Micrographia: containing practical essays on reflecting, solar, oxy-hydrogen gas microscopes, micrometers, eye-pieces, &c. &c

Preface

The importance and utility of the modern improvements in Microscopes being now generally recognized throughout Europe by all practical zoologists and botanists*, the present publication seems imperatively called for as a necessary sequel to the “ Microscopic Illustrations” and “ Microscopic Cabinet,” and is intended to supply all the practical information microscopies can require, which is not afforded by those works.

The first chapter contains a description of all the reflecting Microscopes that are worthy of note, and in particular an account of the most improved form of that of Professor Amici, of Modena (the only one which has come into use) : and it may fearlessly be maintained, that Dr. Goring has omitted no argument which could be adduced in its favour. To this is attached full and perspicuous directions for its management, in every possible manner in which it can be mounted. In my humble opinion, this chapter is an excellent model for this kind of writing.

* The recent discoveries of the fossil remains of Infusoria, Plants, &c. will cause the geologist also to call to his assistance the aid of the Microscope.

IV

PREFACE.

The tract on Micrometers, whatever may be its faults, is the best practical treatise that has yet appeared ; and it is worthy of remark, that few theoretical or even practical opticians can determine the foci or powers of small lenses with any degree of precision : put a minute lens into their hands, and ask them to tell you what its solar or visual focus is, and ten to one if they can give any thing but a rude approximation to the truth. Now, all difficulties of this kind are obviated ; for by his tract, the foci, powers, &c. of all sorts of lenses, simple and compound, as well as those of Engiscopes, can be determined with accuracy. To the description of the various constructions of Micrometers is added that of a new one, for single or compound magnifiers.

The chapter on Monochromatic Illumination contains many curious particulars : that on Solar Engiscopes may be said to be written on a new species of instrument, or one at least not as yet introduced into use, but which may be expected one day to become highly popular with the more refined and fastidious admirers of the wonders displayed by Solar Microscopes.

The chapters* on trying Microscopes and Engiscopes against each other, and that on Eye-pieces, contain much valuable information, and are written in a very forcible style. The tract (Chapter VII.), illustrative of the effects of large angles of aperture on the vision of objects, as contradistinguished from

* The opinions expressed in each chapter are those of the writers whose initials are severally subjoined.

PREFACE.

V magnifying power, touches the very nerve of the question respecting the importance of the modern improvements of object-glasses, metals, doublets, and triplets ; the extension of their angular aperture, or the taking in of a large 'pencil of light (free from aberrations) from every part of the object , being, in fact, nearly the sum and substance of every thing which has been effected in ameliorating the optical part of Microscopes and Engi scopes ; for so far from increasing the magnifying power (as is generally imagined by persons not acquainted with the subject), the principal improvement may be said to consist in so augmenting the penetrating powers of these instruments , that a very high degree of amplification becomes in a manner unnecessary, There is also some useful information on Objects in this chapter.

My own essay on Solar and Oxy-hydrogen Gas Microscopes will, I doubt not, receive due attention from the public, being the first account of the latter which has appeared ; it will be found to contain a vast body of very useful practical information respecting these highly popular, attractive, and amusing instruments, which may be said to have conferred on mankind a new source of pleasure.

In Chapter XI. I have briefly described a few contrivances for facilitating the observation of Microscopic Objects, and perfecting the mechanical part of instruments.

Correct Drawings of Microscopic Objects are exceedingly valuable ; they are every day sought after

VI

PREFACE.

with increasing eagerness; and as my friend, Mr. Bauer, has made more of them than any other person in England, it appeared to me highly desirable to present the reader with an account of the method pursued by him in their production. For this purpose, I have been favoured with a short essay written by himself, and feel confident it will be perused with much interest. The other paper in the Appendix, by the Rev. J. B. Reade, on his new method of Illuminating Microscopic Objects, will be found highly useful, either for investigation or amusement ; with it, even the most fastidious may employ a miscroscope without excitement from light, as in the ordinary illumination, while it is equally avciilable for the larger class, or for the scales of the Podura.

ANDREW PRITCHARD.

263, Strand, London.

On the construction and management of Solar and Oxy-hydrogen Gas Microscopes, &c 168

Chap. IX

I. — On Cuvier’s method of dissecting Microscopic Subjects under Fluid ; with Additions 215

II. — On a new method of obtaining a delicate adjust-

ment to the focus of Microscopes • , • 217

III. — On an improved mode of supporting a candle or

lamp for Microscopes 218 appendix.

