Study of the Double Scknates of the Series E 2 M(Se0. 1 ) i ,,6H.,0. 69 



ground parallel to the symmetry plane, with reference to the normal 

 to the basal plane : 



Section 1 2 0' 



Section 2 .. 27 



Mean 2 3 



The direction is behind the normal, towards the vertical morpho- 

 logical axis. This axis of the optical indicatrix is again the second 

 median line. 



The first median line thus lies in the obtuse morphological axial 

 angle ac, and is inclined 2 3' to the axis a. The second median line 

 lies also in the obtuse angle ac, and is inclined 13 13' to the vertical 

 axis c. 



Refractive Indices. Six prisms, ground on six different crystals 

 belonging to various crops, were employed in the determinations. The 

 results are given in the accompanying table : 



Kefractive Indices of Eubidium Zinc Selenate. 



The intermediate refractive index of rubidium zinc selenate, cor- 

 rected to a vacuum, is expressed as far as F, for any wave-length X, 

 by the formula : 



B = 1-5067 + 592 314 - 1 397 60 00 000 + 

 The a indices are very closely reproduced by the formula if the con- 



