A SILVERED GLASS TELESCOPE. 5 



The process is like a burnishing. Put the rubber carefully away for another 

 occasion. 



Fig. 2. 



Polishing Strokes. 



The thickness of the silver thus deposited is about aiyo^o uo ^^ ^^ inch. Gold 

 leaf, when equally transparent, is estimated at the same fraction. The actual value 

 of the amount on a 15| iach mirror is not quite a cent — the weight being less than 

 4 grains (239 milligrammes on one occasion when the silver was unusually thick), 

 if the directions above given are followed. 



Variations in thickness of this film of silver on various parts of the face of the 

 mirror are consequently only small fractions of g-o o^^tjo't °^ ^^ inch, and are therefore 

 of no optical moment whatever. If a glass has been properly sUvered, and shows 

 the sun of the same color and intensity through all parts of its surface, the most 

 delicate optical tests will certainly fail to indicate any difierence in figure between 

 the sUver and the glass underneath. The faintest peculiarities of local surface 

 seen on the glass by the method of M. Foucault, wiU be reproduced on the silver. 



The durability of these silver films varies, depending on the circumstances under 

 which they are placed, and the method of preparation. Sulphuretted hydrogen 

 tarnishes them quickly. Drops of water may split the silver off. Under certain 

 circumstances, too, minute fissures will spread aU over the surface of the silver, and 

 it will apparently lose its adhesion to the glass. This phenomenon seems to be 

 connected with a continued exposure to dampness, and is avoided by grinding the 

 edge of the concave mirror flat, and keeping it covered when not in use with a sheet 

 of flat plate glass. Heat seems to have no prejudicial efiect, though it might have 

 been supposed that the diff'erence in expansibility would have overcome the mutual 

 adhesion. 



Generally silvered mirrors are very enduring, and will bear polishing repeatedly, 

 if previously dried by heat. I have some which have been used as diagonal re- 

 flectors in the Newtonian, and have been exposed during a large part of the day 

 to the heat of the sun concentrated by the 15^ inch mirror. These small mirrors 

 are never covered, and yet the one now in the telescope has been there a year, and 

 has had the dusty film — like that which accumidates on glass — polished off it a 

 dozen times. 



In order to guard against tarnishing, experiments were at first made in gilding 

 silver films, but were abandoned when found to be unnecessary. A partial con- 

 version of the silver film into a golden one, when it wiU resist sulphuretted hydrogen, 



