NOTES. 



421 



in the same plane. We see the surface by means of the reflected light, 

 which would otherwise be invisible. Whatever the reflecting surface may 

 be, and however obliquely the light may fall upon it. the angle of reflection 

 is always equal to the angle of incidence. Thus 1C, 1' C, being rays in- 

 cident on the surface at C, they will be reflected into CS, C S', so that 

 the angle 8 C P will be equal to the angle I C P, and S' Cf equal to I' C P. 

 That is by no means the case with the refracted rays. The incident 

 rays I C, I' C, are bent at C, toward the perpendicular, in the direction 

 CR, CR' ; and the law of refraction is such, that the sine of the angle 

 of incidence has a constant ratio to the sine of the angle of refraction ; 

 that is to say, the number expressing the length of I m. the sine of I C P, 

 divided by the number expressing the length of R n, the sine of RC/>, is 

 the same for all the rays of light that can fall upon the surface of any one 

 substance, and is called its Index of refraction. Though the index of re- 

 fraction be the same for any one substance, it is not the same for all sub- 

 stances. 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 sub- 

 stance has a higher refractive power than any other known. Light fall- 

 ing perpendicularly on a surface, passes through it without being refract- 

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

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

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

 ular instead of toward it. When the incidence is very oblique, as rC, 

 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' : that frequently 

 happens at the second surface of glass. When a ray 1C falls from air 

 upon a piece of glass A B, it is in general refracted at each surface. At 

 C it is bent toward the perpendicular, and at R from it, and the 'ray 

 emerges parallel to 1C ; 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 re- 

 fracted. 



NOT 185, p. 148. Atmospheric rtfraction. Let a ft, a *, Ac., flg. 49, be 

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



rity toward win, the surface of the earth. A ray coming from a star 

 meeting the surface of the atmosphere at 8, would be refracted at the 

 surface of each layer, and would consequently move in the curved line 

 Bvv 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 AS, would seem to a 

 NN 