No. 1. — On making Drawings of Microscopic Subjects, in a letter, by Francis Bauer, Esq. F.R.S. F.L.S. &c. 221

No. 2. — On a new method of illuminating Microscopic Objects. By the Rev. J. B. Reade, M.A. of Caius College, Cambridge, and Honorary Member of the Halifax Philosophical Society 227

THE

MICROGRAPHIA.

Chapter I. History And Description Of The Amician Reflecting Engiscope

While one sect of Microscopists is pragmatically employed in the production of fancied novelties , and not unfrequently of downright retro gradations in the structure of Microscopes, another party seems sturdily determined that there shall be nothing new under the sun in these latter times : we may rack our brains till they are addled, invent and excogitate all sorts of wonders, ay, and get rewarded by some learned body for our pains, and lo ! it is certain to be found out in the end that we have merely resuscitated some old thing which the ignorance, or want of recollection, of the public had overlooked, and that our utmost endeavours have only succeeded in dressing it out in new habiliments, or engaging it in some new occupation.

When Professor Amici, of Modena, invented his admirable reflecting Engiscope, (for it cannot be pretended that we ever had a reflector really capable of practical application before,) the spirit of detraction instantly exclaimed that he had merely reversed a Newtonian telescope made on a small scale , and of course refused to concede the merit he had so richly earned in

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inventing and introducing into use what every candid person must allow to be a real novelty . Sir Isaac Newton never recommended any such adaptation of his telescope to the purpose of acting upon diverging rays, but merely a concave metal to form an image, and an eye-glass to view it*, being in fact the simplest kind of reflecting microscope which can possibly be constructed, and the best also, if it could but be easily applied to the examination of minute objects, which, however, is not the case. (Vide fig. 25.) In fact, it is in the exterior only of the Amician microscope that there is a prima facie resemblance to a Newtonian telescope, for the form of the concave metal and the situation of the radiant point are so totally different, as to constitute a new instrument.

The severe but simple rebuke given by Columbus to those invidious persons who denied him any merit in discovering the New World, because any one might have done as much, should never be forgotten. Having requested them to try to set up an egg on a plate, which they endeavoured to effect in vain ; by giving the egg a slight blow on one end, thereby rendering a portion of its surface flat, he caused it to stand upright, fe Any fool might have done so simple a thing as that,” said his enemies. “ Why did not you do it, then?” answered Columbus.

It has always been remarked that the English invent nothing, but improve upon every thing ; and the Amician reflecting Engiscope may, in its perfect state, certainly be considered as a fine sample of English improvement on a foreign invention. For, though Professor Amici joins to his acquirements as a man of science an admirable genius for mechanics, and is, in fact, one of the best workmen the continent can produce, yet the instrument was turned out of his hands in a very rough and ineffective state, owing to the concave metal being of too long a focus, and too small an angle of aperture, and the diagonal one of too large a diameter, which caused it to intercept too

* Vide his correspondence with Gregory of Aberdeen, published at the end of Gregory’s Optics.

REFLECTING ENGISCOPE.

large a quantity of light from the other, leaving only a narrow rim of reflection to enter the retina, which occasioned a disagreeable nebulosity in the middle of the field of view, unless the eye-glass was of great depth ; the mounting was, moreover, very deficient, and greatly disabled and cramped the powers of the instrument. The original dimensions of Amici’s Engiscope were the following : — Elliptic metal 1 inch aperture, and ^To inch focus ; diameter of the diagonal \ an inch ; length of tube 1 foot ; and however well such metals may be worked, it will be found impracticable to render them capable of exhibiting even ordinary minute objects as they ought to be shewn, much less tests.

Mr. Cuthbert and myself became the parents of the instrument in its effective form, as follows : —

Mr. C. had devoted a very large portion of his time to the working of metals for dumpy Gregorian telescopes, frequently having an aperture of 3 inches to a focus of only 5 ; he was, therefore, exactly cut out for a maker of engiscopes. I chanced to be introduced to him when he had just made a microscope on Amici’s principle, having an aperture of an inch and a half to a focus of three inches, with a body one foot in length. Just about this period I had discovered the peculiar properties of the feathers and scales of insects, as test-objects : we tried the instrument with them, but though the figures of its metals were excellent, we found its performance quite unsatisfactory : I therefore recommended Mr. C. to endeavour to construct a pair of metals of which the concave should be only lialf-an-inch focus, and to shorten the body to four or five inches. He accordingly succeeded in making a pair of 0-6 of an inch focus, and O’ 3 of aperture, which gave a delightful foretaste of what might be expected from reflecting engiscopes when perfected. Its body was inserted into the cleft socket of the stand of a small spy-glass Mr. C. happened to have by him, and I lent him the stem of a single microscope made by Samuel Varley, to attach as a dependent bar to the objective

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end of the body, which happened to suit it very well, and this arrangement constituted the original pattern of the instrument. I had the honour also to suggest a variety of minor accompaniments and nicknacks which Mr. C , with that liberality of spirit which characterizes true genius, adopted, though his head, more fertile in mechanical expedients and resources than mine, generally enabled him to improve upon what I recommended.

