plates, A, B, C, which are parallel to each other; the wires D and E are riveted into the upper and lower plates, which are by this means united to each other; 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 may be had if required. This little microscope is the most portable of any. Its principal merit is its simplicity.

7. Botanical Lenses or Magnifiers. The haste with which botanists, &c. have frequently occasion to view objects, renders an extempore pocket-glass indispensably necessary. The most convenient of any yet constructed appears to be that contrived, in regard to the form of the mounting, by the late Mr. Benjamin Martin; and is what he called a hand megalascope, because it is well adapted for viewing all the larger sort of small objects universally, and by only three len.es it has seven different magnifying powers.

Fig. 7. represents the case with the three frames and lenses, which are usually of 1, 14, and 2 inches focus: they all turn over each other, and shut into the case, and are turned out at pleasure.

The three lenses singly afford three magnifying powers; and by combining two and two, we make three more for d with e makes one, d with fanother, and e with f a third; which, with the three singly, make six; and lastly, all three combined together make another; so that. upon the whole, there are seven powers of magnifying with these glasses only.

When the three lenses are combined, it is better to turn them in, and look through them by the small apertures in the sides of the case. The eye in this case is excluded from extra light; the aberration of the superfluous rays through the glasses is cut off; and the eye coincides more exactly with the common axes of the lenses.

A very useful and easy kind of microscope (de scribed by Joblot, and which has been long in use), adapted chiefly for viewing, and confining at the same time, any living insects, small animals, &c. is shewn at fig. 8. where A represents a glass tube, about 1 inches diameter, and 2 inches high. B, a case of brass or wood, containing a sliding tube, with two or three magnifying glasses that may be used either separately or combined. In the inside, at the bottom, is a piece of ivory black and white on opposite sides, that is occasionally removed, and admits a point to be screwed into the centre. cap unscrews at D, to admit the placing of the object: the proper distance of the glasses from the object is regulated by pulling up or down the brass tube E at top containing the eye-glasses.

The

This microscope is particularly useful for exhibiting the well-known curious curculio imperialis, vulgarly called the diamond beetle, to the greatest advantage; for which, as well as for other objects, a glass bottom and a polished reflector at the top are often applied, to condense the light upon the object. In this case, the stand and brass bot

tom F, as shewn in the same figure, are taken away by unscrewing.

9. Mr. Lyonet's Single Anatomical Dissecting Microscope.-Fig. 9. represents a curious and extremely useful microscope, invented by that gentleman for the purpose of minute dissections and microscopic preparations. This instrument must be truly useful to amateurs for the minutiae of issects, &c. being the best adapted of any for the purposes of dissection. With this instrument Mr. Lyonet made his very curious microscopical dissection of the chenille de saule, as related in his Traité Anatomique de la chenilie qui ronge le bois de saule, 4to.

AB is the anatomical table, which is supported by a pillar NO; this is screwed on the foot CD. The table AB 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 centre of the mirror EF, that is to reflect the light on the object; the hole G is designed to receive a flat or concave glass, on which the objects for examination are to be placed.

RXZ is an arm formed of several balls and sockets, by which means it may be moved in every possible situation; it is fixed to the board by means of the screw II. The last arm 12 bas a female screw, into which a magnifier may be screwed, as at Z. By means of the screw H, & small motion may be occasionally given to the arm IZ, 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 likewise 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 AB.

To use the dissecting table.-Let the operator sit with his left side near a light window; the instru ment being placed on a firm table, the side DH towards the stomach, the observations should be made with the left eye. In dissecting, the two elbows are to be supported by the table on which the instrument rests, the hands resting against the board AB; and 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 forefingers.

II. Of Double Microscopes, commonly called Compound Microscopes.

Double microscopes are so called, from being a combination of two or more lenses.

The particular and chief advantages which the compound microscopes have over the single are. that the objects are represented under a larger field of view, and with a greater amplification of reflected light.

