Study of the Double Selenatcs of the Series RoM(Se0 4 );>,6H 3 0. 77 

 Determination of True Optic Axial Angle of Caesium Zinc Selenate. 



Dispersion of tlie Median Lines. When the true angle is observed in 

 methyl salicylate in white light, the brushes are seen to be highly 

 coloured in accordance with the following scheme : 



blue | red 



1st M.L. 



red | blue. 



The angle is thus indubitably the larger for red. If the section is 

 arranged so that the obtuse angle of the morphological axes ac is 

 situated to the left, the optic axis to the right is found to be less dis- 

 persed between C and Tl light by about 10' than the left-hand axis. 

 Hence the median lines are dispersed by about 5' between the same 

 wave-lengths, and so that the first median line lies nearer to the mor- 

 phological axis a for red than for green. 



Effect of Pdse of Temperature on the Optic Axial Angle. Repeated 

 measurements of 2E with the best of the sections at 60, indicated that 

 the angle in air decreases about 3 for 50 rise of temperature. 



