66 Mi. A. !!. Ttitton. A O/////>"/ "///v CryttaUographieal 

 Determination of True Optic Axial Angle of Potassium Zinc Selenate. 



Dispersion of the Median Lines. In cedar oil no appreciable move- 

 ment of the right brush, that corresponding to the optic axis situated 

 in the acute morphological angle ac, was detected on altering the 

 wave-length of the light, the total movement occurring at the left 

 brush. Hence the first median line lies nearer to the morphological 

 axis a for red than for blue. The amount does not exceed 5'. 



Effect of Rise of Temperature on the Optic Axial Angle. Measurements 

 at 65 indicated that 2E increases 2 for 50 rise of temperature. 



RUBIDIUM ZINC SELENATE, Rb 2 Zn(Se0 4 )2,6H 2 0. 



An estimation of zinc in a specimen of the crystals employed 

 afforded the following guarantee of purity: 1-1130 grammes yielded 

 0'1426 gramme ZnO, which corresponds to 10'28 percent, of zinc. Cal- 

 culated Zn = 10-33. 



Goniometry. 



Ten suitable small crystals were employed, belonging to four differ- 

 ent crops. 



Habit : thick tabular, sometimes prismatic. 



