Essays on the Microscope / Containing a Practical Description of the Most Improved Microscopes, a General History of Insects, etc., etc.

Plate V. Fig. 1, represents the solar opake microscope, placed together for exhibiting opake objects.

Fig. 2, is that part called the single tooth and pinion microscope, which is used for shewing transparent objects; the cylindrical tube, Y, thereof, being made to fit into the tube E F, Fig. 1. It may be occasionally used as a hand single, or Wilson’s microscope, and for which purpose, the handle, c, is fitted by a screw to the body at g, and the tube, Y, screwed away.

Fig. 3, the slider which contains the six magnifiers; it fits into a dove-tail under P, Fig. 2, at the upper part of the microscope.

Fig. 4 represents a brass dove-tail slider, containing a small lens: it is called a condenser. There are three in number, marked 1 and 2, &c. corresponding to the number of the magnifiers used: they serve to condense the sun’s rays strongly upon the object, and enlarge the circle of light. They slide in at h, Fig. 2.

A B C D E F, Fig. 1, represents the body of the solar microscope; one part thereof, A B C D, is conical, the other, C D E F, is cylindrical. The cylindrical part receives the tube, G, of the opake object box, or the tube, Y, of the single microscope, Fig. 2. At the large end, A B, of the conical part there is a convex lens to receive the rays from the mirror, and refract them convergingly into the box, H I K L.

N O P is a brass frame which is fixed to the moveable circular plate, a b c; in this frame there is a plane mirror, to reflect the solar rays through the afore-mentioned lens. This mirror may be moved into the proper positions for reflecting the solar rays, by means of rack-work turned by the nuts Q and R. By the nut Q, it may be moved from right to left; it maybe elevated or depressed by the nut, R. d e, two screws to fasten the microscope to a window-shutter, or a board fitted entirely before the window.

The box for opake objects is represented as open at H I K L; it contains a plane mirror, M, for reflecting the light that it receives from the large lens to the object, and thereby illuminating it; S is a screw to adjust this mirror to its proper angle for reflecting the light. V X, two tubes of brass, one sliding within the other, the exterior one in the box, H I K L; these carry two magnifying lenses: the interior tube is sometimes taken out, and the exterior one is then used by itself. Part of this tube may be seen in the plate as within the box, H I K L.

At H, is a brass plate, the back part of which is fixed to a tube, h, containing a spiral wire, which keeps the plate always bearing against the side, H, of the brass box H I K L. The sliders, with the opake objects, Fig. 5, pass between this plate and the side of the box; to apply which, the plate is to be drawn back by means of the nut, g. k i, a door to one side of the opake box, to be opened when adjusting the mirror, M.

The foregoing pieces constitute the several parts necessary for viewing opake objects. We shall now proceed to describe the single microscope, which is used for transparent objects; but, in order to examine these, the box, H I K L, must be first removed, and in its place we must insert the tube, Y, of the single microscope, Fig. 2, now to be explained.

Fig. 2 represents a large tooth and pinion microscope; at m, within the body of this microscope, are two thin plates that are to be separated, in order to let the ivory sliders, Fig. 7, pass between them; they are pressed together by a spiral spring, which bears up the under plate, and forces it against the upper one. The slider, Fig. 3, that contains the magnifiers, fits into a hole at n; any of the magnifiers may be placed before the object, by moving the aforesaid slider: when the magnifier is at the center of the hole P, a small spring falls into one of the notches which is on the side of the slider, Fig. 3. At h, slides a condenser, Fig. 4, for condensing the sun’s rays, and enlarging the field of view on the screen: the number must correspond with that of the magnifier used. This microscope is adjusted to the focus, while exhibiting the object, by turning the milled nut O.

APPARATUS TO THE OPAKE SOLAR MICROSCOPE.

The mirror O P, Fig. 1, and square plate, and the tubular body of the microscope, A F. The opake box and its tube, I K G. The tooth and pinion or single microscope, Fig. 2. The slider of magnifiers, Fig. 3. The megalascope magnifier, Fig. 6, fitted to P of Fig. 2. Six ivory sliders with transparent objects, Fig. 7. Twelve wood sliders with opake objects, and a brass frame to hold them, Fig. 5. A brass square-formed slider case, Fig. 8, to hold any animal, piece of ore, or other opake object, and is to be placed like the other slider at H, Fig. 1. A pair of nippers and point, Fig. 9, the pin, a, of which fits into the hole of the slider, Fig. 4, and holds before the magnifiers at P, Fig. 2, any small fly or other complete object to be magnified. A four-glass slider in a brass frame, Fig. 10, for any animalcula, &c. to be placed between the plates at m, Fig. 2. A set of glass fish tubes, Fig. 11. A pair of forceps, Fig. 12. Two brass nuts for the window-shutter or board, Fig. 13; and the two brass fastening screws, d e, Fig. 1, which may be either used with or without the above two nuts.

