Mr. A. K. Tuttoii. A C<>in/ir"/irc (',//*///</, 



(>j'ti>- A nl .Imili'. The optic axial angle in air 2E is so large as to 

 l>c only measurable with some difficulty, even in the cases of the largest 

 and most perfectly transparent sections. Owing to the relatively small 

 double refraction of all the salts of this series, sections require to have 

 a thickness of at least a millimetre in order to afford small ring* md 

 sharp liru.shes. Hence, if the section is not of considerable, and not 

 always attainable, relative breadth the brushes become obscured. Two 

 of the sections prepared enabled trustworthy measurements to be 

 dlit. lined, but the others, although excellent for 2H, did not exhibit 

 adequately clear brushes in air. 



Apparent Angle in Air of Caesium Zinc Selenate. 

 Light. Section 1. Section 2. Mean 2. 



O / O / 01 



Li 162 50 165 20 164 5 



C 163 12 165 33 164 22 



Na 165 43 166 30 166 6 



Tl 168 21 167 23 167 :.i- 



No measurements were obtainable beyond the green, the angle 

 becoming too large. 



From the results given in the accompanying table for the true angle, 

 it will be observed that the dispersion of the optic axes is considerably 

 greater than in the cases of the potassium and rubidium salts. To 

 confirm its nature, and determine the dispersion of the median lines, 

 a liquid was sought for whose refraction was the same as that of the 

 crystals, and which was without action on them. Pure methyl 

 salicylate fulfils these conditions, and the interference figures afforded 

 by sections perpendicular to the first median line were observed, while 

 immersed in a cell of this liquid. Measurements in C and F light 

 afforded angles almost identical with those calculated from 2Ha and 

 2Ho as measured in monobromonaphthalene, and showing the same 

 amount and order of dispersion. 



Four pairs of section plates were employed in the determinations of 

 the true angle. 



