430 OPTICAL INSTRUMENTS AND SILVERING GLASS SURFACES, 
danger is obviated. My own experience is not singular in this respect, for Mr. 
Brashear relates a similar occurrence. 
The silver film is not always of the same quality, and experiments are needed 
to get more information as to what determines the greater density and coherence 
of some films over others. I have had surfaces of glass silvered experimentally 
where the film would not wash off with any amount.of wet rubbing, these mostly 
on surfaces that kad been silvered many times. Probably the glass in this case 
was in the best state to receive the new deposit; certainly the condition of the 
surface does affect the coherence of the silver as well as the amount of the deposit, 
as judged by the way in which certain parts on a mirror that has been incomplete- 
ly cleaned show that the deposition has begun long before other parts, necessarily 
resulting in an unequal thickness of film. With the most careful cleaning of a 
mirror I have often found that the first application did not succeed, but the 
second on the surface just cleaned off with nitric acid was all right. The nature 
of the liquid other than distilled water last in contact with the surface of the 
mirror seems to be the determining thing. 
Many variatians of the sugar process have been used and each man has his own 
particular formula. Mr. F. G. O. Wadsworth gives in the ‘“ Astrophysical 
Journal’ for March, 1895, an article on silvering, in which there are several 
interesting points, he gives a slight modification for the reducing agent and a 
table of quantities to be used for different sized mirrors which it is not necessary 
to introduce as it is on the basis of 120 grains nitrate of silver, 60 grains of 
potash and about one and a half ounces of reducing solution for an 83 inch 
mirror. Ammonia being used as required—he mentions the method of supporting 
the mirror by a suction pad which I successfully used some 13 years ago with a 
three-foot mirror. 
