2 Prof. A. Kundt on the Indices 



true for metals. But the term u index of refraction " is to 

 be used here also to denote the ratio of the velocity of light 

 in vacuo to its velocity in the metal. The values calculated 

 by Beer and Yoigt agree in general. Both find the indices 

 for gold and silver to be less than unity, for copper about equal 

 to unity, and for the other metals values greater than unity. 



A continued occupation with the optical properties of very 

 thin metallic films induced me to investigate whether it would 

 not be possible to prepare metallic prisms of very small angle 

 and sufficiently transparent to permit of determining the 

 prismatic deviation produced, and so arrive at a knowledge of 

 the velocity of light in the most direct way possible. 



I have, in fact, succeeded in preparing such metallic prisms, 

 and have determined for seven metals not only the mean refrac- 

 tive index, but for six of them the direction and approximate 

 value of the dispersion. I believe that measurements with 

 better optical instruments than those at my disposal, and with 

 the necessary perseverance, might be carried out with much 

 greater accuracy than I have yet attained. If, nevertheless, 

 I conclude my experiments in the present preliminary stage, 

 my excuse may be found in the fact that the work has already 

 occupied two years, that the small number of usable prisms 

 as recounted in the following pages had to be chosen out of 

 more than 2000 made, and that, lastly, at least 2000 attempts 

 were made before I learnt how to prepare the platinized glass 

 upon which the prisms were electrolytically deposited in such 

 perfection as was necessary for the purpose. 



In the following pages I give, first of all, a short description 

 of the method of preparing the prisms, then the method of 

 observation and the results, together with a short discussion 

 of them. Then follow some determinations of the index of 

 refraction of metallic oxides; and, lastly, a comparison of my 

 results with the values calculated by Beer and Yoigt. In 

 conclusion, some observations to which my results naturally 

 invite. It appears that the velocity of light in the metals 

 stands in close relationship to their power of conducting elec- 

 tricity and heat ; for in respect to the velocity of light the 

 metals range themselves in the same order as in respect of 

 their conductivity for electricity and heat. In order to es- 

 tablish a reliable and final relationship neither the extent nor 

 the accuracy of my observations is sufficient. If, notwith- 

 standing, I venture to draw wide-reaching conclusions from 

 this general relationship, my object is chiefly to open the way 

 to further investigations. 



Preparation of the Prisms. 

 Most of the prisms were electrolytically deposited upon 



