exterior tube or case, in which the other slides up and down in an easy and steady manner. This mo. tion of the tube C is useful to increase or decrease the magnifying power of the body-glass when thought necessary, as before mentioned. E is a pipe or snout screwed on to the body of the microscope D, and at its lower part, over the several magnifying lenses hereafter described. FGHI is the square stem of the microscope, upon which the stage R moves in an horizontal position, upwards or downwards, by means of the fine rackwork of teeth and pinion. KL is a strong solid joint and pillar, by which the position of the instrument is readily altered from a vertical one to an oblique or to a perfectly horizontal one, as may be required; it is thus well adapted to the ease of the observer either sitting or standing; and as it is very often convenient to view objects by direct unreflected light, when the square stem FI is placed in an horizontal position for this purpose, the mirror T is then to be taken off in order to prevent the obstruction of the rays. M is a circular piece of brass, serving as a base to the pillar. NOP, the tripod or foot by which the whole body of the microscope is steadily supported; it folds up when packed inW is a brass frame, that contains the condensing lens, and acts in conjunction with the large concave and plane mirrors below at T; the reflected rays from which, either of the common light or of that of a candle or lamp, it agreeably modifies, and makes steady in the field of view.

to the case.

brass arm.

The particulars of the apparatus to this microscope are as follow: Q is a circular brass box, containing six magnifiers or object lenses, numbered 1, 2, 3, 4, 5, 6; the digits of which appear severally through a small round hole in the upper plate of it. To the upper side is fixed a small circle of brass, by which it is connected with, and screwed into, the round end of the arm abcd; which is a long piece of brass, and moves through either by teeth or pinion, or not, as may be desired, in ef; which is a socket on the upper part of the pillar, and admits, with a motion both easy and steady, the R is a fixed stage, upon which the objects to be viewed are to be placed; it is firmly fastened to the square pillar, which is moved by the rackwork. In the middle is a large circular hole, for receiving concave-glasses, with fluids, &c. it has also a sliding spring-frame to fasten down slips of glass or other things: at a b c are three small sockets or holes, intended to rec.ive several parts of the apparatus. S is the refractor, or illuminating lens, for converging the sun's rays upon opaque objects laid upon the stage R. To this purpose it moves on a semicircle upon a long shank g, in a spring socket h, in the arm i; this arm moving every way by a stout pin k in the socket a of the stage. In this manner it is easily adjusted to any position of the sun, candle, &c.-T, the reflecting-glass frame, containing a concave and plane speculum, which is moved upon the square pillar by the hand. The use of it is to illuminate all transparent objects that are applied to the stage above.

Fig 14, plate 110, No 1, is an auxiliary moveable stage; which by means of a pin k is placed in the hole a of the stage R. and can be moved in an horizontal direction over the whole field of the stage. In this stage there are three circular holes with shouldered bottoms: a large one in the middle, and on each side a small one, for the reception of the three following necessary articles: No 2, a watchglass to be placed in the large hole, to hold fluids containing animalcules, &c.; a circular piece of ivory, No 3, one side of which is black, the other

white, to support opaque objects of different conftrasted colours; and circular plane and concave glasses, No 4, for extemporaneous transparent objects-The same use is made of the other small hole as of the large one, only in a lesser degree, to receive small concave glasses, plates, &c.

No 5, is the silvered speculum, called a liberkuhn, which makes the single opaque microscope, by being screwed to the slider a bed (fig. 13. instead of the box of lenses Q, and the body A E above it. The chief use of this is to view very small objects strongly illuminated near the compounded focus of the mirror T (fig. 13.) N° 6, is the forceps, or pliers, for holding such kind of objects, and by which they can be applied very readily to the focus of the lens in the liberkuhn. They have a motion all ways by means of the spring socket a, the joint b, and the shank c; they are placed in the socket c of the fixed stage R (fig. 13.). No 7, is a small piece of ivory, to be placed upon the pointed end of the pliers; it is black upon one side, and white upon the other, to receive opaque objects.

No 8, is a liberkuhn of a larger size than that first mentioned, with a hole in its centre: this is screwed into No 9, the hole a of a brass ring, fastened to a long wire b; which moves up and down in the spring socket b of the stage R, in which it also moves sideways; and thus, with the body AE above, forms an aquatic compound microscope for showing all sorts of objects in water and other fluids placed under it in the watch-glass No 2,on the stage.

