NOTES. 499 



NOTK 198, p. 198. Fig. 43 represents the phenomena in question, where S S 

 is the surface, and I the centre of incident waves. The reflected waves are the 

 dark lines returning towards I, which are the same as if they had originated 

 in C on the other side of the surface. 



Fig. 62. 



NOTE 199, p. 201. Fig. 62 represents a prismatic crystal of tour- 

 maline, whose axis is A X. The slices that are used for polarising 

 light are cut parallel to A X. 



NOTE 200, p. 203. Double refraction. If a pencil of light R r, fig. 63, falls 

 upon a rhombohedron of Iceland spar A B X C, it is separated into two equal 



Fig. 63. 



pencils of light at r, which are refracted in the directions r O, r E : when 

 these arrive at O and E they are again refracted, and pass into the air in the 

 directions O o, E o, parallel to one another and to the incident ray R r. The 

 ray r O 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 R r, r O, O o, are all in the same plane. The pencil r E, 

 on the contrary, is bent aside out of that plane, and its refraction does not 

 follow the constant ratio of the sines ; r E is therefore called the extraordinary 

 ray , and r O 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 rl ; and when 

 the crystal is turned round, the image E revolves about O, which remains sta- 

 tionary. 



K K 2 



