492 



L. P. Wheeler — Dispersion of Metals. 



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 n/c are in general 

 parallel though not coincident; and also that the departures 

 from parallelism are in the main much less than the dis- 



FlG. 1. 





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Fig. 1. Silver. 



50yO, 



7S/U, 



crepancies in the absolute values. In copper alone is there any 

 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 farther 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 of the 

 reflecting surface. These arise from small differences 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 to be open to objec- 

 tion. See Voigt, Ann. Phys., xxix, p. 956, 1909. 