The figures on the plate are about half the original size, and the apparatus now made by Messrs. Jones packs into a case thirteen inches long, nine inches broad, and four inches deep. For price, see the list at the end.

TO USE THE SOLAR MICROSCOPE.

Make a round hole in a window-shutter or window-board, that is opposite to the meridian sun, or as nearly so as possible, a little larger than the circle a b c; pass the mirror, N O P, through this hole, and apply the square plate to the shutter; then mark with a pencil the places which correspond to the two holes through which the screws are to pass; take away the microscope, and bore two holes at the marked places, large enough to admit the milled screws, d e, to pass through them. These screws are to pass from the outside of the shutter, to go through it, and being then screwed into their respective holes in the square plate, they will, when screwed home, hold it fast against the inside of the shutter, and thus support the microscope.

Another way, and perhaps more convenient, is to previously screw the two brass nuts, Fig. 13, to the shutter or window-board, at the inside at a suitable distance, to receive the two milled screws; these nuts will always be ready for use, and the operator may in a minute, within his room, fasten the plate, a b c, to the shutter by the two milled screws, being placed contrarywise.

Screw the conical tube, A B C D, to the circle, a b c, and then slide the tube, G, of the opake box into the cylindrical part, C D E F, of the body, if opake objects are to be examined; but if transparent objects are intended to be shewn, then place the tube Y, Fig. 2, within the tube C D E F. The room is to be darkened as much as possible, that no light may enter but what passes through the body of the microscope; for, on this circumstance, together with the brightness of the sun, the perfection and distinctness of the image in a great measure depend.

We shall first consider the microscope as going TO BE USED FOR OPAKE OBJECTS. Adjust the mirror, N O P, so as to receive the solar rays, by means of the two finger-screws or nuts, Q, R; the first, Q, turns the mirror to the right or left; the second, R, raises or depresses it: this you are to do, till you have reflected the sun’s light through the lens at A B, strongly upon a white-paper screen or cloth, from four to eight feet square (about the latter dimensions for transparent objects) placed from about five to eight feet distance from the window, and formed thereon a round spot of light: a white wainscot or wall at a suitable distance answers very well. An unexperienced observer will find it more convenient to obtain the light by first forming this spot, before he puts on either the opake box, or the tooth and pinion microscope, Fig. 2.

Now apply the opake box, and place the object between the plates at H; open the door, k i, and adjust the mirror, M, till you see you have illuminated the object strongly. If you cannot effect this by the screw S, you must move the screws Q, R, in order to get the light reflected strongly from the mirror, N O P, on the mirror M; without which the latter cannot illuminate the object. The object being strongly illuminated, shut the door, k i, and a distinct view of the object will soon be obtained on your screen, by adjusting the tubes V X, with the magnifiers, which is effected by moving them backwards or forwards.

A perfectly round spot of light cannot always be procured in northern latitudes, the altitude of the sun being often too low; neither can it be obtained when the sun is directly perpendicular to the front of the room. As the sun is continually changing its place, it will be necessary, in order to keep his rays full upon the object, to keep them continually directed through the axis of the instrument, by turning the two screws Q and R.

To view transparent objects, remove the opake box, and insert the tube, Y, of Fig. 2, in its place; put the slider, Fig. 3, into its place at n, a condenser, Fig. 4, at h, and the slider with the objects between the plates at m; then adjust the mirror, N O P, as before directed, by the screws, Q, R, so that the light may pass through the object; regulate the focus of the magnifier by the pinion, O. The most pleasing magnifiers in use are the fourth and fifth. The size of the object is generally from four to eight feet, and may be increased or diminished by altering the distance of the screen from the microscope; five or six feet is a convenient distance.

The effect by this sort of microscope is stupendous, and never fails to excite wonder in an observer at the first view, in seeing a flea, &c. augmented in appearance to SEVEN, EIGHT, or even TEN FEET in length, with all its colours, motions, and animal functions, distinctly and beautifully exhibited.