1. Culpeper's Microscope.-The compound microscope, originally contrived by Mr. Culpeper, is represented at fig. 10. Pl. 109. It consists of a large external brass body A, B, C, D, supported upon three scrolls, which are fixed to the stage EF; the stage is supported by three larger scrolls that are screwed to the mahogany pedestal GH. There is a drawer in the pedestal, which holds the appe ratus. The concave mirror I is fitted to a socket in the centre of the pedestal. The lower part LMCD of the body forms an exterior tube, into which the upper part of the body ABLM slides, and may be moved up or down, so as to bring the magnitiers,

which are screwed on at N, nearer to or farther from the object.

To use this microscope :-Screw one of the buttons, which contains a magnifying lens, to the end N of the body; place the slider, with the objects, between the plates of the slider-holder. Then, to attaiu distinct vision, and a pleasing view of the object, adjust the body to the focus of the lens you are using, by moving the upper part gently up and down, and regulate the light by the concave mirror.

For opaque objects, two additional pieces must be used. The first is a cylindrical tube of brass (represented at L, fig. 11.), which fits on the cylindrical part at N of the body. The second piece is the concave speculum h; this is to be screwed to the lower end of the aforesaid tube: the upper end of this tube should be made to coincide with the line which has the same number affixed to it as to the magnifier you are using; ex. gr. if you are making use of the magnifier marked 5, slide the tube to the circular line on the tube N, that is marked also with No. 5. The slider-holder should be removed when you are going to view opaque objects, and a plane glass should be placed on the stage in its stead to receive the object; or it may be placed in the nippers, the pin of which fits into the hole in the stage.

The apparatus belonging to this microscope consists of the following particulars; viz. Five magnifiers, each fitted in a brass button; one of these is seen at N, fig. 10. Six ivory sliders, five of them with objects. A brass tube to hold the concave speculum. The concave speculum in a brass box. A fish-pan. A set of glass tubes. A flat glass fit ted to the stage. A concave glass fitted to the stage. A pair of forceps. A steel wire with a pair of nippers at one end and a point at the other. A small ivory cylinder, to fit on the pointed end of the aforesaid nippers. A convex lens, moveable in a brass semicircle; this is affixed to a long brass pin, which fits into a hole on the stage.

The construction of the foregoing microscope is very simple, and it is easy in use; but the advantages of the stage and mirror are too much confined for an extensive application and manage ment of all kinds of objects. Its greatest recommendation is its cheapness; and to those who are desirous of having a compound microscope at a low price, it may be acceptable.

2. Cuff's Microscope.-The improved microscope next in order is that of Mr. Cuff. Besides remedying the disadvantages above mentioned, it contains the addition of an adjusting screw, which is a considerable improvement, and highly necessary to the examination of objects under the best defined appearance from the glasses. It is represented at fig. 11. with the apparatus that usually accompanies it. A, B, C, shows the body of this microscope, which contains an eye-glass at A, a broad lens at B, and a magnifier which is screwed on at C. The body is supported by the arm DE, from which it may be removed at pleasure. The arm DE is fixed on the sliding bar F, and may be raised or depressed to any height within its limits. The main pillar ab is fixed in the box be; and by means of the brass foot d is screwed to the mu hogany pedestal XY, in which is a drawer containing all the apparatus. O is a milled-headed screw, to tighten the bar F when the adjusting screw cg is used. pq is the stage, or plate, which carries the objects; it has a hole at the centre ». Ga concave mirror, that may be turned in any

direction, to reflect the light of a candle, or the sky, upon the object.

To use this microscope:-Screw the magnifier you intend to use to the end C of the body, place the slider-holder P in the hole n, and the slider with the object between the plates of the sliderholder; set the upper edge of the bar D E to coincide with the divisions which correspond to the magnifier you have in use, and pinch it by the milled nut; now reflect a proper quantity of light upon the object, by means of the concave mirror G, and regulate the body exactly to the eye and the focus of the glasses by the adjusting screw cg.

