplies rapidly; or, if we estimate by the angle of the admitted -pencil, which is more easily ascertained, the intensity will be as the -square of twice the tangent of half the angle. To explain this, let D -B (Fig. 7) represent the diameter of the lens, or of that part of it -which is really employed; C A the perpendicular drawn from its -centre, and A B, A D, the extreme rays of the incident pencil of light -DAB. Then the diameter being 2 C B, the area to which the intensity of -vision is proportional will be (2 C B) squared, and C B is evidently the -tangent of the angle C A B, which is half the angle of the admitted -pencil D A B. Or, if _a_ be used to denote the angular aperture, the -expression for the intensity is (2 tan. 1/2_a_) squared which increases so -rapidly with the increase of _a_ as to make the loss of light by -reflection and absorption of little consequence. - -[Illustration: Fig. 7.] - -The combination of three lenses approaches, as has been stated, very -close to the object; so close, indeed, as to prevent the use of more -than three; and this constitutes a limit to the improvement of the -simple microscope, for it is called a simple microscope, although -consisting of three lenses, and although a compound microscope may be -made of only three or even two lenses; but the different arrangement -which gives rise to the term compound will be better understood when -that instrument is explained. - -Before we proceed to describe the simple microscope and its -appendages, it will be well to explain such other points in reference -to the form and materials of lenses as are most likely to be -interesting. - -A very useful form of lens was proposed by Dr. Wollaston, and called -by him the Periscopic lens. It consisted of two hemispherical lenses, -cemented together by their plane faces, having a stop between them to -limit the aperture. A similar proposal was made Mr. Coddington, who, -however, executed the project in a better manner, by cutting a groove -in a whole sphere, and filling the groove with opaque matter. His -lens, which is the well-known Coddington lens, is shown in Fig. 8. It -gives a large field of view, which is equally good in all directions, -as it is evident that the pencils A A and B B pass through under -precisely the same circumstances. Its spherical form has the further -advantage of rendering the position in which it is held of -comparatively little consequence. It is therefore very convenient as -a hand-lens, but its definition is of course not so good as that of a -well-made doublet or achromatic lens. - -[Illustration: Fig. 8.] - -Another very useful form of doublet was proposed by Sir John Herschel, -chiefly like the Coddington lens, for the sake of a wide field, and -chiefly to be used in the hand. It is shown in Fig. 9; it consists of -a double convex or crossed lens, having the radii of curvature as 1 to -6, and of a plane concave lens whose focal length is to that of the -convex lens as 13 to 5. - -Various, indeed innumerable, other forms and combinations of lenses -have been projected, some displaying much ingenuity, but few of any -practical use. In the Catadioptric lenses the light emerges at right -angles from its entering direction, being reflected from a surface cut -at an angle of 45 degrees to the axes of the curved surfaces. - -[Illustration: Fig. 9.] - -It was at one time hoped, as the precious stones are more refractive -than glass, and as the increased refractive power is unaccompanied by -a correspondent increase in chromatic dispersion, that they would -furnish valuable materials for lenses, inasmuch as the refractions -would be accomplished by shallower curves, and consequently with -diminished spherical aberration. But these hopes were disappointed; -everything that ingenuity and perseverance could accomplish was tried -by Mr. Varley and Mr. Pritchard, under the patronage of Dr. Goring. It -appeared, however, that the great reflective power, the -doubly-refracting property, the color, and the heterogeneous structure -of the jewels which were tried, much more than counterbalanced the -benefits arising from their greater refractive power, and left no -doubt of the superiority of skillfully made glass doublets and -triplets. The idea is now, in fact, abandoned; and the same remark is -applicable to the attempts at constructing fluid lenses, and to the -projects for giving to glass other than spherical surfaces--none of -which have come into extensive use. - -By the term _simple_ microscope is meant one in which the object is -viewed directly through a lens or combination of lenses, just as we -have supposed an arrow or an insect to be viewed through a glass held -in the hand. When, however, the magnifying power of the glass is -considerable, in other words, when its focal length is very short, and -its proper distance from its object of consequence equally short, it -requires to be placed at that proper distance with great precision: it -cannot, therefore, be held with sufficient accuracy and steadiness by -the unassisted hand, but must be mounted in a frame having a rack or -screw to move it towards or from another frame or stage which holds -the object. It is then called a microscope, and it is furnished, -according to circumstances, with lenses and mirrors to collect and -reflect the light upon the object, and with other conveniences which -will now be described. - -One of the best forms of a stand for a simple microscope is shown in -Fig. 10, where A is a brass pillar screwed to a tripod base; B is a -broad stage for the objects, secured to the stem by screws, whose -milled heads are at C. By means of the large milled head D, a -triangular bar, having a rack, is elevated out of the stem A, carrying -the lens-holder E, which has a horizontal movement in one direction, -by means of a rack worked by the milled head F, and in the other -direction by turning on a circular pin. A concave mirror G reflects -the light upwards through the hole in the stage, and a lens may be -attached to the stage for the purpose of throwing light on an opaque -object, in the same way that the forceps H for holding such objects is -attached. This microscope is peculiarly adapted, by its broad stage -and its general steadiness, for dissecting; and it is rendered more -convenient for this purpose by placing it between two inclined planes -of mahogany, which support the arms and elevate the wrists to the -level of the stage. This apparatus is called the dissecting rest. When -dissecting is not a primary object, a joint may be made at the lower -end of the stem A, to allow the whole to take an inclined position; -and then the spring clips shown upon the stage are useful to retain -the object in its place. Numerous convenient appendages may be made to -accompany such microscopes, which it will be impossible to mention in -detail; the most useful are Mr. Varley's capillary cages for -containing animalculae in water, and parts of aquatic plants; also his -tubes for obtaining and separating such objects, and his phial and -phial-holder for preserving and exhibiting small living specimens of -the Chara, Nitella, and other similar plants, and observing their -circulation. The phial-microscope affords facilities for observing the -operations of minute vegetable and animal life, which will probably -lead to the most interesting discoveries. The recent volumes of the -Transactions of the Society of Arts contain an immense mass of -information of this sort, and to these we refer the reader. - -[Illustration: Fig. 10.] - -The mode of illuminating objects is one on which we must give some -further information, for the manner in which an object is lighted is -second in importance only to the excellence of the glass through which -it is seen. In investigating any new or unknown specimen, it should be -viewed in turns by every description of light, direct and oblique, as -a transparent object and as an opaque object, with strong and with -faint light, with large angular pencils and with small angular pencils -thrown in all possible directions. Every change will probably develop -some new fact in reference to the structure of the object, which -should itself be varied in the mode of mounting in every possible way. -It should be seen both wet and dry, and immersed in fluids of various -qualities and densities, such as water, alcohol, oil, and Canada -balsam, for instance, which last has a refractive power nearly equal -to that of glass. If the object be delicate vegetable tissue, it will -be in some respects rendered more visible by gentle heating or -scorching by a clear fire placed between two plates of glass. In this -way the spiral vessels of asparagus and other similar vegetables may -be beautifully displayed. Dyeing the objects in tincture of iodine -will in some cases answer this purpose better. - -But the principal question in regard to illumination is the magnitude -of the illuminating pencil, particularly in reference to transparent -objects. Generally speaking the illuminating pencil should be as large -as can be received by the lens, and no larger. Any light beyond this -produces indistinctness and glare. The superfluous light from the -mirror may be cut off by a screen having various-sized apertures -placed below the stage; but the best mode of illumination is that -proposed by Dr. Wollaston, and called the Wollaston condenser. A tube -is placed below the stage of the instrument containing a lens A B -(Fig. 11), which can be elevated or depressed within certain limits at -pleasure; and at the lower end is a stop with a limited aperture C D. -A plane mirror E F receives the rays of light L L from the sky or a -white cloud, which last is the best source of light, and reflects them -upwards through the aperture in C D, so that they are refracted, and -form an image of the aperture at G, which is supposed to be nearly -the place of the object. The object is sometimes best seen when the -image of the aperture is also best seen; and sometimes it is best to -elevate the summit G of the cone A B G above the object, and at others -to depress it below: all which is done at pleasure by the power of -moving the lens A B. If artifical light (as a lamp or candle) be -employed, the flame must be placed in the principal focus of a large -detached lens on a stand, so that the rays L L may fall in parallel -lines on the mirror, or as they would fall from the cloud. This will -be found an advantage, not only when the Wollaston condenser is -employed, but also when the mirror and diaphragm are used. A good mode -of imitating artificially the light of a white cloud opposite the sun -has been proposed by Mr. Varley; he covers the surface of the mirror -under the stage with carbonate of soda or any similar material, and -then concentrates the sun's light upon its surface by a large -condensing lens. The intense white light diffused from the surface of -the soda forms an excellent substitute for the white cloud, which, -when opposite the sun, and of considerable size, is the best daylight, -as the pure sky opposite to the sun is the worst. - -[Illustration: Fig. 11.] - -_The Compound Microscope_ may, as before stated, consist of only two -lenses, while a simple microscope has been shown to contain sometimes -three. In the triplet for the simple microscope, however, it was -explained that the effect of the two first lenses was to do what might -have been accomplished, though not so well, by one; and the third -merely effected certain modifications in the light before it entered -the eye. But in the compound microscope the two lenses have totally -different functions; the first receives the rays from the object, and, -bringing them to new foci, forms an image, which the second lens -treats as an original object, and magnifies it just as the single -microscope magnified the object itself. - -[Illustration: Fig. 12.] - -The annexed figure (12) shows the course of the rays through a -compound microscope of two lenses. The rays proceeding from the object -A B are so acted upon by the lens C D, near it, and thence called the -object glass, that they are converged to foci in A' B', where they -form an enlarged image of the object, as would be evident if a piece -of oiled paper or ground glass were placed there to receive them. They -are not so intercepted, and therefore the image is not rendered -visible at that place; but their further progress is similar to what -it would have been had they really proceeded from an object at A' B'. -They are at length received by the eye-lens L M, which acts upon them -as the simple microscope has been described to act on the light -proceeding from its objects. They are bent so that they may enter the -eye at E in parallel lines, or as nearly so as is requisite for -distinct vision. When we say that the rays enter the eye in nearly -parallel lines, we mean only those which proceed from one point of the -original object. Thus the two parallel rays M E have proceeded from -and are part of the cone of rays C A D, emanating from the point A of -the arrow; but they do not form two pictures in the eye, because any -number of parallel rays which the pupil can receive will be converged -to a point by the eye, and will convey the impression of one point to -the mind. In like manner the rays L E are part of the cone of rays -emanating from B, and the angle L E M is that under which the eye will -see the magnified image of the arrow, which is evidently many times -greater than the arrow could be made to occupy in the naked eye at any -distance within the limits of distinct vision. The magnifying power -depends on two circumstances: first, on the ratio between the anterior -distance A C or B D and the posterior focal length C B' or D A'; and -secondly, on the power of the eye-lens L M. The first ratio is the -same as that between the object A B and the image A' B'; this and the -focal length or power of the eye lens are both easily obtained, and -their product is the power of the compound instrument. - -Since the power depends on the ratio between the anterior and -posterior foci of the object-glass, it is evident that by increasing -that ratio any power may be obtained, the same eye-glass being used; -or having determined the first, any further power may be obtained by -increasing that of the eye-glass; and thus, by a pre-arrangement of -the relative proportions in which the magnifying power shall be -divided between the object-glass and the eye-glass, almost any given -distance (within certain limits) between the first and its object may -be secured. This is one valuable peculiarity of the compound -instrument; and another is the large field, or large angle of view, -which may be obtained, every part of which will be nearly equally -good; whereas with the best simple microscopes the field is small, and -is good only in the centre. The field of the compound instrument is -further increased by using two glasses at the eye-end; the first being -called, from its purpose, the field-glass, and the two constituting -what is called the eye-piece. This will be more particularly explained -in the figure of the achromatic compound microscope presently given. - -For upwards of a century the compound microscope, notwithstanding the -advantages above mentioned, was a comparatively feeble and inefficient -instrument, owing to the distance which the light had to traverse, and -the consequent increase of the chromatic and spherical aberrations. To -explain this we have drawn in Fig. 12 a second image near A' B', the -fact being that the object-glass would not form one image, as has been -supposed, but an infinite number of variously-colored and -various-sized images, occupying the space between the two dotted -arrows. Those nearest the object-glass would be red, and those nearest -the eye-glass would be blue. The effect of this is to produce so much -confusion, that the instrument was reduced to a mere toy, although -these errors were diminished to the utmost possible extent by limiting -the aperture of the object-glass, and thus restricting the angle of -the pencil of light from each point of the object. But this involved -the defects, already explained, of making the picture obscure, so that -on the whole the best compound instruments were inferior to the simple -microscopes of a single lens, with