CONICAL REFRACTION. 17 



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 phenomenon 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 difference in the mag- 

 nitude of the circular sections at different distances from the 

 crystal. 



When the adjustment was perfect, the light of the entire an- 

 nulus 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 colours.* The light 

 of the circle was polarized, according to the law which I had be- 

 fore observed in the other case of conical refraction. In this in- 

 stance, however, the law was anticipated from theory by Professor 

 Hamilton. 



I measured the angle of incidence by a method similar to that 

 already employed for the emergent ray in the former case ; and 

 found it to be 15 40'. This determination is, for many reasons, 

 capable of much greater accuracy than the other ; and was probably, 

 in this instance, very near the truth. 



In order to compare it with the result of theory, it is to be 

 observed that the optic axis is a normal to the wave-surface, and, 

 therefore, the corresponding incident ray will be given by the 

 ordinary law of the sines, the index of refraction being the mean 

 jin/c.r of the crystal. Now, the angle which the normal to the cir- 

 cular section of the surface of elasticity, or the optic axis, makes 

 with the axis of x, or the perpendicular to the surface, is equal to 



tan' 1 / C T ; and its numerical value, in the case of arragonite, 



\fl a -b-i 



* This part of the phenomenon appears to be explained by the non-coincidence of tho 

 optic axes for the rays of different colours. 







