1877.] 



seen through a Crystalline Plate. 



393 



"When they are the same, the plane of the plate may be either of the prin- 

 cipal planes containing that common direction ; and, indeed, it might be 

 any plane containing it, only that the case of a plate cut obliquely is not 

 at present under consideration. The apparent indices obtained by 

 focusing will accordingly be arranged as follows : — 







Direction of line brought 

 to focus. 







- 



y- 



z. 



Direction of normal to 

 plane of polarization. 



X. 





tl 





y- 



il 







z. 



'2 



A" 



< 





It may be well to give the numerical results for aragonite and topaz, 

 as calculated from Eudberg's indices. I have chosen the same fixed lines 

 as before. 





Fixed 



Aragonite. 



Topaz. 





lines. 



*• 



y> 





X. 



y- 



z. 





C. 



1-5282 



1-8513 



1-8420 



1-6093 



1-6284 



1-6135 





D. 



1-5301 



1-8576 



1-8481 



1-6116 



1-6296 



1-6148 





E. 



1-5326 



1-8653 



1-8554 



1-6145 



1-6338 



1-6189 





C. 



1-6862 



1-6778 



1-3919 



1-6262 



1-6114 



1-6072 



'i 



D. 



1-6902 



1-6816 



13922 



1-6285 



1-6137 



1-6115 





E. 



1-6953 



1-6863 



1-3929 



1-6315 



1-6167 



1-6123 





C. 



1-6736 



1-3885 



1-6820 



1-6040 



1-5999 



1-6188 



■■{ 



D. 



1-6773 



1-3887 



1-6859 



1-6063 



1-6041 



1-6211 





E. 



1-6818 



1-3892 



1 6908 j 



1-6093 



1-6056 



1-6241 



The numbers in the squares xx, yy, zz are, of course, the real principal 

 indices. 