which, indeed, all the important -observations of the last century were made. - -Even after the improvement of the simple microscope by the use of -doublets and triplets, the long course of the rays, and the large -angular pencil required in the compound instrument, deterred the most -sanguine from anticipating the period when they should be conducted -through such a path free both from spherical and chromatic errors. -Within twenty years of the present period, philosophers of no less -eminence than M. Blot and Dr. Wollaston predicted that the compound -would never rival the simple microscope, and that the idea of -achromatizing its object-glass was hopeless. Nor can these opinions be -wondered at when we consider how many years the achromatic telescope -had existed without an attempt to apply its principles to the compound -microscope. When we consider the smallness of the pencil required by -the telescope, and the enormous increase of difficulty attending every -enlargement of the pencil--when we consider further that these -difficulties had to be contended with and removed by operations on -portions of glass so small that they are themselves almost microscopic -objects, we shall not be surprised that even a cautious philosopher -and most able manipulator like Dr. Wollaston should prescribe limits -to improvement. - -Fortunately for science, and especially for the departments of animal -and vegetable physiology, these predictions have been shown to be -unfounded. The last fifteen years have sufficed to elevate the -compound microscope from the condition we have described to that of -being the most important instrument ever bestowed by art upon the -investigator of nature. It now holds a very high rank among -philosophical implements, while the transcendant beauties of form, -color and organization, which it reveals to us in the minute works of -nature, render it subservient to the most delightful and instructive -pursuits. To these claims on our attention, it appears likely to add a -third of still higher importance. The microscopic examination of the -blood and other human organic matter will in all probability afford -more satisfactory and conclusive evidence regarding the nature and -seat of disease than any hitherto appealed to, and will of consequence -lead to similar certainty in the choice and application of remedies. - -We have thought it necessary to state thus at large the claims of the -modern achromatic microscope upon the attention of the reader, as a -justification of the length at which we shall give its recent history -and explain its construction; and we are further induced to this -course by the consideration that the subject is entirely new ground, -and that there are at this time not more than two or three makers of -achromatic microscopes in England. - -Soon after the year 1820 a series of experiments was begun in France -by M. Selligues, which were followed up by Frauenhofer at Munich, by -Amici at Modena, by M. Chevalier at Paris, and by the late Mr. Tulley -in London. In 1824 the last-named excellent artist, without knowing -what had been done on the Continent, made the attempt to construct an -achromatic object-glass for a compound microscope, and produced one of -nine-tenths of an inch focal length, composed of three lenses, and -transmitting a pencil of eighteen degrees. This was the first that had -been made in England; and it is due to Mr. Tulley to say, that as -regards accurate correction throughout the field, that glass has not -been excelled by any subsequent combination of three lenses. Such an -angular pencil, and such a focal length, would bear an eye-piece -adapted to produce a gross magnifying power of one hundred and twenty. -Mr. Tulley afterwards made a combination to be placed in front of the -first mentioned, which increased the angle of the transmitted pencil -to thirty-eight degrees, and bore a power of three hundred. - -While these practical investigations were in progress, the subject of -achromatism engaged the attention of some of the most profound -mathematicians in England. Sir John Herschel, Professor Airy, -Professor Barlow, Mr. Coddington, and others, contributed largely to -the theoretical examination of the subject; and though the results of -their labors were not immediately applicable to the microscope, they -essentially promoted its improvement. - -For some time prior to 1829 the subject had occupied the mind of a -gentleman, who, not entirely practical, like the first, nor purely -mathematical, like the last-mentioned class of inquirers, was led to -the discovery of certain properties in achromatic combinations which -had been before unobserved. These were afterwards experimentally -verified; and in the year 1829 a paper on the subject, by the -discoverer, Mr. Joseph Jackson Lister, was read and published by the -Royal Society. The principles and results thus obtained enabled Mr. -Lister to form a combination of lenses which transmitted a pencil of -fifty degrees, with a large field correct in every part; as this paper -was the foundation of the recent improvements in achromatic -microscopes, and as its results are indispensable to all who would -make or understand the instrument, we shall give the more important -parts of it in detail, and in Mr. Lister's own words. - -"I would premise that the plano-concave form for the correcting flint -lens has in that quality a strong recommendation, particularly as it -obviates the danger of error which otherwise exists in centering the -two curves, and thereby admits of correct workmanship for a shorter -focus. To cement together also the two surfaces of the glass -diminishes by very nearly half the loss of light from reflection, -which is considerable at the numerous surfaces of a combination. I -have thought the clearness of the field and brightness of the picture -evidently increased by doing this; it prevents any dewiness or -vegetation from forming on the inner surfaces; and I see no -disadvantage to be anticipated from it if they are of identical -curves, and pressed closely together, and the cementing medium -permanently homogeneous. - -"These two conditions then, that the flint lens shall be -plano-concave, and that it shall be joined by some cement to the -convex, seem desirable to be taken as a basis for the microscopic -object-glass, provided they can be reconciled with the destruction of -the spherical and chromatic aberrations of a large pencil. - -"Now in every such glass that has been tried by me which has had its -correcting lens of either Swiss or English glass, with a double convex -of plate, and has been made achromatic by the form given to the outer -curve of the convex, the proportion has been such between the -refractive and dispersive powers of its lenses, that its figure has -been correct for rays issuing from some point in its axis not far from -its principal focus on its plane side, and either tending to a -conjugate focus within the tube of a microscope, or emerging nearly -parallel. - -"Let A B (Fig. 13) be supposed such an object-glass, and let it be -roughly considered as a plano-convex lens, with a curve A C B running -through it, at which the spherical and chromatic errors are corrected -which are generated at the two outer surfaces; and let the glass be -thus free from aberration for rays F D E G issuing from the radiant -point F, H E being a perpendicular to the convex surface, and I D to -the plane one. Under these circumstances, the angle of emergence G E H -much exceeds that of incidence F D I, being probably nearly three -times as great. - -"If the radiant is now made to approach the glass, so that the course -of the ray F D E G shall be more divergent from the axis, as the -angles of incidence and emergence become more nearly equal to each -other, the spherical aberration produced by the two will be found to -bear a less proportion to the opposing error of the single correcting -curve A C B; for such a focus therefore the rays will be -over-corrected. - -[Illustration: Fig. 13.] - -"But if F still approaches the glass, the angle of incidence -continues to increase with the increasing divergence of the ray, till -it will exceed that of emergence, which has in the meanwhile been -diminishing, and at length the spherical error produced by them will -recover its original proportion to the opposite error of the curve of -correction. When F has reached this point F'' (at which the angle of -incidence does not exceed that of emergence so much as it had at first -come short of it), the rays again pass the glass free from spherical -aberration. - -"If F be carried from hence towards the glass, or outwards from its -original place, the angle of incidence in the former case, or of -emergence in the latter, becomes disproportionately effective, and -either way the aberration exceeds the correction. - -"These facts have been established by careful experiment: they accord -with every appearance in such combinations of the plano-convex glasses -as have come under my notice, and may, I believe, be extended to this -rule, that in general an achromatic object-glass, of which the inner -surfaces are in contact, or nearly so, will have on one side of it two -foci in its axis, for the rays proceeding from which it will be truly -corrected at a moderate aperture; that for the space between these two -points its spherical aberration will be over-corrected, and beyond -them either way under-corrected. - -"The longer aplanatic focus may be found, when one of the plano-convex -object-glasses is placed in a microscope, by shortening the tube, if -the glass shows over-correction; if under-correction, by lengthening -it, or by bringing the rays together, should they be parallel or -divergent, by a very small good telescope. The shorter focus is got at -by sliding the glass before another of sufficient length and large -aperture that is finely corrected, and bringing it forwards till it -gives the reflection of a bright point from a globule of quicksilver, -sharp and free from mist, when the distance can be taken between the -glass and the object. - -"The longer focus is the place at which to ascertain the utmost -aperture that may be given to the glass, and where, in the absence of -spherical error, its exact state of correction as to color is seen -most distinctly. - -"The correction of the chromatic aberration, like that of the -spherical, tends to excess in the marginal rays; so that if a glass -which is achromatic, with a moderate aperture, has its cell opened -wider, the circle of rays thus added to the pencil will be rather -over-corrected as to color. - -"The same tendency to over-correction is produced, if, without varying -the aperture, the divergence of the incident rays is much augmented, -as in an object-glass placed in front of another; but generally in -this position a part only of its aperture comes into use; so that the -two properties mentioned neutralize each other, and its chromatic -state remains unaltered. If, for example, the outstanding colors were -observed at the longer focus to be green and claret, which show that -the nearest practicable approach is made to the union of the spectrum, -they usually continue nearly the same for the whole space between the -foci, and for some distance beyond them either way. - -"The places of these two foci and their proportions to each other -depend on a variety of circumstances. In several object-glasses that I -have had made for trial, plano-convex, with their inner surfaces -cemented, their diameters the radius of the flint lens, and their -color pretty well corrected, those composed of dense flint and light -plate have had the rays from the longer focus emerging nearly -parallel; and this focus has been not quite three times the distance -of the shorter from the glass: with English flint the rays have had -more convergence, and the shorter focus has borne a rather less -proportion to the longer. - -"If the surfaces are not cemented, a striking effect is produced by -minute differences in their curves. It may give some idea of this, -that in a glass of which nearly the whole disk was covered with color -from contact of the lenses, the addition of a film of varnish, so thin -that this color was not destroyed by it, caused a sensible change in -the spherical correction. - -"I have found that whatever extended the longer aplanatic focus, and -increased the convergence of its rays, diminished the relative length -of the shorter. Thus by turning to the concave lens the flatter -instead of the deeper side of a convex lens, whose radii were to each -other as 31 to 35, the pencil of the longer aplanatic focus, from -being greatly divergent, was brought to converge at a very small -distance behind the glass; and the length of the shorter focus, which -had been one-half that of the longer, became but one-sixth of it. - -"The direction of the aplanatic pencils appears to be scarcely -affected by the differences in the thickness of glasses, if their -state as to color is the same. - -"One other property of the double object-glass remains to be -mentioned, which is, that when the longer aplanatic focus is used, the -marginal rays of a pencil not coincident with the axis of the glass -are distorted, so that a coma is thrown outwards; while the contrary -effect of a coma directed towards the centre of the field is produced -by the rays from the shorter focus. These peculiarities of the coma -seem inseparable attendants on the two foci, and are as conspicuous in -the achromatic meniscus as in the plano-convex object-glass. - -[Illustration: Fig. 14.] - -"Of several purposes to which the particulars just given seem -applicable, I must at present confine myself to the most obvious one. -They furnish the means of destroying with the utmost ease both -aberrations in a large focal pencil, and of thus surmounting what has -hitherto been the chief obstacle to the perfection of the microscope. -And when it is considered that the curves of its diminutive -object-glasses have required to be at least as exactly proportioned as -those of a large telescope to give the image of a bright point equally -sharp and colorless, and that any change made to correct one -aberration was liable to disturb the other, some idea may be formed of -what the amount of that obstacle must have been. It will, however, be -evident that if any object-glass is but made achromatic, with its -lenses truly worked and cemented, so that their axes coincide, it may -with certainty be connected with another possessing the same -requisites and of suitable focus, so that the combination shall be -free from spherical error also in the centre of its field. For this -the rays have only to be received by the front glass B (Fig. 14) from -its shorter aplanatic focus F'', and transmitted in the direction of -the longer correct pencil F A of the other glass A. It is desirable -that the latter pencil should neither converge to a very short focus -nor be more than very slightly if at all divergent; and a little -attention at first to the kind of glass used will keep it within this -range, the denser flint being suited to the glasses of shorter focus -and larger angle of aperture. - -"The adjustment of the microscope is then perfected, if necessary, by -slightly varying the distance between the object-glasses; and after -that is done, the length of the tube which carries the eye-pieces may -be altered greatly without disturbing the correction, opposite errors -which balance each other being produced by the change. - -"If the two glasses which in the diagram are drawn at some distance -apart are brought nearer together (if the place of A, for instance, is -carried to the dotted figure), the rays transmitted by B in the -direction of the longer aplanatic pencil of A will plainly be derived -from some point Z more distant than F'', and lying between the -aplanatic foci of B; therefore (according to what has been stated) -this glass, and consequently the combination, will then be spherically -over-corrected. If, on the other hand, the distance between A and B is -increased, the opposite effects are of course produced. - -"In combining several glasses together it is often convenient to -transmit an under-corrected pencil from the front glass, and to -counteract its error by over-correction in the middle