66± Prof. Lynde P. Wheeler on tlie 



and, with less certainty, copper, there is a minimum in the 

 carve followed by a feeble maximum further up in the 

 spectrum. Thus, for silver there is a distinct " transmis- 

 sion " band in the ultra-violet with its centre at about 

 0*316/4 ; for gold, a less distinct one in the visible spectrum 

 central at about 0*480/4 ; while for copper there are appar- 

 ently two minima of absorption, one in the ultra-violet at 

 about 0*3/a, and the other in the visible spectrum at about 

 0*5 >/4. 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 observa- 

 tions extend ; both of these metals show a more or less 

 steady decrease in nte 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, furthe?*, that the reflexion coefficient (not 

 shown in tables or figures) shows a minimum value at the 

 same points in the spectrum as does rue. 



The indices of refraction also show marked minima in the 

 cases of the three better conductors. That for silver occurs 

 at 0*395/4 ; that for copper at about 0*62/4 (Ingersoll's 

 results would place it beyond 0*7/t) ; and that for gold is 

 apparently located just beyond the longest wave-length for 

 which we have measurements (0*7/4). 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 minimum in the case of cobalt seems imminent, however, 

 jnst 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 

 conductivity and a dielectric constant, 



^ K .. 



V * VE = Q 2 E + 47rcrE, 



where E is the electric force, a the conductivit}', K the 

 dielectric constant, C the velocity of light (which enters 

 owing to the use of electromagnetic units), and V ' V M 



