Attachment of TIdn Galvanometer Mirrors. 189 



operation which taxes the skill of the optician to the utmost. 

 More important, however, is the fact that the silver backing 

 of the mirror has to be coated with some kind of varnish to 

 protect the silver from the action of the impurities in the air. 

 Although numberless kinds of varnish have been tried for 

 this purpose none has yet been discovered which does not 

 distort the mirror to some extent. A further difficulty in 

 the case of the glass mirror is the attachment of the mirror 

 to the stem which carries the magnets, the cement employed 

 almost always producing some distortion. While designing 

 a special form of mirror for use in a magnetograph I have 

 been led to a method of constructing galvanometer mirrors 

 which I believe entirely obviates the difficulties mentioned 

 iibove. 



Prof. Threlfall has already pointed out the advantages of 

 quartz as a material from which to construct galvanometer 

 mirrors, but he, I believe, employed silvered mirrors, and hence 

 had to use some form of varnish, and also he cemented the 

 mirrors to the rod which carried the magnets. He tried 

 both crystalline quartz and fused silica, and states he found 

 them equally suitable. In my case I was obliged to use 

 fused silica, and when I attempted to use thin slices silvered 

 on the back I found that there was a very considerable loss 

 of light owing to the small bubbles which are always present 

 in any but the smallest pieces of fused silica. It then occurred 

 to me to try and use some material for the reflecting surface 

 which would be unaffected by the air. Platinum naturally 

 was the material first tried, and after testing practically all 

 the methods of which I could hear for giving a bright film 

 of platinum, I obtained from Messrs. Johnson and Matthey 

 a platinizing solution which is entirely satisfactory, and gives 

 without any polishing a perfectly bright surface of platinum. 

 The film of platinum obtained is bright on the surface turned 

 away from the silica, so that the refiexion takes place at the 

 air-platinum surface, and hence the silica disk requires 

 polishing on one surface only, a circumstance which, as 

 will be seen, very much simplifies the construction of thin 

 mirrors. 



The method I adopt for making the mirrors is as follows : — 

 A stick of fused silica is prepared having a diameter equal to 

 the diameter of the mirror it is wished to prepare. This 

 stick is cemented to a small piece of wood by means of pitch. 

 A disk of tinned iron (No. 2^^ S.w.G.) about 12 centimetres 

 in diameter is mounted on a mandrel and the edge turned 

 true. A little diamond-dust mixed with thick oil is then 

 spread round the edge of the disk, and while rotating the 