To examine transparent objects of a larger size, or to render the instrument what is usually called a megalascope, take out the slider, Fig. 3, from its place at n; screw the cell and lens, Fig. 6, into the hole at P, Fig. 2; remove the glass which is placed at h, and regulate the light and focus agreeable to the foregoing directions.

At C D, is placed a lens for increasing the density of the rays, for the purpose of burning or melting any fusible substance; this lens must be removed in most cases, lest the objects should be burnt. The intensity of the light is also varied by moving the tube G, and Fig. 2, Y, inwards or outwards.

DESCRIPTION OF THE TRANSPARENT SOLAR MICROSCOPE AND APPARATUS. Plate VI. Fig. 4, to 14.

The foregoing description will, in great part, answer for this microscope; but, the dimensions, apparatus, &c. varying in a small degree from the preceding, a distinct description here, may be acceptable to those, who possess this sort of microscope only.

A B C D, Fig. 4, represents the body of the microscope, consisting of two brass tubes. E F is the end of the inner moveable tube; e f, that of the single tooth and pinion microscope. Fig. 5, screws into the end of this inner tube; at the end, A B, of the external tube there is a convex lens, to receive the sun’s rays from the mirror, K L, and to condense them on the object; the end, A B, screws into the circular plate, G H I. This part may also be used as a single microscope, and may have at m the handle, c, screwed to it. K L, a long frame fixed to the moveable circular plate, with a plane mirror, to reflect the rays of the sun on the lens at A B. An endless worm or screw, which is cut on the lower part of the nut, M, works in a small wheel which is fixed to the frame, K L, so that by turning the nut, the frame, K L, is moved up or down: the nut, N, moves the mirror to the right or left. O, P, two screws to fasten the square plate to the window-shutter.

Fig. 5, the single microscope; e f, the end which screws on to the part, E F, Fig. 4, of the internal tube of the body; q, the dove-tailed slit for receiving the slider, Fig. 8; g, the hole in which the megalascope magnifier, Fig. 6, is to be screwed, when the slider, Fig. 8, is removed. At h, are the moveable plates, between which the object sliders are placed; under the lowermost of these, the lens represented at Fig. 11 is to be placed, when the magnifiers in the slider, Fig. 8, are to be used, a k is a small piece of rack-work, which is moved backwards and forwards by the pinion fixed to the milled nut, b; by the gradual motion of this rack, the objects are adjusted to the foci of the different lenses. Fig. 8 is a brass slider, with six lenses, or magnifying glasses; it is to be inserted into the hole at q; either of the magnifiers may be placed before the object, by sliding it one way or the other: you may perceive when the glass is in the center of the eye-hole by a small spring acting upon a notch which is made on the side of the slider opposite to each lens.

APPARATUS BELONGING TO THIS SOLAR MICROSCOPE.

Square plate and mirror. The body, A D, consisting of two tubes, one within the other. The single microscope, Fig. 5. The megalascope lens, Fig. 6. The slider, Fig. 8, with six lenses. The two screws O, P. Six ivory sliders and a talc box, Fig. 7 and 13. Some glass tubes, Fig. 9. A slider or brass case, Fig. 10, containing a plane piece of glass, and a brass slider with holes, into which are cemented small concave glasses, designed for confining minute insects between the plane and concave glasses, which are thus preserved from being crushed, or from moving out of the field of view. Three condensing lenses to enlarge the field of view, such as Fig. 11, that are fitted to the hole, l, of Fig. 5. Their numbers correspond with the numbers used. Fig. 12, two brass nuts for the window-shutter or board, to receive the two screws, O and P.

To use the transparent solar microscope.

Fasten the square plate against the inside of a window-shutter, by the two screws O, P, which are to go from the outside of the window-shutter through it, and then be screwed into their respective holes in the square plate at G H I. The mirror is to be on the outside of the shutter, passing through a hole made for that purpose. Darken the room; then place a screen at about six or eight feet distance from the window, the farther it is from it the larger is the image: now move the mirror, K L, by the two nuts M N, till the sun’s rays come through the instrument in an horizontal direction to the screen, forming a round spot thereon; screw the microscope, Fig. 5, into its place E F; put the slider with the lenses, Fig. 8, at q, Fig. 5, and the object slider between the plates at h; adjust the object to the focus of the magnifying lens by the screw b, till the object appears distinct and clear on the screen. By moving the internal tube of the body, the object may be placed at different distances from the lens which is fixed at A B, so as to be sufficiently illuminated, and not burnt by the solar rays. If the screws O, P, are to pass inside the room, the two nuts, Fig. 12, must be previously fixed.

