114 Rev. H. Lloyd on the Phenomena of Light 



the wave-surface only in the two points just mentioned ; and 

 consequently that a single ray, proceeding from a point with- 

 out a biaxal crystal, and refracted in the direction of the optic 

 axis, would necessarily be divided into two, determined by the 

 points of contact. Professor Hamilton, however, has shown 

 that the four planes in question touch the wave-surface, not 

 in two points only, but in an infinite number of points, consti- 

 tuting each a small circle of contact, whose plane is parallel to 

 one of the two circular sections of the surface of elasticity ; and 

 that, consequently, a single ray of common light, proceeding 

 from an external point, and refracted in the required direction, 

 ought, if the theory be true, to be divided within the crystal 

 into an infinite number of rays, constituting a conical surface. 



Here then are two singular and unexpected consequences 

 of the undulatory theory, not only unsupported by any phse- 

 nomena hitherto noticed, but even opposed to all the analo- 

 gies derived from experience. If confirmed by experiment, 

 they would furnish a new and almost convincing proof of the 

 truth of that theory ; and if disproved, on the other hand, it 

 is evident that the theory must be abandoned or modified. 



Being naturally anxious to submit the theory of waves to 

 this delicate test, and to ascertain how far these new theoreti- 

 cal conclusions were in accordance with actual phenomena, 

 Professor Hamilton requested me to undertake a series of 

 experiments with that view. I accordingly applied myself to 

 this experimental problem with all the attention which the sub- 

 ject so well deserved, and have fortunately succeeded in verify- 

 ing the first-mentioned species of conical refraction. I hope be- 

 fore long to be able to make similar researches on the second*. 



The mineral I employed in these experiments was arrago- 

 ■nite, which I selected partly on account of the magnitude of 

 the cone which theory indicated in this instance, and partly 

 because the three elasticities in this mineral have been deter- 

 mined, apparently with great care, by Professor Rudberg, and 

 therefore the results of theory could be applied to it at once 

 without further examination. The specimen J used was one 

 of considerable size and purity, procured for me by Mr. Dol- 

 lond, and cut with its parallel faces perpendicular to the line 

 bisecting the optic axes. If we suppose a ray of common 

 light to pass in both directions out of such a crystal, along 

 the line connecting the two cusps in the wave, it is evident 

 that it must emerge similarly at both surfaces: consequently 



* Since we received this paper, we have been informed by the author 

 that lie has now observed phacnomena corresponding to the second species 

 of conical refraction, and of which an account will be given in our next 

 Number. — Edit. . », . 



