426 OPTICAL INSTRUMENTS AND SILVERING GLASS SURFACES. 
APP EN DEX. 
SILVERING GLASS MIRRORS. 
The importance of a good reflecting surface in such instruments as the modern 
silver-on-glass reflecting telescope and the equatorial coudé is obvious. Asa 
rule the silver surface if fairly protected from dust and damp will last many 
years with but slight loss of light, but must be renewed frequently if the best 
results are to be obtained. Many different processes and methods of silvering 
have been from time to time published by different people, and it becomes of 
some interest to examine these with a view of finding out the particular one that 
suits certain cases. 
Having used reflecting telescopes for many years I have had occasion to try a 
great number of experimeuts with a view of getting a good process. It would 
be tedious to give these in detail, but it may be useful to give some few instances 
where satisfactory results have been obtained. 
‘The process of depositing the metallic silver on a glass ae is an empirical 
one; the conditions affecting the reactions are so various that hard-and-fast rules 
cannot be laid down. The temperature in which the process is carried on seems 
perhaps the most important thing to be considered, ranging as it may from 35° 
or 40° to 104° F. according to the reducing agent employed. 
IT have had occasion to look up the processes published from time to time ; 
some of these are of sufficient interest to be given briefly. 
Baron Liebig found in 1835 that on heating aldehyde with an ammoniacal 
solution of nitrate of silver in a glass vessel a brilliant deposit of metallic silver 
was deposited on the surface of the glass. To this observation is due the modern 
process of silvering glass. 
The next important step seems to have been taken by Cimeg, who, in 1861, 
patented a process for silvering mirrors (where of course only the surface against 
the glass is used) by what has since been known as the Rochelle-salt process. 
The patent is No. 619, 1861. After cleaning the glass in the usual way he 
washes the surface with Rochelle-salt solution 1 in 200. For 1 square yard of 
glass he takes 20 grammes of nitrate of silver in solution and adding it to 
ammonia of commerce till a brown precipitate commences to be produced; to this 
is added a solution of 14 grammes of Rochelle salts. Using the mixture in this 
proportion when it becomes turbid he pours it over the glass plate, which has an 
inclination of 1 in 40, for 30 minutes at a temperature of 68°. 
In 1862 Cimeg has another process patented (No. 2314). He uses 20 grammes 
of Rochelle salts in 300 grammes of water, 20 grammes of nitrate of silverin 15 
grammes of water with ammonia to clear; but in place of using a weak solution 
of Rochelle salts on the surface of the glass before silvering he rubs on the juice 
of apples, currants, sorbs, or other berries before silvering. 
In 1878 Woerther uses glucose as the reducing agent. 
In 1876 Pratt patents a process (No. 1259) in which before silvering he treats 
the glass with 1 part of protochloride of tin in 100 parts of water. For large 
plates he uses 1 part of protochloride of tin, 3 drachms of oxalate of ammonia, 
2 lb. putty powder, 4 pints of distilled water; this is rubbed on and allowed to 
dry ; he then uses a solution of 2 parts oxalate of ammonia, 4 parts grape-sugar, 
1 part lime, 1 part potassic cyanide, in 1000 parts water. In silvering he uses 
tartaric acid, but does not give details. 
There are a few more patents for silvering since the last date of no importance. 
In 1881 Piazzi Smith gives, in ‘“ British Journal of Photography Almanac,” 
Martin’s process in full. ‘This is a pretty well-known process, and in some hands 
