in passing through biaxal Crystals. 115 



ban 



the ray which passes along this line, and forms a diverging coi.e 

 of rays at emergence at the second surface of the crystal, must 

 arise from a converging cone incident upon the first surface. 

 Having therefore nearly ascertained the direction of the optic 

 axis by means of the rings, I placed a lens of short focus at 

 the distance of its own focal length from the first surface, and 

 in such a position that the central rays of the pencil might 

 after refraction pass along the axis. Then looking through 

 the crystal at the light of a lamp placed at a considerable 

 distance, I observed, in the expected direction, a point more 

 luminous than the space immediately about it, and surrounded 

 by something like a stellar radiation. Fearing that this ap- 

 pearance might have arisen from some imperfection in the cry- 

 stal, I examined it with polarized light, and was happy to find 

 the system of rings in the same direction. This was after- 

 wards confirmed by numerous observations on different parts 

 of the crystal. 



This result is of some interest in itself, independently of its 

 connexion with theory. It has been hitherto supposed that 

 the only means of determining experimentally the direction 

 of the optic axes, in substances of weak double refraction, was 

 by observation of the rings which appear around them, when 

 the incident and emergent light is polarized. Here, however, 

 it is seen that common or unpolarized light undergoes such 

 modifications in the neighbourhood of the optic axes of biaxal 

 crystals, that the apparent direction of the axes may be at 

 once determined, and with the simplest contrivance. 



But to examine the emergent cone it was necessary to ex- 

 clude the light which passed through the crystal in other di- 

 rections. For this purpose a plate of thin metal, having a 

 minute aperture, was placed on the surface of the crystal next 

 the eye; and the position of the aperture so adjusted that the 

 line connecting it with the luminous point on the first surface 

 might be, as nearly as possible, in the direction of the optic 

 axis. The exact adjustment to this direction was made by 

 subsequent trial. The phenomenon which presented itself, 

 on looking through the aperture, when this adjustment was 

 complete, was in the highest degree curious. There appeared 

 a luminous circle with a small dark space round the centre, 

 and in this dark space (which was also nearly circular) were 

 two bright points divided by a narrow and well-defined dark 

 line. When the aperture in the plate was slightly shifted, 

 the appearances rapidly changed. In the first stage of its 

 change the central dark space became greatly enlarged, and 

 a double cone appeared within it. The circle was reduced to 

 about a quadrant, and was separated by a dark interval from 