one. - -"Slight errors in color may in the same manner be destroyed by -opposite ones; and on the principles described we not only acquire -fine correction for the central ray, but by the opposite effects at -the two foci on the transverse pencil, all coma can be destroyed, and -the whole field rendered beautifully flat and distinct." - -Mr. Lister's paper enters into further particulars, which are not -essential to the comprehension of the subject. It is sufficient to say -that his investigations and results proved to be of the highest value -to the practical optician, and the progress of improvement was in -consequence extremely rapid. The new principles were applied and -exhibited by Mr. Hugh Powell and Mr. Andrew Ross with a degree of -success which had never been anticipated; so perfect indeed were the -corrections given to the achromatic object-glass--so completely were -the errors of sphericity and dispersion balanced or destroyed--that -the circumstance of covering the object with a plate of the thinnest -glass or talc disturbed the corrections, if they had been adapted to -an uncovered object, and rendered an object-glass which was perfect -under one condition sensibly defective under the other. - -This defect, if that should be called a defect which arose out of -improvement, was first discovered by Mr. Ross, who immediately -suggested the means of correcting it, and presented to the Society of -Arts, in 1837, a paper on the subject, which was published in the 51st -volume of their Transactions, and which, as it is, like Mr. Lister's -essential to a full understanding of the ultimate refinements of the -instrument, we shall extract nearly in full: - -"In the course of a practical investigation (says Mr. Ross) with the -view of constructing a combination of lenses for the object-glass of a -compound microscope, which should be free from the effects of -aberration, both for central and oblique pencils of great angle, I -combined the condition of the greatest possible distance between the -object and object-glass; for in object-glasses of short focal length -their closeness to the object has been an obstacle in many cases to -the use of high magnifying powers, and is a constant source of -inconvenience. - -"In the improved combination, the diameter is only sufficient to admit -the proper pencil; the convex lenses are wrought to an edge, and the -concave have only sufficient thickness to support their figure; -consequently the combination is the thinnest possible, and it follows -that there will be the greatest distance between the object and the -object-glass. The focal length is one-eighth of an inch, having an -angular aperture of 60 deg., with a distance of 1-25th of an inch, and a -magnifying power of 970 times linear, with perfect definition on the -most difficult Podura scales. I have made object-glasses 1-16th of an -inch focal length; but as the angular aperture cannot be -advantageously increased, if the greatest distance between the object -and object-glass is preserved, their use will be very limited. - -"The quality of the definition produced by an achromatic compound -microscope will depend upon the accuracy with which the aberrations, -both chromatic and spherical, are balanced, together with the general -perfection of the workmanship. Now, in Wollaston's doublets, and -Holland's triplets, there are no means of producing a balance of the -aberrations, as they are composed of convex lenses only; therefore the -best that can be done is to make the aberrations a minimum; the -remaining positive aberration in these forms produces its peculiar -effect upon objects (particularly the detail of the thin transparent -class), which may lead to misapprehension of their true structure; but -with the achromatic object-glass, where the aberrations are correctly -balanced, the most minute parts of an object are accurately displayed, -so that a satisfactory judgment of their character may be formed. - -[Illustration: Fig. 15.] - -[Illustration: Fig. 16.] - -"It will be seen by Fig. 15, that when a certain angular pencil A O A' -proceeds from the object O, and is incident on the plane side of the -first lens, if the combination is removed from the object, as in Fig. -16, the extreme rays of the pencil impinge on the more marginal parts -of the glass, and as the refractions are greater here, the aberrations -will be greater also. Now, if two compound object-glasses have their -aberrations balanced, one being situated as in Fig. 15, and the other -as in Fig. 16, and the same disturbing power applied to both, that in -which the angles of incidence and the aberrations are small will not -be so much disturbed as where the angles are great, and where -consequently the aberrations increase rapidly. - -"When an object-glass has its aberrations balanced for viewing an -opaque object, and it is required to examine that object by -transmitted light, the correction will remain; but if it is necessary -to immerse the object in a fluid, or to cover it with glass or talc, -an aberration will arise from these circumstances, which will disturb -the previous correction, and consequently deteriorate the definition; -and this effect will be more obvious with the increase of the distance -between the object and the object-glass. - -[Illustration: Fig. 17.] - -"The aberration produced with diverging rays by a piece of flat and -parallel glass, such as would be used for covering an object, is -represented at Fig. 17, where G G G G is the refracting medium, or -piece of glass covering the object O; O P, the axis of the pencil, -perpendicular to the flat surfaces; O T, a ray near the axis; and O -T', the extreme ray of the pencil incident on the under surface of the -glass; then T R, T' R', will be the directions of the rays in the -medium, and R E, R' E', those of the emergent rays. Now if the course -of these rays is continued, as by the dotted lines, they will be found -to intersect the axis at different distances, X and Y, from the -surface of the glass; and the distance X Y is the aberration produced -by the medium which, as before stated, interferes with the previously -balanced aberrations of the several lenses composing the -object-glass. There are many cases of this, but the one here selected -serves best to illustrate the principle. I need not encumber the -description with the theoretical determination of this quantity, as it -varies with exceedingly minute circumstances which we cannot -accurately control; such as the distance of the object from the under -side of the glass, and the slightest difference in the thickness of -the glass itself; and if these data could be readily obtained, the -knowledge would be of no utility in making the correction, that being -wholly of a practical nature. - -"If an object-glass is constructed as represented in Fig. 16, where -the posterior combination P and the middle M have together an excess -of negative aberration, and if this be corrected by the anterior -combination A, having an excess of positive aberration, then this -latter combination can be made to act more or less powerfully upon P -and M, by making it approach to or recede from them; for when the -three are in close contact, the distance of the object from the -object-glass is greatest; and consequently the rays from the object -are diverging from a point at a greater distance than when the -combinations are separated; and as a lens bends the rays more, or acts -with greater effect, the more distant the object is from which the -rays diverge, the effect of the anterior combination A upon the other -two, P and M, will vary with its distance from thence. When therefore -the correction of the whole is effected for an opaque object with a -certain distance between the anterior and middle combination, if they -are then put in contact, the distance between the object and -object-glass will be increased; consequently the anterior combination -will act more powerfully, and the whole will have an excess of -positive aberration. Now the effect of the aberration produced by a -piece of flat and parallel glass being of the negative character, it -is obvious that the above