662 Prof. Lynde P. Wheeler on the 



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 

 m 23ii to 2' 2 5 Jul ; in the case of nickel from 0*42^ to 2'2o/uu ; 

 and for gold from 04//, to O'l/x. The values of the indices of 

 refraction (n) and of the coefficients of absorption (n/c) 

 obtained by these observers for the metals named (with the 

 exception of steel) , are brought together in the second and 

 third columns of Tables I.-V., and are shown graphically in 

 the two lower cuiwes of the corresponding figs. 1-5. The 

 observations at all of the wave-lengths at which measure- 

 ments 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 differ in general by an amount considerably in 

 excess of the errors inherent in the methods of measurement 

 employed; and, second, that in an equally general manner 

 the different observers are in very close agreement as to the 

 amount and trend of the dispersion. Thus it will be observed 

 that for those regions of the spectrum where the results of 

 the different observers overlap, the curves for both n and ?i/c 

 are in general parallel though not coincident ; and also that 

 the departures from parallelism are in the main much less 

 than the discrepancies in the absolute values. In copper 

 alone is there anv real disagreement as to the trend of the 

 dispersion. Here the measures of Tool and Tate would 

 locate the minimum in the curve for n further up in the 

 spectrum than do those of Ingersoll *. 



The discrepancies in the values obtained at any given 

 wave-length are to be attributed to differences in the state 

 ot the reflectino- surface. These arise from small differences 

 in the method or duration of polishing, or possibly (as is 

 suggested by the work of Lummer and Sorge f on glass) 

 they may arise from different states of strain existing in the 



* Bernoulli's values for nickel also form an exception to the general 

 parallelism of the dispersion curves. Too much stress cannot be laid on 

 his measures, however, as his experimental method seems open to 

 objection. See Voigt, Ann. d. Phys. xxix. p. 956 (1909). 



t Lummer and Sorge, Ann. d. Phys. xxxi. p. 325 (1910). 



