in passing along the Axes of Biaxal Crystals. 209 



The light first employed, was that of a lamp placed at some 

 distance ; and in order to procure an incident ray as minute 

 as possible, this light was made to pass through two small 

 apertures ; one of which was in a screen placed near the flame, 

 and the other perforated in a thin plate of metal close to the 

 first surface of the crystal. Observing the two rays into which 

 the incident ray is generally divided, I turned the crystal 

 slowly, so as to alter the incidence very gradually. After 

 some trials, in which I was partly guided by the changes in 

 the relative position of these rays, I at length succeeded in 

 obtaining an incidence at which the two rays were seen to 

 spread into a continuous circle ; the diameter of which was ap- 

 parently equal to the interval between them when near the 

 ultimate position. 



The emergent light in this instance was received directly 

 by the eye, assisted by a lens. On repeating the experiment 

 with the sun's light, I was enabled to receive the emergent 

 cylinder upon a small screen of silver paper, and to see that 

 there was no sensible difference in the magnitude of the sec- 

 tion at different distances from the crystal. 



When the adjustment was perfect, the light of the entire 

 annulus was white, and of equal intensity throughout. But 

 on a very slight deviation from the exact incidence, two op- 

 posite quadrants of the circle appeared more faint than the two 

 others ; and the two pairs were of complementary colours. 



The theoretical incidence is easily calculated. The ray 

 which proceeds within the crystal in the direction of the optic 

 axis being a normal to the wave-surface, the direction of the 

 corresponding incident ray will be given by the ordinary law 

 of the sines, assuming as the refractive index the mean index 

 of the crystal. The angle which the optic axis makes with 

 the axis of x 9 or with the perpendicular to the surface of in-* 



— 1 / c 2 — b 2 

 cidence, is equal to tang / 2 __ 2 ; and its value in the 



case of arragonite is 9° 1', assuming the values of the three 

 indices as determined for the ray E by Professor Rudberg. 

 The corresponding angle of incidence is 15° 19', the refrac- 

 tive index being 1*6863. Now the observed angle of incidence, 

 which was obtained by measuring the angle between the in- 

 cident and reflected rays, was 15° 40'; differing from the com- 

 puted angle by 21'. 



In order to determine the angle of the cone, I measured the 

 diameter of its section made by the second surface of the cry- 

 stal ; and found it to be *016 of an inch. The thickness of the 

 crystal was *49 of an inch, and the inclination of the conical 



Third Series. Vol. 2. No. 9. March 1833. 2 E 