considerations suggest the means of -correction by moving the lenses nearer together, till the positive -aberration thereby produced balances the negative aberration caused by -the medium. - -"The preceding refers only to the spherical aberration, but the effect -of the chromatic is also seen when an object is covered with a piece -of glass; for, in the course of my experiments, I observed that it -produced a chromatic thickening of the outline of the Podura and -other delicate scales; and if diverging rays near the axis and at the -margin are projected through a piece of flat parallel glass, with the -various indices of refraction for the different colors, it will be -seen that each ray will emerge separated into a beam consisting of the -component colors of the ray, and that each beam is widely different in -form. This difference, being magnified by the power of the microscope, -readily accounts for the chromatic thickening of the outline just -mentioned. Therefore to obtain the finest definition of extremely -delicate and minute objects, they should be viewed without a covering; -if it be desirable to immerse them in a fluid, they should be covered -with the thinnest possible film of talc, as, from the character of the -chromatic aberration, it will be seen that varying the distances of -the combinations will not sensibly affect the correction; though -object-lenses may be made to include a given fluid or solid medium in -their correction for color. - -[Illustration: Fig. 18.] - -"The mechanism for applying these principles to the correction of an -object-glass under the various circumstances, is represented in Fig. -18, where the anterior lens is set in the end of a tube A A, which -slides on the cylinder B containing the remainder of the combination; -the tube A A, holding the lens nearest the object, may then be moved -upon the cylinder B, for the purpose of varying the distance according -to the thickness of the glass covering the object, by turning the -screwed ring C C, or more simply by sliding the one on the other, and -clamping them together when adjusted. An aperture is made in the tube -A, within which is seen a mark engraved on the cylinder, and on the -edge of which are two marks, a longer and a shorter, engraved upon the -tube. When the mark on the cylinder coincides with the longer mark on -the tube, the adjustment is perfect for an uncovered object; and when -the coincidence is with the short mark, the proper distance is -obtained to balance the aberrations produced by glass one-hundredth of -an inch thick, and such glass can be readily supplied. - -"It is hardly necessary to observe, that the necessity for this -correction is wholly independent of any particular construction of the -object-glass; as in all cases where the object-glass is corrected for -an object uncovered, any covering of glass will create a different -value of aberration to the first lens, which previously balanced the -aberration resulting from the rest of the lenses; and as this -disturbance is effected at the first refraction, it is independent of -the other part of the combination. The visibility of the effect -depends on the distance of the object from the object-glass, the angle -of the pencil transmitted, the focal length of the combination, the -thickness of the glass covering the object, and the general perfection -of the corrections for chromatism and the oblique pencils. - -"With this adjusting object-glass, therefore, we can have the -requisites of the greatest possible distance between the object and -object-glass, an intense and sharply defined image throughout the -field from the large pencil transmitted, and the accurate correction -of the aberrations; also, by the adjustment, the means of preserving -that correction under all the varied circumstances in which it may be -necessary to place an object for the purpose of observation." - -In the annexed engraving, Fig. 19, we have shown the triple achromatic -object-glass in connection with the eye-piece consisting of the -field-glass F F, and the eye-glass E E, forming together the modern -achromatic microscope. The course of the light is shown by drawing -three rays from the centre and three from each end of the object O. -These rays would, if left to themselves, form an image of the object -at A A, but being bent and converged by the field-glass F F, they form -the image at B B, where a stop is placed to intercept all light except -what is required for the formation of the image. From B B therefore -the rays proceed to the eye-glass exactly as has been described in -reference to the simple microscope and to the compound of two glasses. - -[Illustration: Fig. 19.] - -If we stopped here we should convey a very imperfect idea of the -beautiful series of corrections effected by the eye-piece, and which -were first pointed out in detail in a paper on the subject published -by Mr. Varley in the 51st volume of the Transactions of the Society of -Arts. The eye-piece in question was invented by Huyghens for -telescopes, with no other view than that of diminishing the spherical -aberration by producing the refractions at two glasses instead of one, -and of increasing the field of view. It was reserved for Boscovich to -point out that Huyghens had by this arrangement accidentally corrected -a great part of the chromatic aberration, and this subject is further -investigated with much skill in two papers by Professor Airy in the -_Cambridge Philosophical Transactions_, to which we refer the -mathematical reader. These investigations apply chiefly to the -telescope, where the small pencils of light and great distance of the -object exclude considerations which become important in the -microscope, and which are well pointed out in Mr. Varley's paper -before mentioned. - -[Illustration: Fig. 20.] - -Let Fig. 20 represent the Huyghenean eye-piece of a microscope; F F -and E E being the field-glass and eye-glass, and L M N the two extreme -rays of each of the three pencils, emanating from the centre and ends -of the object, of which, but for the field-glass, a series of colored -images would be formed from R R to B B; those near R R being red, -those near B B blue, and the intermediate ones green, yellow, and so -on, corresponding with the colors of the prismatic spectrum. This -order of colors, it will be observed, is the reverse of that -described in treating of the common compound microscope (Fig. 12), in -which the single object-glass projected the red image beyond the blue. -The effect just described, of projecting the blue image beyond the -red, is purposely produced for reasons presently to be given, and is -called over-correcting the object-glass as to color. It is to be -observed also that the images B B and R R are curved in the wrong -direction to be distinctly seen by a convex eye-lens, and this is a -further defect of the compound microscope of two lenses. But the -field-glass, at the same time that it bends the rays and converges -them to foci at B' B' and R' R', also reverses the curvature of the -images as there shown, and gives them the form best adapted for -distinct vision by the eye-glass E E. The field-glass has at the same -time brought the blue and red images closer together, so that they are -adapted to pass uncolored through the eye-glass. To render this -important point more intelligible, let it be supposed that the -object-glass had not been over-corrected, that it had been perfectly -achromatic; the rays would then have become colored as soon as they -had passed the field-glass; the blue rays, to take the central pencil, -for example, would converge at _b_ and the red rays at _r_, which is -just the reverse of what the eye-lens requires; for as its blue focus -is also shorter than its red, it would demand rather that the blue -image should be at _r_ and the red at _b_. This effect we have shown -to be produced by the over-correction of the object-glass, which -protrudes the blue foci B B as much beyond the red foci R R as the sum -of the distances between the red and blue foci of the field-lens and -eye-lens; so that the separation B R is exactly taken up in passing -through those two lenses, and