THE SCREW BARREL, OR WILSON’S SINGLE POCKET MICROSCOPE. Plate II. B. Fig. 1 and 2.

This microscope of Mr. Wilson’s is an invention of many years standing, and was in some measure laid aside, till Dr. Lieberkühn introduced the solar apparatus to which he applied it, there being no other instrument at that time which would answer his purpose so well; it is much esteemed in particular cases. The body of the microscope is represented at A B, Fig. 1, and is made either of silver, brass, or ivory. C C is a long fine-threaded male screw, that turns into the body of the microscope. D, a convex glass at the end of the said screw, on which may be placed, as occasion requires, one of the two concave apertures of thin brass to cover the said glass, and thereby diminish the aperture when the greatest magnifiers are used. E, three thin plates of brass within the body of the microscope, one whereof is bent to an arched cavity for the reception of a tube of glass. F, a piece of wood or brass, curved in the manner of the said plate, and fastened thereto. G, the other end of the microscope, where a female screw is adapted to receive the different magnifiers. H, a spiral spring of steel, between the said end, G, and the plates of brass, E, intended to keep the plates in a due position, and counteract against the long screw, C. I, a small ivory handle. To this microscope belong seven different magnifying glasses, six of which are set in cells, as in Fig. K, and are marked from 1, to 6: the lowest numbers to the greatest magnifiers. L is the seventh magnifier, set in the manner of a little barrel, to be held in the hand for viewing any large object. M is an ivory slider with the objects. Six of these, and one of brass, are usually sold with this microscope. There is also a brass slider not shewn in the figure, to confine any small object, that it may be viewed without crashing or destroying it. N, a pair of forceps, or pliers, for the taking up of insects or other objects, and applying them to the sliders or glasses. O, a camel hair brush, to take up and examine a small drop of liquid, brush the dust away, &c. P is a glass tube to confine living objects, such as frogs, fishes, &c.

When you view an object, push the ivory slider, in which the said object is placed, between the two flat brass plates, observing always to put that side of the slider, where the brass rings are, farthest from the eye; then screw in the magnifying glass you intend to use at the end of the instrument G, and looking through it against the light, turn the long screw, C C, till your object is brought to appear distinct, or to the true focal distance. To examine any object accurately, view it first through a magnifier that will shew the whole at once, and afterwards inspect the several parts more particularly with one of the greatest magnifiers; for thus you will gain a true idea of the whole, and all its parts: and, though the greatest magnifiers can shew but a minute portion of any object at once, such as the claw of a flea, the horn of a louse, &c. yet by gently moving the slider that contains your object, the eye will gradually see the whole; and if any part should be out of the focal distance, the screw, C C, will easily bring it to the true focus. As objects must be brought very near the glass, when the greatest magnifiers are used, be particularly careful not to rub the slider against the glasses as you move it in or out. A few turns of the screw, C C, will easily obviate this.

DESCRIPTION OF A SCROLL FOR FIXING WILSON’S POCKET MICROSCOPE, AND A MIRROR FOR REFLECTING LIGHT INTO IT.

A B C, Fig. 2, is a brass scroll, which, for the better conveniency of carriage, is made to unscrew into three parts, and may be put into the drawer upon which it stands, with its reflecting mirror D, and Wilson’s pocket microscope, G. The upper part of the scroll is taken off at B, by unscrewing half a turn of the screw; then, if lifted up, it will come out of the socket. The lower part unscrews at C, and the base at E. The mirror lifts out at F, which, with the scroll, lies in one partition of the box.

To apply this scroll for use, fix the body of the microscope to the top thereof by the screw, A, as in Fig. 2, by screwing it in the same hole as the ivory handle was applied to before. The brass or ivory slider being fixed as before described, and the microscope placed in a perpendicular position, move the mirror, D, in such a manner as to reflect the light of the sky, of the sun, or a candle, directly upwards through the microscope; by which means the object will be most conveniently viewed. It is further useful for viewing opake objects, by screwing the arm, Q R, Fig. 1, into the body of the microscope at G; then screwing into the round hole, R, that magnifier which you think will best suit your object, and putting the concave speculum, S, on the outside of the ring, R, you will observe in the body of the microscope, between the wood or brass, F, and the end of the male screw, C C, a small hole, u, through which slides the long wire, T, which has a point at one end, and forceps at the other, that may be used occasionally as your objects require. When you have fixed this, and your object on it, turn the arm, R, till the magnifier is brought over the object; it may be then adjusted to the true focus, by turning the screw, as before. It must also be brought exactly over the speculum, by turning the upper part of the scroll to one side, till your object and the two specula are in one line, as will be found by trial; and then fix it by the screw, B, at which time the upper surface of the object will be enlightened by the light reflected from the mirror, D, to the concave speculum.

