470 PHYSICAL SCIENCES. 



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 



Fig- 63. (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 re- 

 fraction does not follow the 

 constant ratio of the sines ; r E 

 is therefore called the -extraor- 

 dinary ray, and r the ordi- 

 nary ray. In consequence of 

 this bisection of the light, a 

 spot of ink at is seen double 

 at and E, when viewed from 

 r I ; and when the crystal is 

 turned round, the image E revolves about 0, which remains stationary. 



NOTE 206, p. 182. Both of the parallel rays Oo and E o, fig. 63, are 

 polarised on leaving the doubly refracting crystal, and in both the particles 

 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 perpendicular to 

 the horizon. 



NOTE 207, p. 183. 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 direction; 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 A X 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 208, p. 184. If 1C, fig. 48, be the incident and CS the re- 

 flected rays, then the particles of polarised light make their vibrations at 

 right angles to the plane of the paper. 



NOTE 209, p. 184. Let A A, fig. 48, be the surface of the reflector, 

 I C the incident and C S the reflected rays ; then, when the angle S C B is 

 57, and consequently the angle PCS equal to 33, the black spot will 

 be seen at C by an eye at S. 



NOTE 210, p. 185. Let AB, fig. 48, be a reflecting surface, 1C the 

 incident and C S the reflected rays ; then, if the surface be plate-glass, the 

 angle S C B must be 57, in order that C S may be polarised. If the 

 surface be crown-glass or water, the angle S C B must be 56 55' for 

 the first, and 53 11' for the second, in order to give a polarised ray. 



NOTE 211, p. 186. A polarising apparatus is represented in fig, 64, 

 where Rr is a ray of light falling on a piece of glass r at an angle of 57 : 