To view opaque objects, take away the sliderholder P, and place the object on a flat glass under the centre of the body, or on one end of the jointed nippers op. Then screw the silver concave speculum h to the end of the cylinder L, and slide this cylinder on the lower part of the body, so that the upper edge thereof may coincide with the line which has the same mark with the magnifier that is then used; reflect the light from the concave mirror G to the silver speculum, from which it will again be reflected on the object. The glasses are to be adjusted to their focal distance, as before directed.

Ta

The apparatus consists of a convex lens H, to collect the rays of light from the sun or a candle, and condense them on the object. La cylindrical tube, open at each side, with a concave speculum screwed to the lower end h. P the slider-holder: this consists of a cylindrical tube, in which an inner tube is forced upwards by a spiral spring; it is used to receive an ivory slider K, which is to be slid between the plates h and i. The cylinder P fits the hole in the stage; and the hollow part at k is designed to receive a glass tube. R is a brass cone, to be put under the bottom of the cylinder P, to intercept occasionally some of the rays of light. S a box containing a concave and a flat glass, between which a small living insect may be confined: it is to be placed over the hole n. flat glass, to lay any occasional object upon; there is also a concave one for fluids. O is a long steel wire, with a small pair of pliers at one end, and a point at the other, designed to stick or hold objects; it slips backwards and forwards in the short tube o; the pin p fits into the hole of the stage. W a little round ivory box, to hold a supply of tale and rings for the sliders. V a small ivory cylinder, that fits on the pointed end of the steel wire: it is designed for opaque objects. Lightcoloured ones are to be struck upon the dark side, and vice versa. M a fish-pan, whereon to fasten a small fish, to view the circulation of the blood: the tail is to be spread across the oblong hole k at the small end, and tied fast by means of a ribband fixed thereto: the knob is to be shoved through the slit made in the stage, that the tail may be brought under the magnifier.

3. This microscope has received several material improvements from Mr. Martin, Mr. Adams, &c. By an alteration, or rather an enlargement, of the body of the tube which contains the eyeglasses, and also of the eye-glasses themselves, the field of view is made much larger, the mirror below for reflecting light is made to move upon the same bar with the stage; by which means the distance of it from the stage may be very easily and suitably varied. A condensing glass is applied under the stage in the slider-holder, in order to modify and increase the light that is reflected by the mirrors below from the light of a candle or lamp. It is

furnished also with two mirrors in one frame, one concave and the other plane, of glass silvered; and by simply unscrewing the body, the instrument, when desired, may be converted into a single microscope. Fig. 12. is a representation of the instrument thus improved; and the following is the description of it, as given by Mr. Adams in his Essays.

AB represents the body of the microscope, containing a double eye-glass and a body-glass: it is here shown as screwed to the arm CD, from whence it may be occasionally removed, either for the convenience of packing, or when the instrument is to be used as a single microscope.

The eye-glasses and the body-glasses are contained in a tube which fits into the exterior tube AB; by pulling out a little this tube when the microscope is in use, the magnifying power of each lens is increased.

The body AB of the microscope is supported by the arm CD; this arm is fixed to the main pillar CF, which is screwed firmly to the mahogany pedestal GH; there is a drawer to this pedestal, which holds the apparatus.

NIS, the plate or stage which carries the sliderholder KL: this stage is moved up or down the pillar CF, by turning the milled nut M; this nut is fixed to a pinion, that works in a toothed rack cut on one side of the pillar. By means of this pinion, the stage may be gradually raised or depressed, and the object adjusted to the focus of the different lenses.

KL is a slider-holder, which fits into a hole that is in the middle of the stage NIS; it is used to confine and guide either the motion of the sliders which contain the objects, or the glass tubes that are designed to confine small fishes for viewing the circulation of the blood. The sliders are to be passed between the two upper plates, the tubes through the bent plates.

L is a brass tube, to the upper part of which is fixed the condensing lens before spoken of; it fits into the under part of the slider-holder KL, and may be set at different distances from the object, according to its distance from the mirror or the candle.

