of Refraction of the Metals. 3 



platinized glass. At the commencement of my investigations 

 I had two kinds of such glass at my disposal, viz. some pieces 

 of French platinized glass taken from an apparatus made by 

 Konig of Paris, and some pieces which Dr. Lohmann of 

 Berlin had platinized for me. Only a very small portion of 

 them possessed a sufficiently plane surface. Further, though 

 the layer of platinum on these glasses is no doubt coherent, 

 yet, as the microscope shows, it is far from uniform. This 

 latter circumstance is of no consequence when it is desired to 

 deposit another metal in considerable thickness upon the 

 platinum; but for the preparation of the prisms it is very 

 prejudicial. As the stock of platinized glass was soon ex- 

 hausted, and more was not to be obtained, either from Dr. 

 Lohmann or from Paris, nothing remained but to prepare 

 the platinized glass myself. After much trouble I succeeded 

 in finding the composition of a platinizing solution which 

 permitted the burning-in of the platinum at a low red heat, 

 so that the plates remained perfectly even. Further, the 

 film of metal was so perfectly uniform that even with high 

 powers no want of homogeneity could be detected under the 

 microscope. The plate-glass employed was about 6 millim. 

 thick. The film of platinum wasburnt-in in a small muffle-oven. 

 It is unnecessary here to en^er particularly into the details of 

 the method, which will be described in another place. The 

 metallic prisms were formed upon the platinized glass in the 

 following manner : — Upon a strip of glass about 3 centim. 

 broad, placed horizontally, a vertical electrode of the metal to 

 be deposited, of equal breadth, was placed so that it did not 

 touch the platinum. In the angle between the glass and 

 metal a capillary layer of the electrolytic fluid was placed and 

 decomposed by a current of suitable strength. A double 

 wedge of metal was deposited whose greatest thickness lay 

 directly against the metallic electrode. Whether the surfaces 

 of the double wedge were in any degree plane, or whether 

 they were strongly convex or concave, depended upon so 

 many variable circumstances that the result was purely a 

 matter of chance. Only those double wedges were of any 

 use in which the surfaces were so far plane that they sharply 

 reflected a fine wire cross when seen through a telescope pro- 

 vided with a Gauss's eyepiece. Often 50, and not unfre- 

 quently many more, prisms had to be made before one at all 

 usable was obtained. 



At first, when I used the thin (1*5 to 2 millim. thick) Paris 

 and Berlin glass, they were, for the most part, not double 

 prisms but single, which were obtained by plunging the 

 metallic electrode vertically into the electrolytic bath, and 

 bringing the platinized glass horizontally against it. There 



B 2 



