MICROSCOPE, in optics. By micro scopes are understood instruments, of whatever structure or contrivance, that can make small objects appear larger than they do by the naked eye. This is effected by means of convex glasses. When only one convex glass or lens is used, the instrument is called a single microscope; but if two or more are employed constantly to magnify objects, it is then called a•dgabte or compound microscope. When objefts are seen through a perfectly flat glass, the rays of light pass through it from them to the eye in a straight direction, and parallel to each other, and consequently the objects appear very little either diminished or enlarged, or nearer or further off, than to the naked eye. But if the glass through which they are seen has any degree of convexity, the rays of light are directed from the circumference towards the cen tre, in an angle proportionate to the con vexity of the glass, and meet in a point, at a greater distance from the glass, as it is less or more convex. This point, where the rays meet, is called the focus, which is nearer or further off, according to the convexity of the glass ; for a small degree of convexity throws it to a considerable distance ; when the convexity is much, the focus is near. The magnifying power is in proportion to its convexity; for, as a fiat glass magnifies scarcely at all, the less a glass departs from flatness, the less it magnifies ; and the more it approaches towards a globular figure, the nearer its focus is, and the more its magnifying power. We shall refer to Orrice for a more particular account of the principles, and proceed to describe some instruments of which we have made drawings. Plate Microscope, &c.
The body of the microscope, AA, be ing a large tube, is supported by brass pillars, b b, rising from a wooden pedestal, e, in which pedestal is a drawer to hold the object glasses, and other parts of the apparatus ; a lesser tube, e e, slides into the greater, and is moved up and down therein by a rack and pinion turned by a milled-beaded nut, 5; into this tube is pushed the body of the microscope, as shewn in the section fig. 1, and moving up and down with it; it has at its bottom another tube much smaller than itself, f with a male screw, g, at the end whereon to screw the object-glasses, or magnifiers: there are five of these magni fiers, numbered 1, 2, 3, 4, 5, which num bers are also marked on the inner tube, e e, to direct where about to place it, ac cording to the magnifier made use of; but if it does not fit the eye exactly, move the inner tube gently, higher or lower, by the nut t, till the object appears distinct. The
greatest magnifiers have the smallest a pertures, and the lower numbers. L, is a circular plate of brass, fixed horizon tally between the three brass pillars, b b, and in the centre thereof a round hole. II, is adapted to receive a proper con trivance, fig. 4, for holding ivory sliders, wherein objects are placed, which contri vance consists of a spiral steel wire, con fined between three brass circles, one whereof is moveable for the admission of a slider. Q, is a concave mirror set in a box of brass, and turning in an arch, R, upon two small screws. From the bottom of the arch comes a pin, which being let down into a hole in the centre of the pe destal, enables it to turn vertically or hori zontally, and reflect the light either of a candle or the sky, directly upwards on the object to be viewed. V, is a plano-convex lens, which, by turning on two screws when the pin at the bottom of it is placed in the hole made for its reception in the circular plate L, will transmit the light of a candle, to illuminate any opaque object that is put on the round piece of ivory, or on ebony, for examination, and it may be moved higher or lower as the light re quires: this glass Is useful to point the sunshine, or the light of a candle, upon any opaque object ; but in plain day light is of nogreat use. The brass fish pan, fig. 3, is to fasten a smelt, gudgeon, or any such small fish upon, to see the blood cir culate in its tail ; for which purpose, the tail of a fish must be spread across the ob long hole, or the end of the pan, by slipping the button on the backside of the pan, into a slit, through the circular plate, L ; a spring that is beneath the plate presses the button, and will make it steady, and present it well ; but if it be a frog, a newt, or eel, in which the circula tion is desired to be shewn, a glass tube, fig. 6, is fittest for the purpose. The tail of a newt, or eel, or, in a frog, the web between the toe of the hind feet, are the parts where it may be seen best. When the object is well expanded on the inside of the tube, slide the tube along under the circular brass plate, L, where there are two springs, and a cavity made in the shank to hold it, and bring the object di rectly under the magnifier.