Study of the Double Sclcnates of the Series EoM^SeOJo.BILO. 63 



line. The first median line is consequently situated in the obtuse 

 angle of the morphological axes ac, and is inclined 5 3' to the axis a. 

 The second median line lies also in the obtuse angle af, and is inclined 

 9 9' to the vertical axis c. 



Refractive Indices. The results of the determinations with six prisms, 

 ground on six different crystals, are given in the accompanying table. 

 The values obtained by Topsoe and Christiansen are appended in the 

 last column. The (3 values, which were alone determined directly by 

 them, are observed to agree well with the authors' values. 



The intermediate refractive index of potassium zinc selenate, cor- 

 rected to a vacuum, is accurately expressed as far as the neighbourhood 

 of F, for any wave-length A, by the following formula : 



B = 1 -5010 + 694 10 - 3 5 70 00 00 + 

 X 2 A 4 



As the dispersion increases with the numerical value of the index, 

 the a and y indices are not precisely reproduced by diminishing and 

 increasing the constant 1-5010 by fixed amounts ; but they are on the 

 average less and greater respectively than the (3 values by 0-0060 and 

 0-0154. 



Alteration of Refraction lij Rise of Temperature. Determinations 

 carried out at 60 indicated that the indices are diminished by about 

 0'0020 for 45 rise of temperature. 



Axes of the Optical Ellipsoid. The calculated values of the axial 

 ' ratios of the two optical ellipsoids are as follows : 



Axes of optical indicatrix : 



a: /3:y = 0'9960 : 1 : 1-0101. 

 Axes of optical velocity ellipsoid : 



a : fa : t = 1-0040 : 1 : 0'9900. 



