CONICAL REFRACTION. Q 



converging cone at the first. Having therefore nearly ascertained 

 the required direction by means of the system of rings in polarized 

 light, I placed a lens of short focus at its focal distance from the 

 first surface, and in such a position that the central part of the 

 pencil might have an incidence nearly corresponding to the cusp- 

 ray within. 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 resembling a stellar radiation. 

 Fearing that this singular appearance might have arisen from 

 some imperfection in the crystal, I transmitted the light in the 

 same manner through several different parts of its substance, and 

 always with the same result. The connexion of the phenomenon 

 with the optic axis was proved by the system of rings which 

 appeared in the same direction, when the light was examined with 

 a polarizing and analyzing plate. 



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

 connexion with theory. It has been hitherto supposed that the 

 only method of determining experimentally the direction of the 

 optic axes, in most doubly-refracting substances, consisted in 

 observing the system of coloured rings, which appear around them 

 when the incident and emergent light is polarized. Here, how- 

 ever, we find that common, or unpolarized light, undergoes such 

 modifications in the neighbourhood of one of the optic axes, that 

 the apparent direction of that axis may be at once determined, 

 and with the aid of the simplest contrivance.* 



But to examine the emergent cone, it was necessary to exclude 

 the light which passed through the crystal in all but one direction. 

 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 cusp-ray. The exact adjustment 

 to this direction was made by subsequent trial. The phenomenon 



* This fact is here mentioned, rather as a matter of curiosity than as one likely co 

 be of practical value in determining the optical elements of crystals. It is to be 

 observed, moreover, that the direction thus determined is that of the normal to the 

 circular section of the ellipsoid of Fresnel's theory ; while the rings (there is strong 

 reason to believe) are related to the normals to the circular sections of the surface 

 iif elasticity. 