No 11, is a cone with a proper aperture a to exclude superfluous light, that would disturb a critical observation of a curious object; it is placed on the under side of the fixed stage R.

No 12, is what is usually called a bug-box, consisting of a concave glass with a plane one screwed over it; by means of which a bug, louse, flea, &c. may be secured and viewed alive. It is to be placed on either of the stages R (fig. 13.), or No I (fig. 14.).

No 13, is the fish-pan. In the long concave body ab, a fish may be so confined by the ribband c, that the transparent tail may be in part over the slit or hole at a. In this state, it is placed on the stage R, with the pin d in the hole c of the stage, and moves freely and horizontally for viewing the circulation of the blood, &c.

No 14, is the slider-holder that is placed on the stage R: it receives the sliders and tubes when filled with transparent objects, to be viewed either by the compound or single microscope.

No 15, represents the ivory slider, to hold the objects between the tales as usual.

No 16. is a useful auxiliary slider framed in brass. In this slider small concave glasses are cemented; and a slip of plane glass slides over them; by which any small living object, as mites, &c. may be confined without injury, and deliberately viewed.

No 17, represents a set of glass tubes, three in number, one within another; they are useful for small tadpoles, water-newts, eels, &c. when the circulation of the blood is to be viewed. There is a small hole at one end of each tube, that serves to admit the air; for, when they are filled with water, the other end is stopped with a cork.

No 18, is a small ivory box, containing spare tales and wires, to supply the sliders with occasionally.

No 19, a brass cell or button, containing a very small lens, properly set between two small plates of brass, that it may be brought very near to the object when viewed therewith as a single microscope.

This magnifier is screwed into the same hole as the wheel of six magnifiers Q are (fig. 13.).

No 20, is a lens adapted to view and examine objects, by magnifying them sufficiently, so as to be able to apply them to the microscope for inspection on this account it is called the explorator.

The preceding are the chief articles of the apparatus; which, on account of their being somewhat different from what is applied to other microscopes, we have been thus particular in describing. In using the microscope, and while viewing objects by either the single or compound instrument, the focal distances of the magnifiers are made perfectly exact by turning of the pinion at the nut we, in one way or the other, very gently in the teeth of the rack-work at X (fig. 13.).

It is necessary that the centres of the object lenses or magnifiers, the stage, and the mirrors at bottom, should all be in a right line in the axis of the microscope, when opaque objects are to be viewed, that are placed upon the ivory piece No 7, or the forceps No 6, and all other such sort of objects which are placed in the centre of the stage R, or slider-holder No 14: but when aquatic or living objects, which require a great space to move in, are to be viewed, then the horizontal motion at ef (fig. 13.) is made use of, and the view may be extended laterally over the whole of the diameter of the object or field of view: and by putting the arm abcd forward or backward in its socket ef, the view is extended in the contrary direction equally well; and in this manner the whole of the objects may be viewed without the least disturbance.

As the brass arm abcd may be brought to the height of three or four inches above the stage R; so, by means of the rack-work motion of the stage, a lens of a greater focal distance than the greatest in the wheel Q may be occasionally applied in place of the wheel, and thereby the larger kind of objects be viewed; the instrument becoming, in this case, what is called a megalascope.

In viewing moving living objects, or even fixed ones, when nice motions are requisite, a rack-work and pinion is often applied to the arm abcd, the arm is cut out with teeth; and the pinion, as shown at Y, is applied to work it. This acts but in one direction; and, in order to produce an equally necessary motion perpendicular to this, rack-work and pinion is applied tangent-wise to the stage, which is then jointed.

What has been related above respects the construction of those denominated parlour microscopes, in contradistinction to those which are portable: their dimensions, however, have been considerably reduced by opticians, in order to render them fit for the pocket; and as they are for the most part constructed on nearly the same principles as those which have been already described, what has been said will sufficiently instruct our readers in using any pocket microscope whatever. Only it may be observed, that in those reduced instruments, both the field of view and the magnifying power are proportionably diminished.

III. Of Solar Microscopes. This instrument, in its principle, is composed of a tube, a looking-glass or mirror, a convex lens, and Wilson's single microscope before described. See Plate 110. The sun's rays being reflected through the tube by means of the mirror upon the object, the image or picture of the object is thrown distinctly and beautifully upon a screen of white paper or a white linen sheet, placed at a proper distance to receive the same; and may be mag

to a size not to be conceived by those who

have not seen it; for, the farther the screen is removed, the larger will the object appear; insomuch that a louse may thus be magnified to the length of five or six feet, or even a great deal more; though it is more distinct when not enlarged to above half that size.

