152 



investigation of the mineral. It is consequently only the com- 

 mon columnar forms that are available for this purpose. These 

 are, however, in most cases too opaque. The opacity is chiefly 

 owing to the fact that the mineral is split into hair-like fibres, 

 so that sections oriented parallel to the longitudinal axis present 

 quite an asbestus-like appearance. Only such columns whose 

 colour shows a tinge of yellow are tolerably free from cracks 

 and translucent. Very thin sections of such individuals are 

 perfectly transparent. 



Sections oriented parallel to the two vertical pinacoids show 

 in polarized Ught extinction parallel to the с -axis, and in the 

 same direction they also show, as Nordenskiöld states, the 

 greatest velocity of light. Tn a section parallel to the first 

 pinacoid (100) one observes under the conoscope an axial image 

 whose axes, however, are not visible in the field of the micro- 

 scope. The axial plane is parallel to the second pinacoid (010), 

 Owing to the thinness of the section only the trace of a 

 lemniscate is observable in the centre of the field of vision. 

 It has not been possible to produce a translucent section 

 parallel to the third pinacoid; I have not, therefore, been able 

 to measure the angle of the optic axes. 



From the aforesaid yellowish columns prisms were pre- 

 pared for the measurement of the refractive indices of the 

 mineral. One of the prisms is bounded by a face cut parallel 

 to the first pinacoid, and its refracting edge is parallel to the 

 ô-axis. In this prism by Ught falling perpendicularly against 

 the aforesaid face the indices a, parallel to the c-axis, and ß^ 

 parallel to the ô-axis , were determined. The other prism is 

 bounded by a cut face parallel to the second pinacoid, and its 

 refracting edge is parallel to the a-axis. In this prism the index y, 

 parallel to the a-axis, was determined and, besides, a once 

 more. The prisms are not, it is true, translucent except on 

 the extreme thin edge; however, 1 succeeded in making the 

 determinations with tolerable accuracy. 



