191 



made in measuring angles on the transverse section under the 

 microscope, but without much success; in the few cases, where 

 it could be positively determined, that it was a single individual 

 that appeared , the edges were as a rule more or less rounded 

 off or in other ways damaged. 



Among the numerous measured values for edge angles 

 there are especially two, which appear frequently. The average 

 values are 28° 11' and 30° 2'. On the transverse sections 

 edge angles were frequently measured, whose average value 

 was the sum of the above mentioned angles, = 58° 13'. 

 Attention was then directed to the possibility of discovering 

 a case , where the two above mentioned angles followed each 

 other in a regular order and where there at the same time 

 was an angle of 90° between the two faces (rhombic pinacoids). 

 Such a case was found, as will be seen from the following 

 statement, which also shows the ordinary variation in the size 

 of the edge angles : 



«1 : 



«2 



= 



25" 58' 



«2 : 



«3 



= 



32° 48' 



«3 : 



a. 



= 



25° 41' 



«4 : 



«5 



= 



28° 57' 



«5 : 



a, 



= 



34° 21' 



«6 : 



a 7 



= 



31° 26' 



a, : 



«8 



= 



31° 10' 



»s : 



«9 



= 



31°31' 



«9 : 



«10 



= 



31° 43' 



«10 : 



«11 



= 



30° 7' 



«ц: 



«12 



= 



28° 5' 



«12: 



«1 



= 



28° 14' 



89° 55' 



On the transverse sections, as above mentioned, the angle 

 58° 13', corresponding to a■^^,■.a^^ above, was measured several 

 times and the optic axial plane was found to make a right 

 angle with one of the faces, which included this angle. The 



