Rays in Crystals "with one a7id iwo Axes of Double Refraction, 5 



With regard to the question of the dispersion of the two 



rays in rock crystal, we find by comparing the ordinary and 



the extraordinary indices for the different colours, that the 



double refraction is greater for the violet light, and less for 



the red light ; or in general, that the double refraction is as 



much stronger, as the individual refrangibility of the ray itself 



is greater; for by calling ?i' the index of the ordinary ray, and 



»" that of the extraordinary ray, we shall have the following 



w" 

 values of —r- for the different colours : 

 «' 



H 1-00613 



G 1-00605 



F 1-00599 



E 1-00594. 



D 1-00589 



C 1-00586 



B 1-00584. 



Whence it follows that the ratio ,- goes on increasing from 



the red to the violet extremity of the spectrum, and conse- 

 quently that the difference of the velocities of the two rays 

 increases for the different colours in the same direction. 



2. Calcareous Spar. — From this crystal I caused to be cut 

 two prisms having their edge parallel to the axis of crystalli- 

 zation. I could use only one of them, the angle of which was 

 66°-577, or 59° 55' 9". 



In the extraordinary spectrum it was the line H which was 

 reduced to a minimum of deviation, and the prism remained 

 in this position during the measurement of the other lines of 

 the spectrum. In the ordinary spectrum, on the contrary, 

 which had an extent three times greater, the violet light was 

 very feeble, and the line H very wide ; and I chose the Hne F, 

 which was reduced to the least deviation. Since the prism 

 remained in this position, it is evident that in the value of the 

 index «', we must make 8 negative for the two lines H and G, 

 but positive for the lines E, D, C, and B. The mean indices 

 are given in the following table. The temperature was+17°| 

 centigrade. 