DESCRIPTION OF A SMALL MICROSCOPE FOR OPAKE OBJECTS. Plate II. B. Fig. 3 and 4.

A, Fig. 3, is a fixed arm, through which passes a screw, B, the other end is fastened to the moveable arm, C. D, a nut fitted to the said screw, which, when turned, will either separate or bring together the two arms, A C. E, a steel spring, that separates the two sides when the nut is unscrewed. F, a piece of brass turning round in a spring socket, moving on a rivet, in which moves a steel wire pointed at the end G, and the other end a pair of pliers, H: these are either to thrust into, or take up and hold any object, and may be turned round as required. I, a ring of brass, with a female screw fixed on an upright piece of the same metal, turning on a rivet, that it may be set at a due distance when the least magnifiers are used, and is adapted to the screws of all the magnifiers.

Fig. 4, K, a concave speculum of polished silver, in the center of which a lens is placed. On the back of this speculum a male screw, L, is made to fit the brass ring I, Fig. 3. Four of these specula of different concavities, with four glasses of different magnifying powers, as the objects may require. The greatest magnifiers have the least apertures. M, a round object plate, one side white and the other black, intended to render objects the more visible, by placing them, if black, upon the white, and if white, on the black side. A steel spring, N, turns down on each side, to secure any object; from the object plate there is a hollow pipe, to screw it on the needle’s point G, Fig. 3. O, a small box of brass, with a glass on each side, to confine any living object in order to examine it, having a pipe to screw upon the end of the needle at G. P, an ivory handle. Q, a pair of pliers to take up any object. R, a soft hair brush.

To view any object, screw the speculum, with the magnifier you intend to use, into the brass ring, I; place your object either on the needle G, in the pliers H, on the object plate M, or in the brass hollow box O, as may be most convenient; then holding up your instrument by the handle P, look against the light through the magnifying lens, and by means of the nut, D, together with moving of the needle at its lower end, the object may be turned about, raised or depressed, brought nearer the glass, or put farther from it, till you have the true focal distance, and the light be seen reflected from the speculum strongly upon the object.[35]

OF ELLIS’S SINGLE OR AQUATIC MICROSCOPE. Plate VII. B.

This instrument takes its name from Mr. John Ellis, author of “An Essay towards a Natural History of Corallines,” and of the “Natural History of many curious and uncommon Zoophytes.” By this instrument he was enabled to explain many singularities in the œconomy and construction of these wonderful productions of nature. To the practical botanist this instrument is recommended by the respectable authority of Mr. Curtis, author of the Flora Londinensis, a work which does credit to the author and the nation. This microscope is simple in its construction, easy in its use, and very portable; these advantages, as well as some others which it also has over other portable microscopes, have accelerated the sale thereof, and caused it to be very much adopted.

DESCRIPTION OF THE VARIOUS PARTS OF THE MICROSCOPE.

K, the box which contains the whole apparatus; it is generally made of fish-skin; on the top of the box there is a female screw, for receiving the screw which is at the bottom of the brass pillar A, and which is to be screwed on the top of the box, K. D, a brass pin which fits into the pillar; on the top of this pin is a hollow socket to receive the arm which carries the magnifiers; the pin is to be moved up and down, in order to adjust the lenses to their focal or proper distance from the object.

In the representation of this microscope, Plate VII. B. Fig. 1, the pin, D, is delineated as passing through a socket at one side of the pillar, A; it is now usual to make it pass down a hole bored through the middle of the pillar.

