Mr. Potter on Conical Refraction. 349 



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

 form a small cylinder of rays in air. This cylinder, it will 

 be seen, is in all cases extremely small ; for the diameter of its 

 section made by the surface of emergence subtends an angle 

 of 1° 55' oiil}', at a distance equal to the thickness of the cry- 

 stal. Hence the experiments required to detect its existence 

 and measure its magnitude, demand more care and precision 

 than those already described. 



" The incident light was that of a lamp placed at some di- 

 stance; and in order to reduce as much as possible the breadth 

 of the incident beam, it was constrained to pass through two 

 small apertures, the first of which was in a screen placed near 

 the flame, and the second perforated in a thin metallic plate 

 adjoining to the first surface of the crystal. Under ordinary 

 circumstances, it is obvious the incident ray will be divided 

 into two within the crystal, and these will emerge parallel 

 from the second surface. I was able to distinguish these two 

 rays by the aid of a lens ; and turning the crystal slowly, so 

 as to vary the incidence gradually, I at length observed that 

 there was a position in which the two rays changed their re- 

 lative places rapidly on any slight change of incidence, and 

 appeared at times to revolve round one another, as the inci- 

 dence was altered. Being convinced that the ray was now 

 near the critical incidence, I changed the position of the cry- 

 stal, with respect to the incident ray, very slowly ; and after 

 much care in the adjustment, I at last saw the two rays spread 

 into a continuous circle, whose diameter was apparently equal 

 to their former interval. 



" This phaenomenon was exceedingly striking. It looked like 

 a small ring of gold viewed upon a dark ground ; and the 

 sudden and almost magical change of the appearance from 

 two luminous points to a perfect luminous ring, contributed 

 not a little to enhance the interest. 



" The emergent light, in this experiment, being too faint to 

 be reflected from a screen, I repeated the experiment with 

 the sun's light, and received the emergent cylinder upon a 

 small piece of silver paper. I could detect no sensible dif- 

 ference in the njagnitude of the circular sections at different 

 distances from the crystal. 



" VVhen the adjustment was perfect, the light of the entire 

 annulus was white, and of equal intensity throughout. But 

 when there was a very slight deviation from the exact position, 

 two opposite quadrants of the circle appeared more faint than 

 the other two, and the two pairs were of complementary co- 

 lours. The light of the circle was polarized, according to 

 the law which 1 had belbrc observed in the other case of 