O is the frame which holds the two reflecting mirrors, one of which is plane, the other concave. These mirrors may be moved in various directions, in order to reflect the light properly, by means of the pivots on which they move, in the semicircle QSR, and the motion of the semicircle itself on the pin S: the concave mirror generally answers best in the day-time; the plane mirror combines better with the condensing lens, and a lamp or candle. At D there is a socket for receiving the pin of the arm Q (fig. 31.), to which the concave speculum, for reflecting light on opaque objects, is fixed. At S is a hole and slit for receiving either the nippers L (fig. 31. pl. 7.) or the fish-pan 1; when these are used, the slider-holder must be removed. T, a hole to receive the pin of the convex lens M, fig. 31.

To use this microscope:-Take it out of the box. Screw the body into the round end of the upper part of the arm CD. Place the brass sliders, which contain the magnifiers into the dove-tailed slit which is on the under side of the aforesaid arm, as seen at E, and slide it forwards until the magnifier you mean to use is under the centre of the body: opposite to each magnifier in this slit there is a notch, and in the dove-tailed part of the arm CD here is a spring, which falls into the above-men

tioned notch, and thus makes each magnifer co cide with the centre of the body. Pass the ivory slider you intend to use between the upper pre of the slider holder KL, and then reflect as sing a light as you can on the object by means of es of the mirrors; after this, adjust the object to thr focus of the magnifier and your eye, by turning t milled screw M, the motion of which raises and de presses the stage NIS. The degree of light nere. sary for each object, and the accuracy required in the adjustment of the lenses to their proper face. distance from the object, will be easily attained by a little practice.

When opaque objects are to be examined, re move the slider-holder, and place the object on a flat glass, or fix it to the nippers L, the pin of these fit into the hole on the stage; screw the concare speculum R into the arm Q (fig. 31), and then pass the pin of this arm through the socket D, fig. 12; the light is now to be reflected from the coscave mirror to the silver speculum, and from this down on the object. No exact rule can be given for reflecting the light on the object; we must thereste refer the reader to the mother of all aptness, practice. The speculum must be moved lower or higher, to suit the focus of the different magnifiers and the nature of the object.

The foregoing directions apply equally to the using of this instrument as a single microscope; with this difference only, that the body A B is then removed, and the eye is applied to the upper smface of the arm CD, exactly over the magnifiers.

This microscope is sometimes made with the following alterations, which are supposed to make it still more convenient and useful. The arm CD that carries the body and magnifiers is made both to turn on a pin, and to slide backwards and forwards in a socket at C; so that instead of moving the objects below on the stage, and disturbing thein, the magnifiers are more conveniently brought over any part of the objects as desired. The con densing glass is made larger, and slides upon the square bar CF quite distinct from the stage, e the mirrors below; and it is thereby made useful for any other objects that may be applied on glasses fitted to the stage, as well as those put into the sder-holder K. It is thereby not confined to this stage alone, as in the preceding. When the body AB is taken away, the arm CD may be slipt away from its bar, with the magnifiers, and the forceps, wire, and joint, applied to it; and it thereby serves the purpose of a small hand single or opaque mi croscope, for any object occasionally applied to this wire. The magnifiers in the slider E are mounted in a wheel case, which perhaps prevents its being in the way so much as the long slider E before described -This contrivance is represented at X, fig. 12.

4. Martin's New Universal Compound Microscope.-This instrument was originally constructed by the late Mr. Benjamin Martin, and intended to comprise all the uses and advantages of the single, compound, opaque, and aquatic microscopes. The following is a description of it as now made, with a few alterations, chiefly suggested (we are told) by Mr. Jones of Holborn.

Fig. 13. is a representation of the instrument placed up for use. A, B, C, D, is the body of the microscope; which consists of four parts, viz AB the eye-piece, or that containing the eye-glasses, and is screwed into C, which is a moveable or sliding tube on the top; this inner tube contains the body-glass screwed into its lower part. Dis the