E, the bar which carries the magnifying lens; it fits into the socket, X, which is at the top of the pillar, D. This arm may be moved backwards and forwards in the socket X, and sidewise by the pin, D; so that the magnifier, which is screwed into the ring at the end, E, of this bar, may be easily made to traverse over any part of the object lying on the stage or plate B. F is a polished silver speculum, with a magnifying lens placed at the center thereof, which is perforated for this purpose. The silver speculum screws into the arm E, as at F. G, another speculum of a different concavity from the former, with its lens. H, the brass semicircle which supports the mirror, I; the pin, R, affixed to the semicircle, H, passes through the hole which is towards the bottom of the pillar, A. B, the stage or the plane on which the objects are to be placed; it fits into a small dove-tailed arm which is at the upper end of the pillar, A. C, a plane glass, with a small piece of black silk stuck on it; this glass is fitted to a groove made in the stage, B. M, a deep concave glass, to be laid occasionally on the stage instead of the plane glass, C. L, a pair of nippers; these are fixed to the hole of the stage, a, by the pin K; the steel wire of these nippers slides backwards and forwards in the socket, and this socket is moveable upwards and downwards by means of the joint, so that the position of the object may be varied at pleasure. The object may be fixed in the nippers, stuck on the point, or affixed by a little gum-water, &c. to the ivory cylinder, N. O, a small pair of brass forceps to take up minute objects by. P, a brush to clean the glasses.

To use this microscope; begin by screwing the pillar, A, to the cover thereof; pass the pin, R, of the semicircle which carries the mirror through the hole that is near the bottom of the pillar, A; push the stage into the dove-tail at B; slide the pin into the pillar, then pass the bar, E, through the socket, X, which is at the top of the pin D, and screw one of the magnifying lenses into the ring at F.

Now place the object either on the stage, or in the nippers L, and in such a manner, that it may be as nearly as possible over the center of the stage; bring the speculum, F, over the part you mean to observe; then get as much light on the speculum as you can, by means of the mirror, I; the light received on the speculum is reflected by it on the object. The distance of the lens, F, from the object is regulated by moving the pin, D, up and down, until a distinct view of it is obtained. The rule usually observed is, to place the lens beyond its focal distance from the object, and then gradually slide it down, till the object appears sharp and well defined. The adjustment of the lenses to their foci, and the distribution of the light on the object, are what require the most attention.

These microscopes are sometimes fitted up with a rack and pinion to the pillar A, and pin D, for the more ready adjustment of the glasses to their proper foci.

DESCRIPTION OF LYONET’S ANATOMICAL MICROSCOPE. Plate VI. Fig. 3.

Fig. 3 represents the instrument with which M. Lyonet made his microscopical and wonderful dissection of the chenille de saule or caterpillar of the goat moth,[36] of which a specimen is given in Plate XII. Fig. 1, &c. of this work. This portable instrument needs no further recommendation. By it, other observers may be enabled to dissect insects in general with the same accuracy as M. Lyonet, and thus advance the knowledge of comparative anatomy, by which alone the characteristic, nature, and rank of animals, can be truly ascertained.

A B is the anatomical table, which is supported by the pillar O N; this is screwed on the mahogany foot, D C. The table A B, is prevented from turning round by means of two steady pins; in this table or board there is a hole, G, which is exactly over the center of the mirror, F E, that is to reflect the light on the object; the hole, G, is, designed to receive a flat or a concave glass, on which the objects are to be placed that you design to examine or dissect. R X Z is an arm formed of several balls and sockets, by which means it may be moved in every possible position; it is fixed to the board by means of the screw, H; the last arm, I Z, has a female screw, into which a magnifier may be screwed, as at Z. By means of the screw, H, a small motion may be occasionally given to the arm I Z, for adjusting the lens with accuracy to its focal distance from the object. Another chain of balls is sometimes used, carrying a lens to throw light upon the object; the mirror is also so mounted, as to be taken from its place at K, and fitted on a clamp, by which it may be fixed to any part of the table, A B.

To use the dissecting table.

Let the operator sit with his left side near a light window; the instrument being placed on a firm table, the side, D L, towards his breast, the observations should be made with the left eye: this position is well adapted for observing, drawing, or writing. In dissecting, the two elbows are to be supported by the table on which the instrument rests, the hands resting against the board, A B, in order to give it greater stability, as a small shake, though imperceptible to the naked eye, is very visible in the microscope; the dissecting instruments are to be held one in each hand, between the thumb and two fore-fingers. Farther directions are given on the mode of dissecting small objects in the following chapter.

DR. WITHERING’S BOTANICAL MICROSCOPE. Plate VI. Fig. 1.

