ON LIGHT. 347 



point, seen through a rhomboid of the mineral, appear 

 unequally raised above their natural level ; that seen by 

 the ordinarily refracted rays, appearing nearer the eye 

 than the other. 



(125.) By the employment of such a prism as here 

 described,- it is easy to insulate either the ordinary or 

 extraordinary refracted ray, and examine it separately. 

 Suppose, for instance, the latter to be stopped by a 

 screen, and the former only allowed to reach the eye. 

 If before doing so it be made to pass through a second 

 such prism, whose refracting edge is parallel to that of 

 the first, it will be refracted singly and ordinarily : if the 

 edge be held perpendicularly to that of the other, then, 

 singly but extraordinarily. In every intermediate posi- 

 tion the image will be doubled, more of the light passing 

 into the extraordinary image, and less into the ordinary, 

 according as the angle at which the edges cross is in- 

 creased from o to 90; and at 45 of inclination the 

 light is equally divided between the two. This, it is 

 obvious, could not be if the ray were indifferently dis- 

 posed with respect to surrounding space. There sub- 

 sists in it a difference of properties depending on situa- 

 tion a 'difference analogous to that between a square 

 rod and a round one. It has acquired sides in its pas- 

 sage through the crystal, which it preserves in its subse- 

 quent course through space till it meets some body whose 

 action on it may bring their existence into ocular evi- 

 dence. It would seem almost as if light consisted of 

 particles having polarity, like magnets ; and that in its 

 passage through a doubly refracting substance these 