The different forms in which the solar microscope is constructed are as follow:

I. The old construction is represented in fig. 21, Plate 110. A is a square wooden frame, through which pass two long screws assisted by a couple of nuts 1, 1. By these it is fastened firmly to a wis dow-shutter, wherein a hole is made for its recep tion; the two nuts being let into the shutter, and made fast thereto. A circular hole is inade in the middle of this frame to receive the piece of wood B, of a circular figure; whose edge, that projects a little beyond the frame, composes a shallow groove 2, wherein runs a catgut S; which, by twisting round, and then crossing over a brass pulley 4, (the handle whereof, 5, passes through the frame), affords an easy motion for turning round the carcular piece of wood B, with all the parts affixed to it. C is a brass tube, which, screwing into the middle of the circular piece of wood, becomes a case for the uncovered brass tube D to be drawn backwards or forwards in. E is a smaller tube, of about one inch in length, cemented to the end of the larger tube D. F is another brass tube, made to slide over the above described tube E; and to the end of this the microscope must be screwed when we come to use it. 5. A convex lens, whose focus is about twelve inches, designed to collect the sun's rays, and throw them more strongly upon the ob ject. G is a looking-glass of an oblong figure, set in a wooden frame, fastened by hinges in the cir cular piece of wood B, and turning about therewith by means of the above-mentioned catgut. H is a jointed wire, partly brass and partly iron; the brass part whereof, 6, which is flat, being fasteðal to the mirror, and the iron part, 7, which is round, passing through the wooden frame, enables the ot server, by putting it backwards or forwards, to elevate or depress the mirror according to the san's altitude. There is a brass ring at the end of the jointed wire 8, whereby to manage it with the greater ease. The extremities of the catgut are fastened to a brass pin, by turning of which it may be braced up, if at any time it becomes too slack.

When this microscope is employed, the room must be rendered as dark as possible; for on the darkness of the room, and the brightness of the sunshine, depend the sharpness and perfection of your image. Then putting the looking-glass G through the hole in your window-shutter, fasten the square frame A to the shutter by its two screws and nuts 1, 1. This done, adjust your lookingglass to the elevation and situation of the sun, by means of the jointed wire H, together with the catgut and pulley, 3, 4. For, the first of these rais ing or lowering the glass, and the other inclising it to either side, there results a twofold motion, which may easily be so managed as to bring the glass ta a right position, that is, to make it reflect the sun's rays directly through the lens 5, upon the paper screen, and form thereon a spot of light exactly round. But though the obtaining a perfect cir cular spot of light upon the screen before you apply the microscope is a certain proof that your mirror is adjusted right, that proof must not always be expected; for the sun is so low in winter, that, if it shine in a direct line against the window, it cannot then afford a spot of light exactly round; but if it be on either side, a round spot may be

obtained, even in December. As soon as this appears, screw the tube C into the brass collar provided for it in the middle of your wood-work, taking care not to alter your looking-glass: then screwing the magnifier you choose to employ to the end of your microscope in the usual manner, take away the lens at the other end thereof, and place a slider, containing the objects to be examined, between the thin brass plates, as in the other ways of using the microscope. Things being thus prepared, screw the body of the microscope over the small end E of the brass tube F; which slip over the small end E of the tube D, and pull out the said tube D less or more as your object is capable of enduring the sun's heat. Dead objects may be brought within about an inch of the focus of the convex lens 5; but the distance must be shortened for living creatures, or they will soon be killed.

If the light fall not exactly right, you may easily, by a gentle motion of the jointed wire and pulley, direct it through the axis of the microscopic lens. The short tube F, to which the microscope is screwed, renders it easy, by sliding it backwards or forwards on the other tube E, to bring the cbjects to their focal distance; which will be known by the sharpness and clearness of their appearance: they may also be turned round by the same means, without being in the least disordered.

The magnifiers most useful in the solar microscope are in general the fourth, fifth, or sixth. The screen on which the representations of the objects are thrown is usually composed of a sheet of the largest elephant paper, strained on a frame which slides up or down, or turns about at pleasure on a round wooden pillar, after the manner of some firescreens. Larger screens may also be made of several sheets of the same paper pasted together on cloth, and let down from the ceiling with a roller like a large map.

