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Of Optics.

ITence, if DBH were a crooked stick put obliquely into thewater, it would appear a straight one, as DBG. Therefore, asthe line BH appears at BG, so the line BG will appeat; at Bg-,and confequentiy, a straight stick DBG put obliquely into water,^ will feem bent at the surface of the water in B, and crooked,as DBg.

When a ray of light pastes out of air into glass, the sine of in-cidence is to the sine of refraction, as 3 to 2; and when out of airinto a diamond, as 5 to 2.

Fig, 5. Glass may be ground into eight disterent shapes at least, for optical

purposes, viz.

1. A plane glass, which is stat on both sides, and of equal thicknefsin all its parts, as A.

Lenscs. 2. A plano convex, which is stat on one side, and convex on the

other, as B.

3. A double convex, which is convex on both sides, as C.

4. A plano-concave, which is flat on one side, and concave on theother, as D.

5. A double concave, which is concave on both sides, as E.

6. A meniscus, which is concave on one side, and convex on theother, as F.

7. A f.at plano convex, whofe convex side is ground into feverallittle stat surfaces, as G.

8. A prifm, which has three stat sides, and when viewed endwife,appears like an equilateral triangle, as H.

Glasses ground into any of the shapes B, C, D, E, F, are ge-nerally called lenfes.

Fig. 6. A right line LIK, going perpendicularly through the middle of a

lens, is called the axis of the lens.

A ray of light Gh, falling perpendicularly on a plane glass E F, willpafs through the glass in the fame direction hi, and go out of it intothe air in the fame right course iH.

A ray of light A B, falling obliquely on a plane glass, will go outof the glass in the fame direction, but not in the fame right line ; for intouching the glass, it will be refracted in the line BG, and in leavingthe glass, it will be refracted in the line C D.

A ray of light CD, falling obliquely on the middle of a convexglass, will go forward in the fame direction D E, as if it had fallenwith the fame degree of obliquity on a plane glass; and will go outof the glass in the fame direction with which it enterest : for it willbe equally refracted at the points D and E, as if it had passest through

a plane

Fig. 7.