the whole of the colors coincide as to -focal distance as soon as the rays have passed the eye-lens. But while -they coincide as to distance, they differ in another respect; the blue -images are rendered smaller than the red by the superior refractive -power of the field-glass upon the blue rays. In tracing the pencil L, -for instance, it will be noticed that after passing the field-glass, -two sets of lines are drawn, one whole, and one dotted, the former -representing the red, and the latter the blue rays. This is the -accidental effect in the Huyghenean eye-piece pointed out by -Boscovich. This separation into colors at the field-glass is like the -over-correction of the object-glass; it leads to a subsequent complete -correction. For if the differently colored rays were kept together -till they reached the eye-glass, they would then become colored, and -present colored images to the eye; but fortunately, and most -beautifully, the separation effected by the field-glass causes the -blue rays to fall so much nearer the centre of the eye-glass, where, -owing to the spherical figure, the refractive power is less than at -the margin, that the spherical error of the eye-lens constitutes a -nearly perfect balance to the chromatic dispersion of the field-lens, -and the red and blue rays L' and L'' emerge sensibly parallel, -presenting, in consequence, the perfect definition of a single point -to the eye. The same reasoning is true of the intermediate colors and -of the other pencils. - -From what has been stated it is obvious that we mean by an achromatic -object-glass one in which the usual order of dispersion is so far -reversed that the light, after undergoing the singularly beautiful -series of changes effected by the eye-piece, shall come uncolored to -the eye. We can give no specific rules for producing these results. -Close study of the formulae for achromatism given by the celebrated -mathematicians we have quoted will do much, but the principles must be -brought to the test of repeated experiment. Nor will the experiments -be worth anything, unless the curves be most accurately measured and -worked, and the lenses centered and adjusted with a degree of -precision which, to those who are familiar only with telescopes, will -be quite unprecedented. - -The Huyghenean eye-piece which we have described is the best for -merely optical purposes, but when it is required to measure the -magnified image, we use the eye-piece invented by Mr. Ramsden, and -called, from its purpose, the micrometer eye-piece. When it is stated -that we sometimes require to measure portions of animal or vegetable -matter a hundred times smaller than any divisions that can be -artificially made on any measuring instrument, the advantage of -applying the scale to the magnified image will be obvious, as compared -with the application of engraved or mechanical micrometers to the -stage of the instrument. - -The arrangement is shown in Fig. 21, where E E and F F are the eye and -field glass, the latter having now its plane face towards the object. -The rays from the object are here made to converge at A A, immediately -in front of the field-glass, and here also is placed a plane glass on -which are engraved divisions of a hundredth of an inch or less. The -markings of these divisions come into focus therefore at the same time -as the image of the object, and both are distinctly seen together. -Thus the measure of the magnified image is given by mere inspection, -and the value of such measures in reference to the real object may be -obtained thus, which, when once obtained, is constant for the same -object-glass. Place on the stage of the instrument a divided scale the -value of which is known, and viewing this scale as the microscopic -object, observe how many of the divisions on the scale attached to the -eye-piece correspond with one of those in the magnified image. If, for -instance, ten of those in the eye-piece correspond with one of those in -the image, and if the divisions are known to be equal, then the image -is ten times larger than the object, and the dimensions of the object -are ten times less than indicated by the micrometer. If the divisions -on the micrometer and on the magnified scale were not equal, it -becomes a mere rule-of-three sum, but in general this trouble is taken -by the maker of the instrument, who furnishes a table showing the -value of each division of the micrometer for every object-glass with -which it may be used. - -[Illustration: Fig. 21.] - -While on the subject of measuring it may be well to explain the mode -of ascertaining the magnifying power of the compound microscope, which -is generally taken on the assumption before mentioned, that the naked -eye sees most distinctly at the distance of ten inches. - -Place on the stage of the instrument, as before, a known divided -scale, and when it is distinctly seen, hold a rule at ten inches -distance from the disengaged eye, so that it may be seen by that eye, -overlapping or lying by side of the magnified picture of the other -scale. Then move the rule till one or more of its known divisions -correspond with a number of those in the magnified scale, and a -comparison of the two gives the magnifying power. - -Having now explained the optical principles of the achromatic compound -microscope, it remains only to describe the mechanical arrangements -for giving those principles their full effect. The mechanism of a -microscope is of much more importance than might be imagined by those -who have not studied the subject. In the first place, steadiness, or -freedom from vibration, and most particularly freedom from any -vibrations which are not equally communicated to the object under -examination, and to the lenses by which it is viewed, is a point of -the utmost consequence. When, for instance, the body containing the -lenses is screwed by its lower extremity to a horizontal arm, we have -one of the most vibratory forms conceivable; it is precisely the form -of the inverted pendulum, which is expressly contrived to indicate -otherwise insensible vibrations. The tremor necessarily attendant on -such an arrangement is magnified by the whole power of the instrument; -and as the object on the stage partakes of this tremor in a -comparatively insensible degree, the image is seen to oscillate so -rapidly, as in some cases to be wholly undistinguishable. Such -microscopes cannot possibly be used with high powers in ordinary -houses abutting on any paved streets through which carriages are -passing, nor indeed are they adapted to be used in houses in which the -ordinary internal sources of shaking exist. - -One of the best modes of mounting a compound microscope is shown in -the annexed view (Fig. 22), which, though too minute to exhibit all -the details, will serve to explain the chief features of the -arrangement. - -A massy pillar A is screwed into a solid tripod B, and is surmounted -by a strong joint at C, on which the whole instrument turns, so as to -enable it to take a perfectly horizontal or vertical position, or any -intermediate angle, such, for instance, as that shown in the -engraving. - -This movable portion of the instrument consists of one solid casting D -E F G; from F to G being a thick pierced plate carrying the stage and -its appendages. The compound body H is attached to the bar D E, and -moves up and down upon it by a rack and pinion worked by either of the -milled heads K. The piece D E F G is attached to the pillar by the -joint C, which being the source of the required movement in the -instrument, is obviously its weakest part, and about which no doubt -considerable vibration takes place. But inasmuch as the piece D E F G -of necessity transmits such vibrations equally to the body of the -microscope and to the objects on the stage, they hold always the same -relative position, and no _visible_ vibration is caused, how much -soever may really exist. To the under side of the stage is attached a -circular stem L, on which slides the mirror M, plane on one side and -concave on the other, to reflect the light through the aperture in the -stage. Beneath the stage is a circular revolving plate