PROPBRTIES OF LIGHT. J 6? 



emergent rays; those which arise from the ordinary and ex- 

 traordinary refraction of the first crystal being refracted 

 each In a single ordinary or extraordinary ray by the second. 

 In fact in this case it may be conceived, that, whether the 

 axes of the crystals be parallel, or placed in opposite di- 

 rections, every ray issuing from the first <?rystal parallel to 

 its principal section is not divided by the second, for its 

 movement takes place round the axis b or the axis c ; and 

 we have seen by the phenomena of reflection, that, when- 

 ever the movement takes place round these axes, the ray is 

 not altered ; all the particles preserving the parallelism of 

 their similar axes. The rotation round the axis a being the . 

 only one, that changes the respective positions of the axes 

 of the particles of a given ray. 



When the incident ray makes any angle whatever with the ExamjileS. 

 principal sections, the rays that proceed fron^ the double re- 

 fraction of the first crystal are divided into two by the 

 second, so that we then obtain four emergent rays. In this 

 circumstance however there are two different cases, in which 

 the phenomena are very distinct: that in which the axes 

 of the crystals are parallel, and that in which they are in op- 

 posite directions. When the axes are parallel, a very vivid 

 light must be employed, and the plane of incidence must be 

 removed to a sensible distance frdm that of the principal 

 sections, to be able to perceive the rays refracted ordinarily 

 by one crystal and extraordinarily by the other. In fact, 

 agreeably to the theory, the maximum of intensity of these ' 



two rays is not the thirtieth part of that of the ray, which 

 proceeds from the ordinary refraction of the two crystals; 

 which has led those who have written on this subject to 

 imagine, that, when the principal sections and the axes are 

 parallel, the light comports itself in the same manner as in 

 the principal section, whatever be the direction of the inci- 

 dent ray : but if we employ a vivid light, under suitable 

 circumstances, observation accords perfectly with the the- 

 ory. The phenomenon is much more evident, when the 

 axes are in opposite directions. 



The extraordinary refraction is produced by a repulsive Extraordinary 

 force, the action of which is proportional to the square of 'efraction, 



the 



