On a Sensitive Photo-Electric Cell. 679 



Summary. 



1. From an examination of the available data on the 

 dispersion of silver, copper, gold, nickel, and cobalt, it is 

 concluded that the inaccuracies in the experimentally deter- 

 mined values of the optical constants of these metals are too 

 great to allow of much dependence to be placed on them 

 except for relative values. These uncertainties limit seriously 

 the information to be derived from the measurements, and 

 suggest the desirability 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 incident radiation ; slowly and uniformly 

 in the infra-red, and more rapidly in the regions where 

 the metals are more transparent. The explanation of 

 this phenomonon is so involved with that of the 

 mechanism of metallic absorption in general, that a 

 wholly satisfactory elucidation of it seems to be im- 

 possible at the present time. 



(b) 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. 



LXVIII. A Sensitive Photo-Electric Cell. By A. Ll. Hughes, 

 D.Sc, P. A., Mackinnon Student of the Royal Society ; 

 Emmanuel College, Cambridge *. 



DURING the course of a research, it was found necessary 

 to measure the transparency of a quartz plate over a 

 considerable region of the spectrum. To effect this, use was 

 made of the well-established law that the photo-electric effect 

 is proportional to the intensity of the light. A photo-electric 

 cell in which some new features were introduced was found 

 to be extraordinarily sensitive. As photo-electric cells are 

 * Communicated bv Professor Sir J. J. Thomson, O.M., F.R.S. 



3 A2 



