148 Mr. Lioypd on Conical Refraction. 
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 erystal 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 direc- 
tion 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 emer- 
gent light is polarized. Here, however, 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 which presented itself when this disposition was complete, was in the 
highest degree curious. There appeared at first a luminous circle, with a small dark 
space in the centre; and in this dark central space were two bright points, separated 
by a narrow and well-defined dark line. ‘These appearances are represented in (figures 
a and 6.) When the aperture in the plate was slightly shifted, the phenomena rapidly 
changed, assuming in succession the forms represented in (figs. c, d, e.) In the first stage 
of its change, the central dark space became greatly enlarged, and a double sector 
appeared in the centre. The circle was reduced to about a quadrant, and was sepa- 
rated by a dark interval from the sector just mentioned. This is represented in (fig. ¢.) 
The remote sector then disappeared, and the circular arch diminished, as in (fig. d) ; 
* This fact is here mentioned, rather as a matter of curiosity than as one likely to 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 Fresne’s theory; while the 
rings (there is strong reason to believe) are related to the normals to the circular sections of the surface 
of elasticity. 
