69 



that 1 have now undertaken has given such results as, with 

 the exception of the calculated axial angles, may be regarded 

 as fairly accurate. 



In sections cut parallel to the three pinacoids the extinc- 

 tion is parallel to the crystallographical axes. A section 

 parallel to the second pinacoid, Ь = {OlO}, shows, though 

 only if it is very thick, in converging polarized light an 

 axial image with a small angle between the optic axes. The 

 orientation of the image shows that the axial plane is parallel 

 to the third pinacoid. To prepare a continuous section 

 parallel to the first pinacoid, a = {ЮО} proved impossible 

 on account of the cleavages of the mineral being at right 

 angles to this direction. However, in the fragments obtained 

 in the attempts to grind, one can perceive that also here the 

 extinction is parallel to the c- and a-axes. In a section parallel 

 to the base it can be ascertained that the direction of the 

 greatest velocity of light is parallel to the ô-axis. Thus, 

 epididymite is optically positive. 



The indices of refraction were determined by means of two 

 prisms. One of these is bounded by a basic cleavage, plane 

 and of a ground and polished plane in the zone [OlOj, forming 

 an angle of 37° 55' with the third pinacoid. The refracting 

 edge is, consequently, parallel to the è-axis. The indices a 

 (the ray vibrating parallel to the a-axis) and y (the ray vibrating 

 parallel to the ô-axis) were here determined by light falling 

 perpendicular to the basal plane. The other prism has its 

 refracting edge parallel to the c-axis and is bounded by the 

 second pinacoid and a plane in the zone [OOl], both were ground 

 and polished and formed with each other an angle of 37° 22'. 

 The indices a and ß were here determined by light falling 

 perpendicular to the first-mentioned prism face. They, how- 

 ever, lie so near to each other, that it presents the greatest 

 difficulty to separate and fix them. The divergence obtained 

 with the nicol in diff'erent positions in the collimator tube is 



