508 L. P. Wheeler — Dispersion of Metals. 



of free electrons. With this added information we should get 

 some light on the magnitude of the part played in the absorp- 

 tion by metals, by the relatively fixed ions ; information which 

 could not fail to be of use in unravelling the mechanism of 

 that process. 



Summary. 



1. From an examination of the available data on the disper- 

 sion of silver, copper, gold, nickel, and cobalt, it is concluded 

 that the inaccuracies in the experimentally determined values 

 of the optical constants of these metals are too great to allow 

 of such dependence to be placed on them except for relative 

 values. These uncertainties limit seriously the information to 

 be derived from the measurements, and suggests the desirabil- 

 ity of obtaining data on the dispersion of mercury, for which 

 metal the causes of the inaccuracies can be eliminated. 



2. From a comparison of the data with the dispersion 

 formulae yielded by the electron theory, it appears 



(a) That the number of free electrons is not a constant but 

 increases with the frequency of the currents set up by the inci- 

 dent radiation ; slowly and uniformly in the infra-red, and 

 more rapidly in the regions where the metals are more trans- 

 parent. The explanation of this phenomenon is so involved 

 with that of the mechanism of metallic absorption in general, 

 that a wholly satisfactory elucidation of it seems to be impossi- 

 ble at the present time. 



(h) That the dispersion of the so-called dielectric constant 

 for these metals can be determined. It is seen to consist of 

 two parts, of which that which depends on the presence of free 

 electrons is more important than the other or pure dielectric 

 term, especially at the shorter wave lengths. It also appears 

 that the form of the dispersion curves found is not inconsistent 

 with that demanded by theory, although the inaccuracies of the 

 data do not permit a satisfactory investigation of this point. 