I also particularly explained to him a method by which a metal of an unlimited angle of aperture might be made to act upon transparent bodies, without increasing the size of the small metal4

In relating these little anecdotes of the origin of the instrument, I am afraid I shall be justly accused of egotism ; but I am sure that by and by, when the invention shall have attained a venerable beard of due length, they will be read with pleasure.

As I have in the paper just alluded to, given sufficient directions for the management of the original instrument, and as Mr. Cuthbert has moreover in the same work published his own method of managing it in its perfected'state, it is not my intention in the present work to give any repetition of those documents, but to describe a variety of it, with the final improvements which my worthy coadjutor Mr. Pritchard and myself have, by clubbing our wise noddles together, (as we imagine,) effected in it. I trust we have played no monkey tricks ; that there will be found in the new form no affected singularities , or alterations made merely for the purpose of differing from our neighbours ; but that, where deviations from the pattern manufactured by Mr. Cuthbert have been adopted, they are in some sense or another for the better.

* Vide my paper in the Quarterly Journal of Science of the Royal Institution, vol. 21. p. 34, and pi. 1. In this tract will be found the first description of the aquatic live box, which, like all other good things, is claimed by half a dozen fathers, while a bad thing is sure to have only one, or to be like a bastard, without any father at all.

REFLECTING ENGISCOPE.

To begin from the foundation : - — The stand or pillar of the Engiscope is not unlike those of small spy-glasses ; it grasps the body of the instrument by means of a split socket, as they do, and will readily serve as a stand to a 20-inch glass, if required : the peculiarities of it I shall now notice. The pillar is screwed on to a solid cruciform stand : — it is well known to every cabinet-maker that a claw table stands much more securely on four legs than on three, and is much less liable to be overturned ; of this any person may easily satisfy himself ; and though a table on three legs is always sure to rest truly on an uneven surface, one on four may be made to do so likewise by means of an adjusting screw applied to one of the legs, made purposely a little too short, and, moreover, will not require to be made so massive, or to have its feet sprawled out so far, in order to procure a given degree of stability. Now the Amician Engiscope requires to be raised to a considerable height, in order to come to the height of the eye without a scaffolding of boxes, books, or pieces of wood, &c. : nothing can be more agreeable to use when it is on a true level with the eye, or more uncomfortable when it is not ; on this account the cruciform stand seems to be peculiarly suited to it. At the same time I freely admit, that if the legs are wanted to be made to fold up together, in order to pack in a small compass, the tripod is the preferable construction, for the cruciform must be made of one piece. Mr. P., of course, makes them to suit the fancy of purchasers : I have little doubt that he would, to oblige a good customer, mount an instrument on a mop-stick, or the top of an old brass candle-stick, as Sir I. Newton did the original of his telescope. It will be observed that there are two joints in the pillar, like those of a pull-out telescope : these are composed of tubes, the recipient part of which is split, and screwed on the outside, and firmly grasps the internal tube by means of the pinching milled heads* so that the whole is as strong and solid as if made of a single tube. As will be seen by reference to the scale, the instrument may be

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elevated to the height of 18 inches, if required. All the particulars I have described are sufficiently well represented in Plate 1, fig. 1 ; the adjusting screw for the fourth foot is seen in front. Fig. 17 is a key for tightening the cradle-joint at the top of the pillar, so that the body may be firmly fixed in any degree of inclination without swerving when turned round in the split socket.

