342 ON LIGHT. 



(120.) As we have already stated when speaking of re- 

 fraction generally, a ray of light incident on any trans- 

 parent crystal (certain specified classes of crystals ex- 

 cepted) is subdivided by refraction into two distinct rays, 

 pursuing different paths within the crystal, and of course 

 emerging from it at different points, and so, of necessity, 

 reappearing, not as a single, but as two distinct and 

 separate rays, each pursuing its subsequent course inde- 

 pendent of the other through space. Of these, when 

 traced, one is found to have been refracted in the \ lane 

 of its incidence on the transmitting surface, and accord- 

 ing to the ordinary simple " law of the sines " already 

 explained. It is therefore said to be ordinarily refracted, 

 and it is called the ordinary ray. The other, except in 

 special cases, deviates after refraction from the plane of 

 its incidence, more or less according to the situation of 

 that plane with respect to the faces of the crystal: and, 

 moreover, in respect of the amount of its flexure, does 

 not conform to the simple law of the sines, but to a rule 

 much more complex in its expression, called the law of 

 extraordinary refraction; this ray being designated as the 

 extraordinary ray. Such is the case if the original, inci- 

 dent ray be one directly emitted from the sun, a candle, 

 or any self-luminous body. But if in place of such a ray, 

 we employ either of the two rays so separated as above 

 described, for transmission through a second crystal of 

 the same kind, the result will be very different. If it fall 

 upon such second crystal in the same manner, according 

 to the same angles and with the same relative situation 

 as to its plane of incidence with the sides of the crystal, 



