428 



NOTES. 



NOTE 198, p. 171. Fig. 43 represents the phenomeaa in question, where 

 S S is the surface, and I the center of incident waves. The reflected 

 waves are the dark lines returning toward I, which are the same as if 

 they had originated in C on the other side of the surface. 



NOTE 199, p. 173. Fig. 62 represents a prismatic crystal of tourma- 

 line, whose axis is A X. The slices that are used for polarizing light are 

 cut parallel to AX. 



Fig. 62. Fig. 63. 



A 



NOTE 200, p. 175,-Double refraction. If a pencil of light, Rr, fig. 63, 

 fa-Us upon a rhombohedron of Iceland spar, A B X C, it is separated into 

 two equal pencils of light at r, which are refracted in the directions rO, 

 r E : when these arrive at O a-nd E they are again refracted, and pass 

 into the air in the directions Oo, Eo, parallel to one another and to the 

 incident ray Rr. The ray rO is refracted according to the ordinary law, 

 which is, that the sines of the angles of incidence and refraction bear a 

 constant ratio to one another (see Note 184), and the rays Rr, rO, Oo 

 are all in the same plane. The pencil rE, on the contrary, is bent aside 

 out of that plane, and its refraction does not follow the constant ratio 

 of the sines; rE is therefore called the extraordinary ray, and rO the 

 ordinary ray. In consequence of this bisection of the light, a spot of ink at 

 O is seen double at O and E, when viewed from r ; and when the crystal 

 is turned round, the image E revolves about O, which remains stationary. 



NOTE 201, p. 176. Both of the parallel rays Oo and Eo, fig. 63, are 

 polarized on leaving the doubly refracting crystal, and in both the parti- 

 cles of light make their vibrations at right angles to the lines Oo Eo. 

 In the one, however, these vibrations lie, for example, in the plane of the 

 horizon, while the vibrations of the other lie in the vertical plane per- 

 pendicular to the horizon. 



NOTE 202, p. 177. If light be made to fall in various directions on the 

 natural faces of a crystal of Iceland spar, or on faces cut and polished 

 artificially, one direction, A X, fig. 63, will be found, along which the 

 light passes without being separated into two pencils. A X is the optic 

 axis. In some substances there are two optic axes forming an angle with 

 each other. The optic axis is not a fixed line, it only has a fixed direc- 

 tion ; for if a crystal of Iceland spar be divided into smaller crystals, each 

 will have its optic axis ; but if all these pieces be put together again, their 

 optic axes will be parallel to A X. Every line, therefore, within the 

 crystal parallel to AX is an optic axis; but as these lines have all the 

 same direction, the crystal is still said to have but one optic axis. 



NOTE 203. p. 178. If 1C, fig. 48, be the incident and CS the reflected 



