208 Rev. H. Lloyd on the Phenomena of Light 



the wave. The second takes place within the crystal, when a 

 single ray has been incident externally in such a manner, that 

 one of the refracted rays may coincide with the normal to the 

 circular section of the surface of elasticity, or the optic axis*. 



In the article alluded to, I have entered into an account of 

 some experiments, undertaken at the request of Professor Ha- 

 milton, which establish the existence of the first case of coni- 

 cal refraction ; and go far, therefore, to support the theory of 

 which it is a consequence. I have only to add, on this part 

 of the subject, that additional measurements, taken since that 

 paper was written, indicate a nearer agreement between the 

 observed and computed cones than was at first obtained. 



I have since succeeded in observing also the second species 

 of conical refraction ; and I now propose to give a brief sketch 

 of the results of my experiments, referring fur further detail 

 to the forthcoming volume of the Transactions of the Royal 

 Irish Academy. 



It has been already mentioned, that the existence of this 

 phenomenon depends upon the mathematical fact, — that the 

 wave surface is touched in an infinite number of points, con- 

 stituting a small circle of contact, by a single plane parallel 

 to one of the circular sections of the surface of elasticity. 

 When a ray is incident upon the crystal externally, in such 

 a direction that one refracted ray may be normal to the plane 

 just mentioned, it will be divided into a cone of rays within 

 the crystal, determined by lines connecting the centre of the 

 wave with the points of the periphery of the circle of contact. 



The angle of this cone = tang — 1 ■ 2 — — , c being 



the mean axis; and its value in the case of arragonite, calcu- 

 lated from the elements of this crystal as determined by Pro- 

 fessor Rudberg, is 1° 55 ! . 



Since the rays which compose this cone will be refracted at 

 emergence in a direction parallel to the incident ray, they will 

 form a small cylinder of rays in air; the base of the cylinder 

 being the section of the cone formed by the second surface 

 of the crystal. This cylinder is in all cases extremely small, 

 and the experiments necessary to detect its existence and as- 

 certain its magnitude require more care than those hitherto 

 described. 



nomenclature. Fresnel calls the former, the optic axis, when he is de- 

 fining the term ; but he subsequently applies the same name to the other. 

 I fear that 1 have also made the same double application of the term in 

 my former communication on this subject; though I have generally, with 

 Professor Hamilton, used the word cusp-ray to designate the latter. 

 * See last note. 



