492 



NOTES.' 



can fall upon the surface of any one substance, and is called its index of re- 

 fraction. Though the index of refraction be the same for any one substance, 

 it is not the same for all substances. For water it is 1-336; for crown-glass it 

 is 1-535; for flint-glass, 1-6; for diamond, 2'487; and for chromate of lead it 

 is 3, which substance has a higher refractive power than any other known. 

 Light falling perpendicularly on a surface passes through it without being re- 

 fracted. If the light be now supposed to pass from a dense into a rare medium, 

 as from glass or water into air, then R C, R' C, become the incident rays ; and 

 in this case the refracted rays, C I, C I', are bent from the perpendicular in- 

 stead of towards it. When the incidence is very oblique, as r C, the light never 

 passes into the air at all, but it is totally reflected in the direction C r', so that 

 the angle p C r is equal to p C r j : that frequently happens at the second sur- 

 face of glass. When a ray I C falls from air upon a piece of glass A B, it is in 

 general refracted at each surface. At C it is bent towards the perpendicular, 

 and at R from it, and the ray emerges parallel to I C; but, when the ray is 

 very oblique to the second surface, it is totally reflected. An object seen by 

 total reflection is nearly as vivid as when seen by direct vision, because no 

 part of the light is refracted. 



NOTE 185, p. 172. Atmospheric refraction. Let a b, a b, &c., fig. 49, be 

 strata, or extremely thin layers, of the atmosphere, which increase in density 



towards m n, the surface of the earth. A ray coming from a star meeting the 

 surface of the atmosphere at S, would be refracted at the surface of each layer, 

 and would consequently move in the curved line S vv v A; and as an object 

 is seen in the direction of the ray that meets the eye, the star, which really is 

 in the direction A S, would seem to a person at A to be in s. So that refrac- 

 tion, which always acts in a vertical direction, raises objects above their true 

 place. For that reason, a body at S', below the horizon H A O, would be 

 raised; and would be seen in s'. The sun is frequently visible by refraction 

 after he is set, or before he is risen. There is no refraction in the zenith at Z. 

 It increases all the way to the horizon, where it is greatest, the variation being 

 proportional to the tangent of the angles Z A S, Z AS', the distances of the 

 bodies S, S ', from the zenith. The more obliquely the rays fall, the greater the 

 refraction. 



