4 Prof. A. Kundt on the Indices 



is then deposited from the capillary layer of liquid, between 

 the metal and the glass, a wedge of metal upon the glass and 

 lying close to its edge. With silver the prisms were pre- 

 pared, not only electrolytically, but by the chemical method 

 of reduction. Prof. Quincke first showed how to obtain 

 wedge-shaped films of silver from a solution of silver by 

 placing the glass plate to be silvered upon a glass tube of 

 suitable diameter. It was shown later that the films so ob- 

 tained did not always increase steadily in thickness from the 

 centre, but that the thickness increased periodically. From 

 very many wedges prepared according to Quincke's method, 

 it is, however, possible occasionally to find one in which the 

 thickness increases continuously for a short distance, and 

 where the boundary surface is nearly plane. The reflexion of 

 a fine wire cross was again employed to test whether the sur- 

 face was plane. Since, as is well known, no good deposit 

 can be obtained with platinum electrolytically, it was neces- 

 sary to find some other method. It is known that a platinum 

 wire intensely heated by a current passed through it is disinte- 

 grated, and if a glass plate is brought near it the metallic dust 

 is deposited upon it. In order to prepare prisms by this method 

 a piece of platinum foil 0'015 millim. thick, 6 millim. wide, 

 and about 45 millim. long, was stretched out, its sides vertical, 

 directly above a glass plate placed horizontally. If the foil 

 is then heated nearly to white heat, it becomes strongly dis- 

 integrated, and a double prism is formed upon the glass, which 

 consists of a mixture of platinum and platinum oxide. The 

 intensely hot particles of platinum scattered from the foil 

 appear, partially at least, to become oxydized by the atmo- 

 sphere. The platinum oxide is easily converted into metallic 

 platinum by gently heating. The double prisms, consisting 

 of platinum and oxide, are therefore easily converted into 

 pure platinum by heating. 



Lastly, it was attempted to prepare double prisms of metal 

 by volatilizing a metallic kathode in vacuo. A glass plate 

 was brought as near as possible to, and parallel with, a metallic 

 wire, which served as a kathode, and was intensely heated in 

 vacuo by a powerful induction-current. As with the galvani- 

 cally heated platinum coil, so here, a double wedge of disin- 

 tegrated metal was formed. For the final experiments, re- 

 lated further on, such prisms are, however, not suitable. I give 

 only in passing some of the numbers obtained with them. 



Methods of Ohservation and Results. 



The indices of refraction are determined for the metallic 

 prisms obtained in the manner described, assuming the truth 



