A SILVERED GLASS TELESCOPE. 3 



specula. These were described as possessing great capabilities for astronomical 

 purposes. They reflect more than 90 per cent, of the light that falls upon them, 

 and only weigh one-eighth as much as specula of metal of equal aperture. 



As no details of Steinhcil's or Foucault's processes for silvering in the cold way 

 were accessible at the time, trials extending at intervals over four months were 

 made. A variety of reducing agents were used, and eventually good results 

 obtained with milk sugar. 



Soon after a description of the process resorted to by M. Foucault in his excel- 

 lent experiments was procured. It consists in decomposing an alcoholic solution 

 of ammonia and nitrate of silver by oil of cloves. The preparation of the solutions 

 and putting them in a proper state of instability are very difficult, and the results 

 by no means certain. The silver is apt to be soft and easily rubbed off, or of a 

 leaden appearance. It is liable to become spotted from adherent particles of the 

 solutions used in its preparation, and often when dissolved off a piece of glass with 

 nitric acid leaves a reddish powder. Occasionally, however, the process gives 

 excellent results. 



In the winter of 1861, M. Cimeg published his method of silvering looking- 

 glasses by tartrate of potash and soda (Rochelle salt). Since I have made modifica- 

 tions in it fitting the silver for being polished on the reverse side, I have never on 

 any occasion failed to secure bright, hard, and in every respect, perfect films. 



The operation, which in many details resembles that of M. Foucault, is divided 

 into: 1st, cleaning the glass; 2d, preparing the solutions; 3d, warming the glass; 

 4th. immersion in the silver solution and stay there ; 5th, polishing. It should be 

 carried on in a room warmed to 70 F. at least. The description is for a 15| inch 

 mirror. 



1st. Clean the glass like a plate for collodion photography. Rub it thoroughly 

 with nitric acid, and then wash it well in plenty of water, and set it on edge on 

 filtering paper to dry. Then cover it with a mixture of alcohol and prepared chalk, 

 and allow evaporation to take place. Rub it in succession with many pieces of 

 cotton flannel. This leaves the surface almost chemically clean. Lately, instead 

 of chalk I have used plain uniodized collodion, and polished with a freshly-washed 

 piece of cotton flannel, as soon as the film had become semi-solid. 



2d. Dissolve 560 grains of Rochelle salt in two or three ounces of water and 

 filter. Dissolve 800 grains of nitrate of silver in four ounces of water. Take an 

 ounce of strong ammonia of commerce, and add nitrate solution to it until a brown 

 precipitate remains undissolved. Then add more ammonia and again nitrate 

 of silver solution. This alternate addition is to be carefully continued until the 

 silver solution is exhausted, when some of the brown precipitate should remain in 

 suspension. The mixture then contains an undissolved 

 excess of oxide of silver. Filter. Just before using, mix 

 with the Rochelle salt solution, and add water enough 

 to make 22 ounces. 



The vessel in which the silvering is to be performed may 

 be a circular dish (Fig. 1) of ordinary tinplate, 16 \ inches 

 in diameter, with a flat bottom and perpendicular sides one inch high, and coated 