"This microscope (says Mr. Baker) is the most entertaining of any; and perhaps the most capable of making discoveries in objects that are not too opaque: as it shows them much larger than can be done any other way. There are also several conveniences attending it, which no other microscope can have: for the weakest eyes may use it without the least straining or fatigue: numbers of people together may view any object at the same time; and, by pointing to the particular parts thereof, and discoursing on what lies before them, may be able better to understand one another, and more likely to find out the truth than in other microscopes, where they must peep one after another, and perhaps see the object neither in the same light nor in the same position. Those, also, who have no skill in drawing, may, by this contrivance, eas ly sketch out the exact figure of any object they have a mind to preserve a picture of; since they need only fasten a paper on the screen, and trace it out thereon either with a pen or pencil, as it appears before them. It is worth the while of those who are desirous of taking many draughts in this way to get a frame, wherein a sheet of paper may be put in or taken out at pleasure; for, if the paper be single, the image of an object will be seen al most as plainly on the back as on the fore-side; and, by standing behind the screen, the shade of the hand will not obstruct the light in drawing, as it must in some degree when one stands before it." This construction, however, has now become rather obsolete, and is superseded by the following.

II. The improved Solar Microscope, as used with the improved Single Microscope, with teeth and pi

nion. Fig. 22, represents the whole form of the sin gle microscope; the parts of which are as follows ABCD the external tube; GHIK the internal moveable one; QM part of another tube within the last, at one end of which is fixed a plate of brass hollowed in the middle, for receiving the glass tubes: there is also a moveable flat plate, between which and the fixed end of the second tube the ivory sliders are to be placed. L, a part of the microscope, containing a wire spiral spring, keeping the tube QM with its plates firm against the fixed part IK of the second tube.

EF is the small rack-work of teeth and pinion, by which the tube IG is moved gradually to or from the end AB, for adjusting the objects exactly to the focus of different lengths. NO is a brass slider, with six magnifiers; any one of which may easily be placed before the object. It is known when either of the glasses is in the centre of the eye-hole, by a small spring falling into a notch in the side of the slider made against each of the glasses. Those parts of the apparatus, fig. 14, (Pl. 109.) marked No. 15, 16, 17, 18, 19, 20, 21 and 22, are made use of here to this microscope. GH is a brass cell, which holds an illuminating glass for converging the sun's beams or the light of a candle strongly upon the objects. The aperture of the glass is made greater or less, by two circular pieces of brass, with holes of different sizes, that are screwed separately over the said lens. But, at times, objects appear best when the microscope is held up to the common light only, without this glass. It is also taken away when the microscope is applied to the apparatus now to be described.

Fig. 23, represents the apparatus with the single microscope screwed to it, which constitutes the solar microscope. AB is the inner moveable tube, to which the single microscope is screwed. CD is the external tube, containing a condensing convex glass at the end D, and is screwed into the plate EF, which is cut with teeth at its circumference, and moved by the pinion I, that is fixed with the plate GH. This plate is screwed fast against the window-shutter, or board fitted to a convenient window of a darkened room, when the instrument is used. KL is a long frame, fixed to the circular plate EF; containing a looking-glass or mirror for reflecting the solar rays through the lens in the body of the tube D. O is a brass milled head, fastened to a worm or endless screw; which on the outside turns a small wheel, by which the reflecting mirror M is moved upwards or downwards.

In using this microscope, the square frame GH is first to be screwed to the window-shutter, and the room well darkened: which is best done by cutting a round hole of the size of the moveable plate EF, that carries the reflector in the windowshutter or board; and, by means of two brass nuts a a, let into the shutter to receive the screws PP, when placed through the holes in the square frame GH, at the two holes QQ; which will firmly fasten the microscope to the shutter, and is easily taken away by only unscrewing the screws PP.

The white paper screen, or white cloth, to receive the images, is to be placed several feet distant from the window; which will make the representations the larger in proportion to the distance. The usual distances are from 6 to 16 feet.

The frame KL, with its mirror M, is to be moved by turning the pinion I, one way or the other, till the beams of the sun's light come through the hole into the room: then, by turning of the worm at O, the mirror must be raised or depressed