This small instrument consists of three brass parallel plates, A, B, C; two wires, D and E, are rivetted into the upper and lower plate; the middle plate or stage is moveable on the aforesaid wires, by two little sockets which are fixed to it. The two upper plates each contain a magnifying lens, but of different powers; one of these confines and keeps in their places the fine point F, the forceps G, and the small knife H.

To use this instrument, unscrew the upper lens, and take out the point, the knife, and the forceps; then screw the lens on again, place the object on the stage, and then move it up or down till you have gained a distinct view of the object, as one lens is made of a shorter focus than the other; and spare lenses of a still deeper focus are sometimes added. The principal merit of this microscope is its simplicity.

THE POCKET BOTANICAL AND UNIVERSAL MICROSCOPE.

This pocket instrument is represented at Plate VI. Fig. 2. It is by most naturalists deemed preferable to Dr. Withering’s, being equally simple, more extensive in its application, and the stage unincumbered; though that of M. Lyonet seems better adapted than either to the purposes of dissection only.

A B, a small arm, carrying three magnifiers, two fixed to the upper part, as at B, the other to the lower part of the arm, at C; these may be used separately or combined together, by which you have seven powers. The arm, A B, is supported by the square pillar I K, the lower end of which fits into the socket, E, of the foot, F G; the stage, D L, is made to slide up and down the square pillar. H, a mirror for reflecting light on the object.

To use this microscope, place the object on the stage, L, reflect the light on it from the mirror H, and regulate it to the focus, by moving the stage nearer to or further from the lenses at B C. The ivory sliders pass under the stage, L; other objects may be fixed in the nippers, M N, and then brought under the magnifiers; or they may be laid on one of the glasses fitted to the stage. The apparatus to this instrument consists of three ivory sliders, a pair of nippers, a pair of forceps, a flat glass, and a concave ditto, all fitted to the stage, L. By taking out the pin, M, the pillar, I K, may be turned half round, and the foot, F G, made to answer as an handle.[37]

BOTANICAL MAGNIFIERS.

Since botany has been cultivated with so much ardor, it has been found necessary to contrive some very portable instrument, by which the botanist might investigate the object of his pursuits as it rises before him. Plate VIII. Fig. 7 and 8, represent two of the most convenient sort.

In the tortoiseshell case, Fig. 7, three lenses are contained, d, e, f, of different foci, which are all made to turn into the case, and may be used combined or separately. The three lenses in themselves afford three different magnifying powers; by combining two and two, we make three more; and the three together make, a seventh magnifying power. When the three lenses are used together, it is best to turn them into the case, and look through the hole, for more distinctness, and the exclusion of superfluous light. In the case, Fig. 8, are also three lenses, g, h, i, of different magnifying powers, that all turn up, and shut into the case; but these are not capable of combination.

DESCRIPTION OF A PORTABLE MICROSCOPE AND TELESCOPE. Plate VIII. Fig. 1, to 6.

The telescope is one of those which are composed of several sliding drawers or tubes, for the convenience of being put into the pocket; the sliding tubes are made of thin brass, the outside tube of mahogany. The sliding tubes are contrived to stop, when drawn out to a proper length, so that by applying one hand to the outside tube, and the other hand to the end of the smallest tube, the telescope may at one pull be drawn out to its full length; then any of the tubes (that next the eye is most generally used) may be pushed in gradually, while you are looking through it, till the object is rendered distinct to the eye. To make the tubes slide properly, they all pass through short springs or tubes; these springs may be unscrewed from the ends of the sliding tubes, by means of the milled edges which project above the tubes, and the tubes taken from each other if required, and the springs set closer if at any time they be too weak.

Fig. 5 represents the exterior tube of the telescope, which is to be unscrewed from the rest, at m l, as it does not make any part of the microscope; the cover, k, which protects the object-glass, serves also as a box to contain two ivory wheels, Fig. 1 and 2, with the objects, and a small mirror, Fig. 6.

Fig. 4 is a view of this cover when taken off: unscrew the top part of it, and the mirror, Fig. 6, may be taken out; unscrew the cover of the lower part, and you will find therein the two circular object-wheels above mentioned.

Fig. 3 represents the three internal tubes of the telescope, which constitute the microscopic part thereof. Draw the tubes out in the manner as shewn in the figure; then at the inside, but at the lower end of the exterior tube, a, you will find a short tube, which serves as a stage to hold the object and support the mirror; pull this tube partly out, and turn it, so that a circular hole which is pierced in it may coincide with a similar hole in the exterior tube. This tube is represented as drawn out at Fig. 3, the mirror, Fig. 6, placed therein at b c, and the transparent object-wheel fixed at a.

