In the first column are the names of the bodies; in the fecond column, the fines of incidence and refraction; in the third column, the refracting power; in the fourth column, their denfity, or specific gravity; in the fifth column, the ratio.

From this Table it appears, that those bodies which con

tain oily fulphurous particles differ from the constant ratio, and have a greater refractive power.

SECT.

SECT. II.

OF SINGLE AND COMPOUND MICROSCOPES.

SINGLE microfcopes are fuch as have but one lens; and may be conftructed in an infinite variety of ways. I fhall give the forms of a few of the most useful.

A (fig. 10, plate 5) is a circular piece of wood, metal, or ivory, in the middle of which is a fmall hole, c. Upon this hole is fixed, by means of a wire, a glass spherical lens, of one tenth of an inch radius, whose focal distance is c D. At the focus D is a pair of plyers DE, fixed upon a fliding fcrew B, and which opens by means of the two ftuds a, e: these plyers ferve to hold any small object O, which is to be viewed through the lens by the eye, placed in the other focus F: and according to the focal distance of the lens, the object will be more or lefs magnified, as before described: thus, in a lens of this radius, viz. one tenth of an inch, the focal distance of which is a radius and an half from the centre thereof, as described in the foregoing propofitions; the length of an object, by fuch a lens, will be magnified forty times; the furface, one thousand fix hundred; and folidity, fixty-four thousand times. For a radius and an half of such a sphere, or of an inch, is only the fortieth part of fix inches, (the leaft distance of naked vifion;) the fquare of 40 is 1600, and the cube of 40 64,000. This instrument, from its fize and portability, is very convenient. The beft lenses for these fingle microscopes are, however, those whose focal distance is three tenths, or four tenths of an inch.

Again, when the diameter of a spherical lens is only one twentieth of an inch, the object will fubtend an angle at the

eye

eye as great as if it were only three eightieths of an inch (which is the one hundredth and fixtieth part of fix inches) diftant: therefore, the length of an object feen through fuch a lens will be magnified one hundred and fixty times; the furface, twenty-five thousand fix hundred times; and the folidity, four million and ninety-fix thoufand times; which is fo great a magnifying power as to furpafs conception.

But fpherule lenfes, of fo fmall a diaineter as the latter, are of no use but in viewing tranfparent objects; for if an opake object were to be viewed by fuch a lens, the eye must be applied almoft close to the furface of the lens, by which the lens and object would be so overfhadowed, as to render an opake object too obfcure to be viewed.

These fpherule lenses were ufed by the famous Mr. Leeuwenhock, by which he made fuch wonderful discoveries in the minuter parts of nature. And it must be by a proper arrangement of thefe, that the particles of matter are to be discovered, if ever they can, which Sir Ifaac Newton thought was poffible.

But there are great difficulties attending the use of these very small fpherules: firft, the difficulty of making very good ones, of a very fmall diameter. Secondly, the prejudice done to the eyes in ufing them. Thirdly, the trouble of placing objects at the proper focus. And laftly, the very small part of the object, which can be feen at one time. These inconveniences render this fort of microscopes of very little fervice for common ufe; they have therefore been fuperfeded by others, particularly by the following; in which both tranfparent and opake objects niay be viewed with lefs

trouble.

In this inftrument (fig. 11.) F is a piece of brafs turning round in a focket, at the end of which is a small spring tube moving upon a rivet, through which there runs a steel wire, terminated at one end by a fharp point, G, and the other end hath a pair of plyers, H. The point and plyers are to thrust into, or take up any object. Either of them may be

turned

turned upwards, towards G. I is a ring of brass with a female screw within it: this ring is fixed upon a piece of brass which turns round on a rivet near D, that it may be set to the proper difance, when the leaft magnifiers are used. K (fig. 12.) is a concave fpeculum, or mirror, generally of filver, polished very high, in the centre of which a double convex lens is placed, with a proper aperture, to look through it; on the back of this fpeculum is a male fcrew L, which is made to screw into the female fcrew I (fig. 11).

There are generally four of these concave fpecula, with different lenfes of different magnifying powers, for different objects. The greatest magnifiers must always have the least apertures.

D is a nut adapted to the screw B: one end of this screw is faftened to the fide C, and by turning the nut D, the two fides A and C are gradually brought together, or separated, being held steady by the steel spring E. P is the handle of the inftrument.

To use this inftrument, the fpeculum K is to be fcrewed into the brass ring I, the magnifying lens being first fixed in the fpeculum. Then place the object either on the needle G, in the plyers H, on the object plate M (fig. 13), or in the brafs box O (fig. 14), as is moft convenient; then holding up the inftrument by the handle P, look through the magnifying lens directly against the light; and by the nut D, together with the motion of the needle, the object may be turned about, raised, or depreffed; brought nearer to or further from the glafs, till it be brought to the true focus, and the light is ftrongly reflected upon it, from the speculum: by these means the object will be seen very clearly and distinctly. The light of a candle will ferve as well as day-light, for this inftrument.

O (fig. 14) is a small brass box, with a glass on each fide to confine any living object, in order to be examined; it fcrews upon the needle G, by means of a small pipe at bottom.

M (fig.

M (fig. 13) is a round object plate, black on one fide, and white on the other. N, a steel spring, turning down on each fide, to hold any object faft. A dark-coloured object is to be placed on the white fide; and a light-coloured object on the black fide, to render them more diftinct. This plate also has a small pipe to screw on the needle G (fig. 11).

This microscope is of late invention: and by means of the polished mirror, any opake object may be viewed with a very fmall magnifier. Transparent objects may also be viewed by it: but in this case, it will not be proper to throw on the object all the light reflected from the speculum; left the light tranfmitted through the object, meeting the reflected light, produce too great a glare. For this purpose, a bit of paper may be interpofed between the object and fome part of the fpeculum. A little practice will enable a perfon to know how to regulate the light.

As opake objects are more numerous than others, very great discoveries may be made in fuch objects.

A Microscope on a Stand, which will answer all the Ends of the large Double Reflecting Microscope.

B (fig. 17) is a round frame of wood, on which is fixed the brafs fcroll A for the purpose of holding the microscope fteady. C is a brafs fcrew that paffes through a hole in the scroll, into the microscope D, and ferves to fcrew it fast to the scroll.

E is a concave looking-glafs, or fpeculum, of metal, fet in a brass box, and hanging in the arch G by two small screws: at the bottom of this arch is a small brass pin h, which goes through the stand of the arch into the wooden frame B, by which means the arch is turned round in an horizontal direction; and the mirror fwings in a vertical direction. By these two motions the fpeculum may be fo adjufted as to reflect the light directly upwards, through the body of the microfcope D, which is fixed perpendicularly over it.

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