of Refraction of the Metals. 13 



for iron, nickel, platinum, and bismuth. There is a consider-, 

 able difference between the absolute value of n as calculated 

 from ray observations and from those of Voigt. This pro- 

 bably also arises from differences in the materials used. 



Voigt has further shown, in a separate treatise*, that 

 according to his theory the refractive index of strongly ab- 

 sorbent media can be correctly determined in the ordinary 

 way by prismatic deflection, if only the refracting angle of 



n 2 k 2 

 the prism is sufficiently small. The quantity — — sin 2 S, in 



which n denotes the index of refraction, k the coefficient of 

 absorption, and 8 the angle of the prism, may be neglected. 

 This is always the case with our prisms ; and thus, according 

 to Voigt, also our assumption made above is justified that the 

 values obtained for n are the reciprocals of the velocity of 

 light. 



It is not possible to enter upon a discussion of the measure- 

 ments made by Herr Dessau | of the dark rings seen in coni- 

 cal metallic layers, with the values of refractive index here 

 found, since he was not able to measure the rings with normal 

 incidence. I will only remark that the anomalous dispersion 

 in platinum, iron, and nickel does not necessarily require 

 an anomalous order of the rings in the red and blue, since, 

 according to the above values for n in the metals named, the 

 wave-lengths for red always remain greater than for blue. 



Indices of Refraction of Oxides. 



In the course of this investigation opportunity occurred of 

 determining the indices of refraction of some compounds of 

 the metals — and in particular of the oxides — chiefly in order 

 to check the method of observation in different ways. Since 

 for many of the substances examined the index of refraction 

 has not hitherto been determined, I give some of the num- 

 bers obtained. I add some determinations on metals which 

 were impure by reason of the presence of oxides. 



* Wied. Ann. xxiv. pp. 144-156. 

 t Wied. Ann. xxix. pp. 353-370. 



