Imatges de pàgina
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9. When the object and its image are on different fides of the lens, as the object approaches, the lens, the image recedes from it: or, if the object recedes from the lens, the image approaches it.

10. If the object and image be both on one fide of the lens, and the object move towards the lens, the image also moves towards the lens: and if the object moves from it, the image alfo moves from it.

11. In a convex lens, if the object be further off than the principal focus, its image will be on the other fide of the glafs, inverted. But if the object be nearer than the principal focus, the image will be on the fame fide of the glafs, erect.

12. In a concave lens, the image and object are always on one fide of the lens.

13. If an object be placed in the principal focus of a lens, its apparent magnitude at any place whatever, beyond the lens, will be invariably the fame; and equal to the apparent magnitude, when feen from the centre of the lens, with the naked eye.

14. The apparent magnitude of a body, placed in the principal focus, will always continue the fame, however the eye is moved backward or forward, from the lens.

15. The nearer the eye is to the lens, the more of the object appears; and the farther off the eye is from the leus, the lefs of the object is feen.

16. If the object be nearer than the principal focus, its apparent magnitude becomes lefs, in going from the glass. But if the object be further than the focus, the apparent magnitude will increafe, in going from it.

17. If the eye be fixed in the principal focus, the apparent magnitude of an object will be invariably the fame, wherever the object is placed before the glass.

18. If the eye and object be fixed, and a convex lens be moved from the object to the eye, the apparent mag

nitude of the object increases to the middle, and then decreases to the eye. When it comes into fuch a position that the eye and object are conjugate foci, the object is infinitely confused.

19. If the eye and object be fixed, and a concave lens be moved from either of them to the other; the apparent magnitude of the object will decrease to the middle, and then increase again. The apparent diftance is reciprocally as the apparent magnitude; and, in general, we judge the apparent distance of an object to be the very fame as we fuppofe the real distance of an object to be, from which the rays come to our eyes, with the fame degree of confufion.

20. Converging rays are made more converging by a convex lens: and diverging rays are made more diverging

a concave.

21. Parallel rays falling on one fide of a convex lens will be refracted to the focus on the other fide. But parallel rays falling on a concave lens will, by refraction, diverge from the focus on the fame fide.

22. In a convex lens, rays diverging from the focus will emerge parallel on the other fide.

23. In a concave lens, rays converging to the focus will emerge parallel, going out of the lens on the same fide.

24. In a double convex lens of equal radii, the principal focus is diftant from the lens, the radius of the sphere, of which the lens is a fegment.

25. When the lens is a perfect sphere, the principal focus is diftant from the lens half the radius.

26. When the lens is an hemifphere, the principal focus is diftant four thirds of the radius, when the convex fide is expofed to the rays; and twice the radius, when the plane fide is expofed.

27. When the lens is a plano-convex, the distance of the focus is equal to the diameter, or twice the radius of the sphere when the plane fide is expofed to the rays. But when

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the convex fide is exposed to the rays, the focus is distant twice the radius, except two fifths of the thickness.

28. In double concaves of equal radii, the principal focus is diftant the radius of the sphere; and it is virtual.

29. In a plano-concave, the principal focus is diftant twice the radius; and is virtual.

30. If the radiant point and the focus be equidiftant from a lens, they will each of them be diftant twice the principal focal distance.

31. If the radiant point be nearer the lens than the principal focus, the rays after refraction will diverge; but if the radiant point be in the principal focus, they will after refrac tion emerge parallel; and if the point be farther off than the focus, the rays will converge after refraction.

32. The diftance of the radiant point and its focus, made by a convex lens, is the leaft poffible, when they are equidiftant from the lens.

33. If a convex lens be held directly to the rays of the fun, and a combuftible body be held in the principal focus; the heat of the rays of the fun, converging to a point in the focus, will fet the body on fire.

34. The heat of the focus of the lens is to the sun's direct heat, as the area of the glafs to the area of the image in the focus.

35. The degrees of heat in the foci of different lenfes are as the fquares of the diameter directly and the squares of the focal distances reciprocally.

The foregoing propofitions contain all the phenomena of refracting lenses; and may be proved for the most part by actual experiments.

It is neceffary just to obferve, for the fake of those who may use lenses of a different medium, that different mediums have different refracting powers: I fhall, therefore, exhibit a table of the refracting power, as given by Sir Ifaac Newton, and proved by many later experiments.

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 fpecific gravity; in the fifth column, the ratio.

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From this Table it appears, that those bodies which con

tain oily fulphurous particles differ from the conftant 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. 1 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 small 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 screw B, and which opens by means of the two studs 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 less magnified, as before defcribed; 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, sixty-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 inftrument, from its fize and portability, is very convenient. The best lenses for these fingle microscopes are, however, those whofe 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

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