346 Mr. Potter on Conical Refraction. 



the optic axis the ring and spot possessed some trace of this 

 triangular relation. The triangular appearance arose from 

 the well-known optical property of a small hole, which gives 

 on a screen beyond it an imperfect image of a bright object 

 in front of it. In the experiment with the sun-light, although 

 the central spot was round, yet it cannot be considered an 

 image of the sun, in the same way as the two round spots 

 generally seen. 



The change from two spots to a I'lng and a spot was sud- 

 den, upon the direction of the light becoming that requisite for 

 refraction symmetrical to the optic axis ; and in experiments 

 which will be related shortly, when a minute pencil of inci- 

 dent light was transmitted through a very minute aperture 

 in the tinfoil, it required very nice adjustment to obtain the 

 ring and spot. This shows that if a converging pencil were 

 incident at the hole on the first surface, with its axis in the 

 direction requisite for refraction symmetrical to the optic axis 

 of the crystal, then no other ray but that in the axis of the pencil 

 could be subject to the singular refraction in the crystal : but 

 the other rays of the incident pencil giving each two rays within 

 the crystal, these latter would be arranged around the optic 

 axis, as the incident rays were around the axis of the pencil, 

 and the appearances would be symmetrical ; although intro- 

 ducing, in proportion to the magnitude of the pencil, con- 

 fusion and indistinctness into the real phsenomena of conical 

 refraction. Professor Lloyd says, " The first-mentioned 

 species of conical refraction, it has been observed, takes place 

 in air, when a ray of common light is transmitted within the 

 crystal in the direction of the line joining two opposite cusps 

 of the wave. If we suppose such a ray to pass in both di- 

 rections out of the crystal, it is evident that it must emerge 

 similarly at both surfaces ; consequently, the rays which are 

 transmitted along this line within the crystal, and form a di- 

 verging cone at emergence at the second surface, must be 

 incident in a 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," &c. 

 And again, he says, " 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 sur- 

 face of the crystal next the eye, and the position of the aper- 

 ture so adjusted, that the line connecting it with the luminous 



