Study of the Double Seienates of the Series R 3 M(SeO t ) 2 ,6H 3 0. 65 



Molecular Optical Constants. Following are the values of these con- 

 stants : 



Optic Axial Angle. Three excellent pairs of section-plates were 

 ground, perpendicular to the first and second median lines respectively. 



Determination of Apparent Angle in Air of Potassium Zinc Selenate. 





Topsoe and Christiansen give for the angle in air 111 50', and for 

 the true angle 66 8', both being measured in sodium light. 



The dispersion is observed to be extremely small, and the most 

 accurate measurement is required to determine it, employing sections 

 which afford very small rings and sharp brushes. A valuable confirma- 

 tion of the nature of the dispersion is afforded by immersion in cedar 

 oil, whose refraction is almost exactly the same as that of the crystals. 

 A section perpendicular to the first median line shows, in cedar oil, 

 brushes separated at their true angle and frinjed with colour accord- 

 ing to the following scheme : 



red | blue 1st M.L. no colour | no colour. 



The obtuse morphological axial angle ac is assumed to be to the left. 

 The optic axial angle is therefore greater for blue than for red, and 

 measurements in Li and F light gave an angle larger in the latter case 

 by about 5', thus confirming the accuracy of the values given in the 

 table. 



VOL. LXVII. F 



