348 Mr. Potter on Conical Refraction. 



to the optic axis, I found it on emergence giving the ring and 

 central spot. In this experiment the angle of the incident 

 cone was rather more than 4'. 



In this experiment the light was reduced to a small pencil 

 by apertures in thin metal plates, but I have seen the same 

 phccnomena when the incident pencil arose from a luminous 

 point formed by a lens of short focus, at some distance from 

 the crystal. I'he results of Professor Lloyd's method of ex- 

 perimenting were, that in the first instance he found the angle 

 of the emergent cone about double what the calculation from 

 theory gave, but by reducing the diameter of the aperture on 

 the second surface this observed angle was reduced also; 

 until with a very minute aperture an approximation to the 

 calculated angle was obtained ; and a hollow cone was seen, 

 as required, in place of a solid cone, which appeared in the 

 first instance. 



It appears, that in order to obtain any tolerable approxi- 

 mation to the calculated quantities, an aperture was required 

 to be applied to the second surface, so as to cut ofi^all the ring 

 and a large portion of the central spot. Such experiments 

 cannot be considered as investigations of the refraction of 

 biaxal crystals in the neighbourhood of the optic axes. The 

 real nature of this refraction will be seen when we have ex- 

 amined Professor Lloyd's investigation in search of the conical 

 refraction within the crystal giving a cylinder of rays in air. 



He says, " The second kind of conical refraction, whose ex- 

 istence has been anticipated by Professor Hamilton, depends 

 (it will be remembered) on 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. It 

 takes place when a single external ray falls upon a biaxal 

 crystal in such a manner that one refracted ray may coincide 

 with an optic axis. When this is the case, there will be a 

 cone of rays within the crystal, determined by lines connect- 

 ing the centre of the wave with the points of the periphery of 

 the circle of contact. The angle of this cone is equal to 



tan-iJ^'-^''- ^ ^'' 



and its numerical value in the case of arragonite is 1° 55', as- 

 suming the values of the three indices as determined for the 

 ray E by Professor Uudberg (see Trans. R.I. A., vol. xvii. 

 page 151.). 



"As the rays constituting this cone will be refracted at emer- 



