330 Professor Airy on the Double Refraction of Quartz. 



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 white 

 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 

 surprised 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 de- 

 serves (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 conti- 

 nuity. The nappes of the wave surfaces are absolutely separated. 

 This is not the case in the common construction for uniaxal 

 crystals, nor in FresneFs construction for biaxal crystals. There 

 may possibly be a connection of the same kind as that between 

 the change from partial reflexion to total reflexion within glass, 

 and the accompanying change from plane polarized light to el- 

 liptically-polarized light. The cases are at least thus far ana- 

 logous, that the change in the light and the interruption of con- 

 tinuity go together. But we are so much in the dark respect- 

 ing the physical constitution of quartz, that we cannot at pre- 

 sent go farther. 



It might have been desirable to verify my suppositions by 

 more direct experiments on the separate rays of quartz. I can 

 only plead that the duties of my office have not allowed me 

 the necessary time. They would (under all circumstances) have 

 been much more troublesome than those which I have the ho- 

 nour of laying before the Society : and I do not think that 

 they would have been more satisfactory. The appearances 

 presented by depolarization are admirably adapted to the dis- 

 covery of the most delicate differences in the nature and course 



