87 



by Voigt and Hochgesang after the three symmetrical planes 

 of the crystal, as thin as it was possible to make them. The 

 pleochroism of the ilvaite then is seen to be very marked, thus: 



■ф. the a-axis brownish yellow 

 17^ the i-axis brown 

 7^ the c-axis green. 



The absorption is very strong and very different in the 

 three directions; -ф the «-axis it is smallest and not quite so 

 great as in haematite; 7^ the è-axis it is already so strong 

 that the plate is intransparent by common dayhght, and even 

 by direct sun-light shows only a very faint light; гф the c-axis 

 it is still much stronger, and the plate is quite intransparent 

 by direct sun-light with the exception of a few small grooves, 

 where the thickness of the plate is so small that a slight 

 amount of green light could force its way through. 



The examination of the other optical qualities, of course, 

 is very difficult on account of the strong absorption. I have, 

 however, succeeded in substantiating the following optical table : 



a = è 



b == a 



1 



с = с. 



In the section :7^{010} slight traces are found of the optic 

 ones; the axial plane -i- the a-axis. û (= Ъ] is the acute bi- 

 sectrix; accordingly the mineral is optically positive. 



On account of the indistinctness of the axial figure it has 

 been impossible to measure the size of the axial angle. As 

 the rays 9^ the c-axis are almost completely absorbed, the 

 figure must be formed exclusively by the rays ф the a-axis; 

 consequently no difference of phases is found, and sa no rings 

 are formed; only the black cross is distinctly seen; by the 

 turning of the table the hyperbolas are very much drawn out, 

 and become at the same time almost invisible. 



