Hatfs, in Crystals xmlh cme and two Axes of Double Refract ion. 1 45 



These ratios differ so little from each other, that one would 

 be led to regard the differences as only errors of observation. 

 They, hovvever, appear to increase a little from the violet to 

 the red, and consequently do not contradict the result ob- 

 tained for Iceland spar, rock crystal and Arragonite. 



The inclinations of the optical axes calculated by the formula 



n''- — n"'- 

 sin- T a = -73 F7?r are as follows : 



n'- — n"' 



Inclination of the Optical Axes. 



H . . 5A>°5^' 0" 



G . . 55 34 24 



F . . 56 37 24 



E . . 56 40 30 



D . . 56 37 30 



C . . 56 3 



B . . 55 51 58 

 Abstracting the irregularities in these values towards the red 

 extremity of the spectrum, it appears that the inclination of 

 the optical axes goes on diminishing 'with the refrangihility of 

 the rays, whilst the contrary takes place in Arragonite. 



With regard to the value of the inclination, Dr. Brewster 

 has found it = 65°, and M. Biot = 64° 14'. This difference 

 of more than 8°, appears to indicate errors in the determina- 

 tion of the indices, unless the inclination in different speci- 

 mens of colourless topaz is different, as Dr. Brewster found it 

 to be for diffei'ent kinds of Brazil topaz. It is to be observed, 

 that all the prisms with which I made the preceding observa- 

 tions, came from the same topaz. Having after this only thin 

 plates, I could not, on account of the great extent of the ellip- 

 tical rings, measure the inclination of the axes with precision. 



Taking for topaz as for Arragonite the elasticity along the 

 axis A as unity, the following will be the elasticities along the 

 other axes. A. B. C. 



1 1-01186 1-00922 



In his memoir on Double Refraction (Mem. de VInstituty 

 torn, vii,) Fresnel has given from his experiments on diffrac- 

 tion made with colourless topaz, the ratio between the least 

 and greatest velocity. He found it 0*9938. My experiments 



n'" 1 



make the mean result — — = :r^r-'- = 0-99412, which ex- 

 n' 1-00591 ' 



ceeds the former by 0-0003. Assuming the ratio 0-9938, and 



ii" 

 the inclination of the optical axes = 65°, the ratio — j- may be 



fou»j by the ecjuation, 



Thiy-d Series. Vol. 1. No. 2. /lag. 1832. U 



