Study of the Double Selenates of the Series ll 2 M(Se0 4 ) 2 ,6H 2 0. 71 

 Determination of True Optic Axial Angle of Rubidium Zinc Selenate. 



The dispersion is thus seen to be small, but it is distinctly greater 

 than in the case of the potassium salt. In cedar oil, whose refraction 

 is almost exactly the same as that of this salt, a section perpendicular 

 to the first median line exhibits the brushes fringed with colour as 

 follows : 



blue | red 1st M.L. red | blue 



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

 confirming the results of the measurements. 



Dispersion of the Median Lines. In the foregoing representation of 

 the coloured brush-fringes as seen in cedar oil, the obtuse angle of the 

 morphological axes ac is situated to the left. The hyperbolic brush on 

 this side was more faintly tinted at the edges than the right-hand 

 brush, indicating feebler dispersion of the optic axis to the left. 

 Measurements in cedar oil confirmed this, and showed that the differ- 

 ence between the position of this optic axis for C-light and for F-light 

 was 8 10', whilst for the right-hand axis it was 20'. Hence the 

 median lines are dispersed so that the first median line lies nearer to 

 the morphological axis a by 4 5' for red light than it does for blue. 



Effect of Rise of Temperature on the Optic Axial Angle. Very little 

 change is introduced in the optic axial angle of this salt by variation 