The body ( a ) has nothing very peculiar about it ; it has the usual eye-tube (cZ), into which is screwed the eye-piece ( e ), consisting of two glasses on the Huyghenian principle, but in the lower 'powers further separated than in the regular construction, to accomodate them to the metals : it is difficult to procure a field of view of more than 20° with a very shallow eye-piece, acting with a short body, if a maximum of distinctness is required. Not unfrequently the edges of the field of view with low powers are rendered dark and shadowy, or altogether invisible, owing to the hole in the side of the tube of reflectors (fig. 28, c.) not being sufficiently opened out. The same effect is even, to a certain extent, sometimes produced by the hole in the stage being too small, or closed up partially by the traversing plates. The foci of the anterior glasses of the eye-pieces, of which there are usually three, are 3-4ths, 3-8ths,and 3-16ths of an inch; the powers given by which of course depend upon the depths of the metals they are used with, and also upon the elongation or contraction of the length of the body, by means of the eye-tube, b, c, is the tube containing the metals ; a section of it, fig. 28, shews them of the full size for one inch acting focus, in situ : b , is the elliptic one ; a , the plane diagonal, c ; the hole in the side of the tube to admit the rays from the object, cZ, the radiant point, or object itself. The rays proceed from the object in the manner marked in fig. 28, and are reverberated into the opposite focus of the ellipse, where they form an image between the two eye-glasses, which is viewed by the eye through the anterior glass.

On the outside of the tube of reflectors, at 6, will be re-

REFLECTING ENGISCOPE.

marked a lappet, or segment of tube, the use of which is to lap over the hole in the side of the tube, to keep out dust and damp, &c., when the reflectors are not in action : for the same reason a cap is made to cover the open end of the tube, when removed from the body. A circular line is also turned on the tube, passing exactly through the centre of the hole in the side, as a rough guide for placing the object in its proper place.

It here becomes necessary to say somewhat concerning the manner in which the tube of reflectors and the body of the instrument are attached to each other : this is effected in the ordinary manner in which the component tubes of a pull-out spy-glass are connected with each other. Every one must have observed that the tubes of spy-glasses rather get tighter than otherwise from age and use, owing to the partial oxidation of their surfaces ; moreover, if w'ell executed, a tube consisting of a number of pieces is just as stiff and straight as if formed only of one. In this way, therefore, are the tubes of reflectors fastened to the main body ; one of them, of the full size, is seen at fig. 11, with the spring-joint* (d), which will, I doubt not, be instantly recognized by any optical turner-]-. The only peculiar point necessary to be attended to is to form a notch, or a mark of some kind, on the milling of the sliding tube (e), which must be made to coincide exactly with another on the neck of the body, as may be observed in fig. 1, in order that the metals may be always affixed so that the prolongation of their axis and of the body may be in the line of the bar. Too much care cannot be bestowed on this circumstance, for if the plane of the stage and object presents itself obliquely to the metals, their performance will be seriously deteriorated.

* One of these, of course, suffices for the whole of the tubes, and always remains screwed to the neck of the body.

t I think the joints of flutes, clarionets, &c. might be made with great advantage on this principle, and would not be at all liable to get out of order ; a little bee’s-wax and suet would render them perfectly windtight, though I suspect if the tubes were carefully drawn, and merely ■wetted when put together for use, the wax would not be required.

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fig. 1, is the triangular bar of the Engiscope, soldered to the neck of the body, which, with all its apparatus, is much like that of other engiscopes. Its mirror is plane on one side, and should be a dead white surface on the other. 1 do not hold with concave mirrors ; they always produce an indistinctness in vision, along with an increase of light, when used with transparent objects, and for opaque ones they are not wanted, as the condensing lens, fig. 7, acts much better, either used by itself or in conjunction with the plane side of the mirror, as circumstances may require. The stage (l) is undoubtedly the best yet contrived for giving traversing motions, being the celebrated combination of rack and screw work, worked by the two concentric milled heads at m, invented by Mr. Tyrrell. The small anterior head gives a motion to the object in the line of the body, the other in the contrary direction ; both are under the command of the finger and thumb of the right hand, and both may be moved together, giving a diagonal motion. The said stage is removable at pleasure, from the triangular socket ( g ) being made to lift out.

To the triangular socket (g) is appended that which carries the clamping screw (h), and the adjusting screw fij, by which the object is brought into the focus of the Engiscope.

Fig. 2 is the tube which carries the aplanatic object-glasses belonging to the instrument ; the rectangular prism, which enables them to act with the Amician, may be seen in dotted lines. Where several object-glasses are screwed together, they ought to be made of decreasing diameters, as shewn in the plate ; the anterior ones (which are always of shorter foci than the first screwed on) being the smallest, so that whenever one objective is screwed on before another, it shall always be of less diameter than that to which it is attached, and thus leave space for the illumination of opaque objects. When all are made of the same diameter, in the usual manner, it is impossible to illuminate opaque objects shewn by them otherwise than by a cup.

Fig. 3 is a silver cup which is fastened on to the tube of

REFLECTING ENGISCOPE.

reflectors by a clasping piece of brass, the perforation in it being made to coincide with that in the side of the tube. Each pair of metals capable of acting upon opaque objects has one adapted to it.

