12 



Prof. A. Kundt on the Indices 



Comparison of these Results with those calculated from the 

 Indices of Refraction calcxdated by Beer and Voigt. 



In the following table I have brought together the mean 

 of the measurements given above: — 



Comparison of Mean Values of 





Eed. 



White. 



Blue. 



Silver , 



: 38 

 0-45 

 1-76 

 1-81 

 2-17 

 2-61 



0-27 

 0-58 

 0-65 

 1-64 

 1-73 

 201 

 2-26 



1-00 

 0-95 

 1-44 

 1-52 

 1-85 

 2-13 



Gold 









Nickel 



Bismuth 





The velocity of light in silver is nearly four times as great 

 as in vacuo, but the dispersion in silver is not very great. 

 Also in gold and copper the velocity is greater than in vacuo 

 but the dispersion normal; in all the other metals examined 

 the dispersion is greatly abnormal. The values calculated by 

 Beer and Voigt agree with these essentially, one may say in 

 a surprising manner. 



Beer has calculated the indices of refraction, i. e. the ratio 

 of velocities in metal and in vacuo according to Cauchy's 

 theory from Jamin's observations on reflexion. He finds 

 for silver no marked dispersion and a mean refractive index 

 0'25. Copper gave strong normal dispersion, and for the 

 red rays n<l; iron gave anomalous dispersion, 7i re d = 2 , 54, 

 ^violet =1*47*. 



Voigt has elsewhere calculated the value of n according 

 to his theory of the optical properties of the metals, from the 

 observations upon reflexion made by Jamin, JEaughton, and 

 Quincke. 



The values of n obtained for one and the same metal are 

 somewhat different; the reason of this may be that the dif- 

 ferent observers have not used for their experiments metals 

 of equal chemical purity and similar physical condition. 



The values calculated are, however, less than unity for 

 silver and gold and for the red rays in copper. So also Voigt 

 finds normal dispersion for copper, but anomalous dispersion 



* Pogg. Ann. xcii. p. 417, 1854. 



