a Silver ed-GIass Reflecting Telescope. 251 



Many different processes for silvering glass have been pro- 

 posed, some requiring the aid of heat, others doing as well cold. 

 Those of M. Cimeg and Professor A. Martin have given the 

 most satisfactory results. The reducing agents are respectively 

 Rochelle salt and cane-sugar (interverti). As the formulae may 

 be of use to those interested in the matter, they are appended. 

 For silvering a 15^-inch mirror, about 22 ounces of liquid are 

 needed. In Cimeg' s process, 800 grains of nitrate of silver must 

 be dissolved in 4 ounces of water, and ammonia added until the 

 precipitated oxide of silver is almost redissolved. In another 

 vessel, 560 grains of tartrate of potash and soda are to be placed 

 with 17 or 18 ounces of water. The concave surface of the 

 glass must be thoroughly cleaned with nitric acid and water, and, 

 wheD dry, coated with uniodized collodion, and the film polished 

 off with cotton flannel. The liquids being mixed are then to be 

 poured into a shallow vessel of hard india-rubber, and the glass 

 immersed face downwards, the back standing out of the liquid 

 and being freely exposed to daylight or sunlight. The same 

 precautions are necessary to avoid streaks as in the case of a 

 collodion negative. The silver, when polished, should be about 

 2 o q^o o o of an inch thick, and should show the sun by trans- 

 mitted light as a light-blue disk. 



In Professor Martin's process only 100 grains of nitrate of 

 silver are required. The formula is to dissolve the nitrate in 

 water, add ammonia till the brown oxide redissolves completely, 

 then pour in slowly 80 grains of caustic soda in solution. If 

 this produces a precipitate, the quantity of ammonia must be 

 increased. The reducing agent is procured by boiling 12| parts 

 of white sugar in 100 parts of water with 1 part of nitric acid, 

 and adding water to make 500 parts with 50 parts of alcohol. 

 Two ounces and a half of this liquid are to be mixed with the 

 previous solution just before the glass is immersed. The draw- 

 back to this process, as compared with Cimeg' s, is that there are 

 often many minute holes in the silver, particularly if the solu- 

 tions have been freshly prepared. They cannot be avoided by 

 previous filtration. 



The durability of silver films under favourable circumstances 

 is quite surprising. When not exposed to sulphuretted hydrogen, 

 they do not show any disposition to tarnish, and, all things con- 

 sidered, are more lasting than the polished specula that are in 

 the observatory. These latter are apt to accumulate a yellow 

 film, which interferes as seriously with their photographic power 

 as does the reddish colour so strongly seen in Gregorians and 

 Newtonians when the copper and tin are incorrectly propor- 

 tioned. The silvered surfaces however, are, occasionally liable 

 to an accident which does not affect the others. A series of 



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