Fig. 4 is an illuminator for opaque objects, about an inch focus : its stem is passed into the spring tube (n) carried by the arm attached to the under side of the stage ; it has a motion upwards and downwards in the tube, and likewise a rotatory one; the lens also can be turned round in its socket (a), by means of the milled head (b). Nothing, in my opinion, can be a greater blunder than to give these sort of things a greater latitude of motion than is really requisite, by means of cradle-joints and balls and sockets, which only render them more difficult to manage, and increase the expense of their construction. By the by, all sorts of needles and pins, &c. ought to be made of brass, or, what would be better, of gun-metal; if made of steel they soon rust by being handled, and then move very stiffly and awkwardly in their sockets.

Fig. 5 is another illuminator for opaque objects, intended to co-operate with that just described, and also to act by itself along with the object-glasses and the metals of long focus.

It is about 2j inches focus, and is passed into a spring tube travelling on a slide, with a pinching screw underneath, by which it is adjusted at a proper distance from the object, or the second illuminating mirror. It is represented in a detached state, but fixes to the under plate of the stage on that side opposite the milled heads.

Illuminators and burning lenses present much analogy to each other, the caustic and illuminating power of each depending, first, upon their actual diameter, and, secondly, upon their angular aperture : thus a burning lens, even of 2 feet diameter and 4 feet focus, does not exert any great caustic power alone, because, though it draws a vast body of calorific rays together, it does not condense them into a sufficiently small space. But let a second lens be interposed at ten inches from the focal

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point of the first, and of sufficient diameter to catch the whole cone of rays proceeding from it at that point, and converge to a focus perhaps only six inches distant from the small one, and we shall soon see what an enormous increase of caustic power will be the consequence. On this principle the two illuminators are combined ; the first is only of an inch diameter, but might be made larger to increase the effect, for the vision by the Amician Engiscope is so dark and sombre on opaque objects as to require all the help from illuminators which can possibly be given to it. Every one must, I think, have remarked the intense illumination given by silver cups acting with illuminators of large angular aperture, which is, in fact, an example of the intrinsic brightness produced by a primary lens having its light condensed by a cup placed at a proper distance from it ; though a silver cup can never reflect so much light as a lens of the same focus and aperture can transmit, acting in lieu of it.

It might perhaps be worth while to construct an illuminator of two lenses placed at a proper distance from each other with relation to their foci, (somewhat after the fashion of those operaglasses which are worn about the neck,) having no tube, but mounted on a bar, the largest being made to travel on the bar by means of a socket, and the small one to fold down ; the bar must be equipped with a cradle-joint attached to a stem to be passed into the spring tube of the slide.

Fig. 6 is a raised stage to come up to the level of the tube of reflectors, since the primary stage itself cannot easily be made to do so : it has a sunk rim turned to receive a watch or plane glass, black and white discs, &c., like other microscopes, it is also convertible into a black box for viewing certain opaque objects when closed at the bottom by the part (e), which is covered on the inside with black silk velvet, (a) is an arm made for convenience to revolve round on a shoulder at the top of the stage, and even to remove altogether at the pleasure of the observer ; at the end of the arm is a sprung

REFLECTING ENGISCOPE.

tube to receive the needle (d), which is furnished at one end with a miniature vice to grasp pins, at the other with a small cylinder to act with silver cups. At c is a small black velvet ground, which is employed when opaque objects, not attached to discs, are to be seen by silver cups, in order that their minutiae may be displayed to the greatest advantage ; the more sombre and obscure these are, the greater is the necessity of an intense black ground to contrast and draw them out. The aforesaid little ground serves the important purpose of keeping all direct rays from the illuminating lens from getting into the Engiscope, so long as it remains in the line of the axis of the metals ; but if the traversing plates of the stage are called into full action, it must evidently fail in this purpose ; they must therefore be quiescent, or nearly so, and the observer will have to move the object about by means of the needle, which, having no cradle-joint, or ball and socket, I know by experience he may easily do, giving a small final motion only by means of the screws. If, however, it is determined that the traversing plates shall act to their full extent, {which, by the by, will be sure to cut off some of the light coming from the illuminator,') then the wire carrying the disc or black ground must be made to fix to one of the arms of the stage : then it will probably happen that the traversing motions will carry the object away from its influence ; so, upon the whole, 1 think it is best as I have arranged it. Different sizes of discs will be required to suit different cups.

Fig. 7 is the illuminator co-operating with the silver cups.