496 



L. P. Wheeler — Dispersion of Metals. 



Fig. 3. 



the ultra-violet at about 0'3yu, and the other in the visible 

 spectrum at about 0*55/x. In the cases of nickel and cobalt 

 the minima in the absorption curves (if they exist) have not 

 been reached within the limits of the spectrum over which 

 these observations extend ; both of these metals show a more 

 or less steady decrease in n/c throughout the whole range, 

 though in the case of cobalt a minimum seems to be indicated 

 not far beyond the range of the observations in the ultra-violet. 

 It is to be noted further that the reflection 

 coefficient (not shown in tables or figures) 

 show T s a minimum value at the same points in 

 the spectrum as does n/c. 



The indices of refraction also show marked 

 minima in the cases of the three better con- 

 ductors. That for silver occurs at 0*395//. ; 

 that for copper at about 0*62/* (Ingersolls 

 results would place it beyond 0'7/a) ; and that 

 for gold is apparently located just beyond the 

 longest wave length for which we have meas- 

 urements (0'7 fi). For nickel and cobalt there 

 are no such marked minima ; the curves show 

 an index decreasing tolerably uniformly with 

 increasing frequency throughout the range 

 over which the experiments extend. A mini- 

 mum in the case of cobalt seems imminent, 

 however, just beyond that range. For a large 

 range of the spectrum the numerical value of 

 the index is, in the case of the three better 

 conductors, less than unity ; while for the 

 other two it is approaching such a value in 

 the ultra-violet. The fact that the index of 

 refraction is less than unity for silver and 

 gold and copper in the visible spectrum has 

 been verified, it may be remarked, by the 

 direct measurement of the deviation produced by prisms of very 

 acute angle. 



The theoretical relations which it is proposed to discuss are 

 those yielded by the well-known equation of condition for the 

 propagation of waves in a medium possessing both conductiv- 

 ity and a dielectric constant 



K 



C 2 



where E is the electric force, a the conductivity, K the dielec- 

 tric constant, C the velocity of light (which enters owing to 

 the use of electro-magnetic units), and V-V is the Laplacian 



7 

 6 

 5 

 4 

 3 

 2 





v 











\\ 







60 





V 









\ 



\ 





40 



30 



zo 



IO 



H 



t 







V 









V 



X 











t 

















/ 



3.5 

 30 



125 





o\ 



It*-// 



1/ 













0.75 





J 







i PI 











10 



0.5 



c 

 t 













c 



r 











o.^u. o.fyn. 0.7** 



Fig. 3. Gold. 



