378 On Refraction and Dispersion in certain Metals. 



length in air (fig. 4). These curves of anomalous dispersion 

 are seen to have an analogous shape for the three metals. The 

 values for " red " light lie well between those for D and Lia, 

 considering its imperfect homogeneity. 



There appears to be no simple relation between this dis- 

 persion of refraction and that of either Kundt's or Kerr's 

 phenomenon (compare § 2). 



III. Conclusion. 



§ 20. We have from the beginning guarded ourselves 

 against basing in any way upon any of the various elastic or 

 electric optical theories proposed (§ 6). In the present 

 paper we have attempted to restrict ourselves to the com- 

 munication of facts which may be simply inferred from our 

 experiments. We believe that the realm of fact in the optics 

 of metals is as yet too little extended to serve as a successful 

 criterion for the theories ; any slight extension of it, to which 

 we may have contributed, hardly changes this state of affairs. 



A valuable contribution towards the experimental investiga- 

 tion of this part of Optics has been lately published by Dr. 

 Drude ; he determined the two ordinary constants of reflexion 

 For a number of metals with the utmost care. By means 

 of the particular elastic theory advocated by him, he then 

 calculated indices of refraction and of extinction and re- 

 flecting powers which he compared with experimental data 

 by Kundt (A, p. 266), Wernicke*, and one of usf. Of the 

 refractive indices obtained in this way four| are directly 

 comparable with ours, and even show a certain amount of 

 coincidence. 



In answer to our inquiry Dr. Drude w r as good enough to 

 promise measurements on cobalt also, eventually with regard 

 to dispersion. We therefore think it advisable to wait 

 for those further determinations, as these are sure to afford 

 Dr. Drude a safer base for judging any coincidence or dis- 

 crepancy than the data at present to hand can possibly do. 



Physic. Inst. Univ, Berlin, July 20, 1890. 



* Wernicke, Pogg. Ann. Erg. viii. p. 75 (1878). 

 t Rubens, Wied. Ann. xxxvii. p. 267 (1889). 



X Drude, Wied. Ann. xxxix. p. 481 (1890). The values meant are w' 

 (red) and n (sodium light) for nickel and steel in his table on p. 537. 