Fig. 1 represents the slider with transparent objects.

Fig. 2, that with the opake. They are made of ivory, and turn on a pin at the center; the slit end of this pin fits on the edge of the tube, which is then to be pushed up, so that the lower end of the exterior tube may bear lightly on the upper side of the slider, agreeable to the view which is given at a, Fig. 3. Now push down the second tube till the milled part falls on the milled edge of the extreme tube, being careful of the circular hole in the exterior one. Nothing now remains to be done but to adjust for the focus, which is effected by pushing in the tube R, and moving only the first, n.

The instrument may be used in two ways for transparent objects: first, in a vertical position, when the light is to be thrown on the object by the mirror, b c; or it may be examined by looking up directly at the light; in the latter case the mirror must be taken away. In viewing opake objects the mirror is not used; as much light as possible must be admitted on them through the circular holes of the tubes. Any object may be viewed by first pushing in the tube, R, and then bringing the tube, n, to its focal distance from the object. The telescope, when shut up, is about eight inches in length, and when drawn out, is about twenty inches. It is of the achromatic construction.

DESCRIPTION OF AN INSTRUMENT FOR CUTTING THIN TRANSVERSE SECTIONS OF WOOD, Plate IX. Fig. 1.

It consists of a wooden base, which supports four brass pillars; on the top of the pillars is placed a flat piece of brass, near the middle of which there is a triangular hole. A sharp knife which moves in a diagonal direction, is fixed on the upper side of the afore-mentioned plate, and in such a manner, that the edge always coincides with the surface thereof. The knife is moved backwards and forwards by means of the handle, a. The piece of wood is placed in the triangular trough, which is under the brass plate, and is to be kept steady therein by a milled screw which is fitted to the trough; the wood is to be pressed forward for cutting, by the micrometer screw, b. The pieces of wood should be applied to this instrument immediately on being taken out of the ground, or else they should be soaked for some time in water, to soften them, so that they may not hurt the edge of the knife. When the edge of the knife is brought in contact with the piece of wood, a small quantity of spirit of wine should be poured on the surface of the wood, to prevent its curling up; it will also make it adhere to the knife, from which it may be removed by pressing a piece of blotting paper on it.

Fig. 2, is an appendage to the cutting engine, which may be used instead of the micrometer screw, being by some practitioners preferred to it. It is placed over the triangular hole, and kept flat down upon the surface of the brass plate, while the piece of wood is pressed against a circular piece of brass which is on the under side of it. This circular piece of brass is fixed to a screw, by which its distance from the flat plate on which the knife moves may be regulated.[38]

CHAP. IV.
GENERAL INSTRUCTIONS FOR USING THE MICROSCOPE AND PREPARING THE OBJECTS.

As the advantages which are obtained from any instrument are considerably increased, if it be used by a person who is master of its properties, attentive to its adjustments, and habituated by practice to the minutiæ of management, it is the design of this chapter to point out those circumstances which more peculiarly require the attention of the observer, and to give such plain directions, as may enable him to examine any object with ease; to shew how he may place it in the best point of view, and if necessary, prepare it for observation.

A small degree of diligence will render the observer master of every necessary rule, and a little practice will make them familiar and habitual: the pains he takes to acquire these habits will be rewarded by an increasing attachment to his instrument, and the wonders it displays. Let him only persevere till he has overcome the natural indolence that opposes the advancement of every kind of knowledge, and he will most assuredly find himself very amply recompensed, by the gratification arising from the acquisition of a science that has the unlimited treasures of INFINITE WISDOM for the object of its researches: and his mind being strengthened by the victory it has gained, will be more keen in perceiving, and more patient in the investigation of truth.

It has long been a complaint,[39] that many of those who purchase microscopes are so little acquainted with their general and extensive usefulness, and so much at a loss for objects to examine by them, that after diverting themselves and their friends some few times with what they find in the sliders, which generally accompany the instrument, or perhaps two or three common objects, the microscope is laid aside as of little further value: whereas no instrument has yet appeared in the world capable of affording so constant, various, and satisfactory an entertainment to the mind. This complaint will, I hope, be obviated by these Essays, in which I have endeavoured to make the use of the microscope easy, point out an immense variety of objects, and direct the observer how to prepare them for examination.