120 Prof. S. P. Thompson on the 



are respectively at right angles to one another, and which 

 move with different velocities. 



Now assuming, as above, /*i=/*2=l 3 we have 



where /? 2 , p x are respectively the indices of refraction for ordi- 

 nary and extraordinary rays. 



Billet gives (Optique Physique, ip. 619) the following values 

 for the refractive indices of different specimens of tourma- 

 line: — 



(?) 



Colour of 

 ray. 



.Ray used. 



Ordinary 

 index (o 2 ). 



Extraordinary 

 index (p x ). 



Pi. 



Coloui'less ... 

 Green 



RayD 



EayD 



Ked ray 



Eed ray 



1-6366 

 1-6408 

 1-6415 

 1-6435 



1-6193 

 1-6203 

 1-623 

 1-6222 



1-0107 

 101265 

 101311 

 101313 



Green -blue. . . 

 Blue 





Mean 





1-6406 



1-6212 



1-0120 







14. From these we deduce for the two velocities of light in 

 tourmaline : — 



Ordinary Hay. 



TT 3x 10 10 cm. per sec. i oaon 1Aln 



V 2 = = „, *, =1-8286 x 10 10 cm. per sec; 



1*6406 r 



and 



Extraordinary Ray. 



T , 3x 10 10 cm. per sec. . , A , irvm 



V, = ., na ^^ = 1-8504 x 10 10 cm. per sec. 



1*6212 r 



15. The two corresponding dielectric capacities should 

 therefore be 



Ki = /3j = 2*6283 in direction parallel to crystallograpbic 



axis, 

 K 2 =/^= 2*6792 in direction perpendicular to crystallo- 



graphic axis. 



I am not aware of any experimental determination of these 

 quantities having been made. 



16. Case ii. {Conducting Medium). 



Here ^ = 0, and we may neglect the terms containing K ; 

 for, though K>1, C is by hypothesis much greater, and the 

 terms containing C are therefore those by which the mode of 