containing -three apertures of various sizes, to limit the angle of the pencil of -light which shall be allowed to fall on the object under examination. -Besides these conveniences the stage has a double movement produced by -two racks at right angles to each other, and worked by milled heads -beneath. It has also the usual appendages of forceps to hold minute -objects, and a lens to condense the light upon them, all of which are -well understood, and if not, will be rendered more intelligible by a -few minutes' examination of a microscope than by the most lengthened -description. One other point remains to be noticed. The movement -produced by the milled head K is not sufficiently delicate to adjust -the focus of very powerful lenses, nor indeed is any rack movement. -Only the finest screws are adapted to this purpose; and even these are -improved by means for reducing the rapidity of the screw's movement. -For this purpose the lower end of the compound body H, which carries -the object-glass, consists of a piece of smaller tube sliding in -parallel guides in the main body, and kept constantly pressed upwards -by a spiral spring but it can be drawn downward by a lever crossing -the body, and acted on by an extremely fine screw whose milled head -is seen at N, and the fineness of which is tripled by means of the -lever through which it acts on the object-glass. The instrument is of -course roughly adjusted by the rack movement, and finished by the -screw, or by such other means as are chosen for the purpose. One very -ingenious contrivance, but applied to the stage, instead of the body -of the microscope, invented by Mr. Powell, will be found described in -the 50th volume of the Transactions of the Society of Arts. - -[Illustration: Fig. 22.] - -The greater part of the directions for viewing and illuminating -objects given in reference to the simple microscope are applicable to -the compound. An argand lamp placed in the focus of a large detached -lens so as to throw parallel rays upon the mirror, is the best -artificial light; and for opaque objects the light so thrown up may be -reflected by metallic specula (called, from their inventor, -Lieberkhuns) attached to the object-glasses. - -It has been recently proposed by Sir David Brewster and by M. Dujardin -to render the Wollaston condenser achromatic, and they have -accordingly been made with three pairs of achromatic lenses instead of -the single lens before described, with very excellent effect. The -last-mentioned gentleman has also projected an ingenious apparatus, -called the Hyptioscope, attached to the eye-piece for the purpose of -erecting the magnified picture. - -The erector commonly applied to the compound microscope consists of a -pair of lenses acting like the erecting eye-piece of the telescope. -But this, though it is convenient for the purpose of dissection, very -much impairs the optical performance of the instrument. - -[Illustration: Fig. 23.] - -For drawing the images presented by the microscope the best apparatus -consists of a mirror M (Fig. 23), composed of a thin piece of rather -dark-colored glass cemented on to a piece of plate-glass inclined at -an angle of 45 deg. in front of the eye-glass E. The light escaping from -the eye-glass is assisted in its reflection upwards to the eye by the -dark glass, which effects the further useful purpose of rendering the -paper less brilliant, and thus enabling the eye better to see the -reflected image. The lens L below the reflector is to cause the light -from the paper and pencil to diverge from the same distance as that -received from the eye-glass; in other words, to cause it to reach the -eye in parallel lines. - -[Illustration: Fig. 24.] - -Dr. Wollaston's camera lucida, as shown in Fig. 24, is sometimes -attached to the eye-piece of the microscope for the same purpose. In -this instrument the rays suffer two internal reflections within the -glass prism, as will be seen explained in the article "Camera Lucida." -In this minute figure we have omitted to trace the reflected rays, -merely to avoid confusion. - -[Illustration: Fig. 25.] - -[Illustration: Fig. 26.] - -[Illustration: Fig. 27.] - -Annexed are four engravings of microscopic objects, the true character -of which it is, however, impossible to give in wood, and is difficult -indeed to accomplish by any description of engraving. - -[Illustration: Fig. 28.] - -Fig. 25 shows a scale of the small insect called Podura Plumbea, the -common Skiptail, magnified about five hundred times. To define the -markings on this scale clearly is the highest test of a deep -achromatic object-glass; and this drawing is given rather to explain -what the observer should look for, than as a very correct -representation. Fig. 26 is a scale or feather of the Menelaus -Butterfly; Fig. 27 is the hair of a singular insect, the Dermestes; -and Fig. 28 is a longitudinal cutting of fir, showing the circular -glands on the vessels which distinguish coniferous woods. These latter -objects may be seen by half-inch or quarter-inch achromatic glasses. -Opaque objects are generally better exhibited by inch and two-inch -glasses, when a general view of them is required, and by higher powers -when we wish to examine their minute structure. In the latter case the -light must be obtained by condensing lenses instead of the metallic -specula. - -Although the reflecting microscope is now very little used, it may be -expected that we should mention it. In this instrument, at Fig. 29, -the object O is reflected by the inclined face of the mirror M, and -the rays are again reflected and converged by the ellipsoidal -reflector R R, which effects the same purpose as the object-glass of -the compound microscope. It forms an image which is not susceptible of -the over-correction as to color before described, and which therefore -becomes colored in passing through the eye-piece. This fact, and the -loss of light by reflection, will probably always render the -reflecting microscope inferior to the achromatic refracting. - -[Illustration: Fig. 29.] - -The solar microscope has been so nearly superseded by the -oxy-hydrogen, that a brief description of the latter must suffice, -particularly as their optical principles are similar. - -The primary object in both is to throw an intense light upon the -object, which is sometimes done by mirrors, and sometimes by lenses. -In Fig. 30, L represents the cylinder of burning lime, R R the -reflector, which concentrates the light upon the object O O; the rays -from which, passing through the two plano-convex lenses, are brought -to foci upon a screen placed at a great distance, and upon which is -formed the magnified image. - -[Illustration: Fig. 30.] - -Fig. 31 shows a combination of lenses to condense the light upon the -object. In either case the optical arrangements by which the image is -formed admit of the same perfection as those which have been described -for the compound microscopes. A few achromatic glasses for -oxy-hydrogen microscopes have been made, and they will ultimately -become valuable instruments for illustrating lectures on natural -history and physiology. One made by Mr. Ross was exhibited a few -months since at the Society of Arts to illustrate a lecture on the -physiology of woods. It should be observed, however, that the -oxy-hydrogen or solar microscope requires either a spherical screen, -or that the objects should be mounted between spherical glasses, in -order to bring the whole into focus at one time. This latter plan was -adopted on the occasion just mentioned with perfect success. - -[Illustration: Fig. 31.] - - - - - -End of the Project Gutenberg EBook of The Microscope, by Andrew Ross - -*** END OF THIS PROJECT GUTENBERG EBOOK THE MICROSCOPE *** - -***** This file should be named 41958.txt or 41958.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/4/1/9/5/41958/ - -Produced by Chris Curnow, Matthew Wheaton and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - -Updated editions will replace the previous one--the old editions -will be renamed. - -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. 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