L.P. Wheeler — Dispersion of Metals. 491 



Art. XLII. — The Dispersion of Metals ; by Lynde P. 

 Wheeler, Asst. Prof, of Physics, Sheffield Scientific School 

 of Yale University. 



The increase in our experimental knowledge of the disper- 

 sion of metals in the past ten years invites a more complete 

 discussion than it thus far seems to have called forth of those 

 relations between the optical and other physical constants of 

 metals which theory demands. The most detailed investigation 

 of these relations which has been made up to the present time 

 appears to be that of Nicholson.* In his paper he uses the 

 equations of the theory to calculate (in the manner first pointed 

 out by Schuster) the ratio of the number of free electrons to 

 the number of molecules in unit volume ; and also to the cal- 

 culation of the values of the so-called dielectric constants of the 

 metals. He discusses, however, only the classical measure- 

 ments of Drude, and hence the consideration of dispersion was 

 practically excluded. In fact nowhere, to the author's knowl- 

 edge, has there appeared an adequate discussion of the more 

 modern measurements of Minor, f Bernoulli, J Tool,§ IngersollJ 

 and Tate,*[ in the light of the demands of the electron theory. 



As a result of the work of these investigators we possess 

 measures of the optical constants at wave lengths extending in 

 the cases of silver, copper, cobalt and steel from about *23/x to 

 2*25/1- ; in the case of nickel from 0'42/a to 2'25/x ; and for gold 

 from Q'4:/jl to 0'7//>. The values of the indices of refraction 

 (n) and of the coefficients of absorption (iik) obtained by 

 these observers for the metals named (with the exception of 

 steel) are brought together in the third and fourth columns of 

 Tables I-V, and are shown graphically in the two lower curves 

 of the corresponding figures I-Y. The observations at all of 

 the wave lengths at which measurements were made are not 

 included, but none are omitted except where the trend of the 

 curves is so uniform that many points are unnecessary. The 

 data for steel are omitted because the chemical and physical 

 complexity of this material is such as to render all the physical 

 constants required for a comparison with theory so uncertain 

 as to make such a study fruitless. 



A study of these tables or curves reveals two salient facts 

 with regard to the precision of the measurements. First, that 

 the results of different observers at the same wave lengths dif- 

 fer in general by an amount considerably in excess of the errors 



* Nicholson, Phil. Mag. (6), xxi, p. 245, 1911. 

 f Minor, Ann. Phys., x, p. 581, 1903. 

 ' \ Bernoulli, Ann. Phys., xxix, p. 585, 1909. 

 gTool, Phys. Eev., xxxi, p. 1, 1910. 

 || Ingersoll, Astroph. Jour., xxxii, p. 265, 1910. 

 «TTate, Phys. Rev., xxxiii, p. 321, 1912. 



