Mr. Potter on Conical Refraction. S\-5 



The one of one-seventh of an inch thickness does not give suf- 

 ficient separation of the images for convenient use, and the 

 one of five-sixths of an inch had its inconveniences, arising from 

 the fact of crystals of arragonite being formed of twin crystals ; 

 and it is seldom that any component crystal is large enough 

 to allow a ray of light to traverse the optic axis for any great 

 distance without entering the twin one. This was only in 

 some places possible with the specimen referred to, so that 

 the experiments about to be related were generally made with 

 the crystal of half an inch thickness. 



The experiment with sun-light was made as follows : a piece 

 of tinfoil with a small hole in it, made with the point of a pin, 

 being attached to one surface of the crystal, and the whole 

 then attached by means of wax to a moveable adjusting ap- 

 paratus fixed to a stand, I placed the crystal in the sun-light 

 reflected horizontally through a window by a mirror, with the 

 tinfoil towards the incident light, so that the pencil entering 

 the small hole would be in angular diameter 32' nearly, or 

 the sun's apparent diameter ; then placing a piece of tissue- 

 paper on the other surface, I saw generally upon it two round 

 bright spots ; but when the crystal was moved until the light 

 entering it traversed the optic axis, or rather was symmetric- 

 ally refracted with legard to that axis, then there appeared 

 one round bright spot, with a dark ring, and then a broader 

 bright ring round it: now drawing the tissue-paper away 

 from the surface gradually, but keeping it parallel to it, the 

 darker ring gradually disappeared, and the section of the 

 pencil became larger, the central spot becoming more and 

 more faint, and expanding into a conical pencil which blended 

 with the other conical pencil arising IVom the bright ring. 

 These phaenomena show that within the crystal the light had 

 been refracted in the form of a small solid cone, approxi- 

 mating to a cylinder, whose section was the bright spot seen 

 at the second surface, and a larger hollow cone whose section 

 was the bright ring. After emergence into air v/e see that 

 the bright ring expanded into a hollow cone ; but the bright 

 spot, gradually disappearing, gave rise to an indefinite number 

 of conical surfaces of various angles. 



These and other properties of the emergent pencil are 

 much better examined by means of the eye-lens ; and first, 

 placing the crystal in ordinary day-light coming through a 

 small round aperture at some distance, with the second sur- 

 face in the focus of the eye-lens, we perceive the same ap- 

 pearances as were seen on the tissue-paper at the second sur- 

 face of the crystal. Using now a triangular aperture, the 

 two spots appeared triangular images of the aperture, and at 



