122 Professor Airy on the 



light in the two rays of quartz is such as I have described. 

 I do not mean to exclude the possibility of supposing that the 

 form of neither wave (in the construction for determining the 

 course of the rays) is exactly spherical or exactly spheroidal : 

 provided the difference of the forms be nearly the same as that 

 of a sphere and a spheroid. Nor do I mean to assert that each 

 elliptically-polarized ray consists exactly of two plane-polarized 

 rays following each other at the interval of one-fourth of an un- 

 dulation : or that the ratio of the two axes in the two rays is 

 exactly the same. But I conceive it to be perfectly certain that 

 the general character of the light is such as is stated in my hy- 

 potheses. 



I have not made any calculations upon other suppositions, 

 but I can hardly imagine that any other would represent the 

 phenomena to such extreme accuracy. I am not so much struck 

 with the accounting for the continued dilatation of circles, and 

 the general representation of the form of spirals, as with the 

 explanation of the minute deviations from symmetry, as when 

 circles become almost square, and crosses are inclined to the 

 plane of polarization. And I believe that any one who shall 

 follow my investigations and imitate my experiments, will be 

 surprized at their perfect agreement. 



There is one relation between the construction for determin- 

 ing the course of the rays, and the nature of the rays, which 

 deserves (I think) particular attention. It is that (comparing 

 the rays of quartz with those of any other crystal) a change 

 in the nature of the ray is accompanied with an interruption of 

 continuity. The ?mppes of the wave surfaces are absolutely se- 

 parated. This is not the case in the common construction for 

 uniaxal crystals, nor in Fresnel's construction for biaxal crystals. 



