A SILVERED GLASS TELESCOPE 



5 



The process is like a burnishing. Put the rubber care^fully away for another 

 occasion. 



Kie. 2. 



Polishing Strokes. 



The thickness of the silver thus deposited is about iTj5Ti?ooTr of 'M\ inch. Gold 

 leaf, when e([ually transparent, is estimated at the same fraction. Tlie actual value 

 of the amount on a 15i inch mirror is not quite a cent- the weiglit being less than 

 4 grains (2:39 milligrammes on one occasion when the silver was unusually thick), 

 if the directions ahove given are followed. 



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

 mirror are consequently only small fractions of o o o!tfoo "*^ "*^ "^f'^^' '^"'^ ^^'^' tlierefore 

 of no optical mf)ment whatever. If a glass has been properly sihcred, 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 difference in figure between 

 the silver and the glass underneath. The f\iintest peculiarities of local surface 

 seen on the glass by the method of M. Foucault, will be rc^produccd on the silver. 



Tlie durability of these sihcr films varies, d(-pending on the circumstances under 

 which they are placed, and the method of pre])iu-ation. Sulphuretted hydrogen 

 tarnishes them ([uickly. Drops of water may sjjlit tli(^ silver off. Under certain 

 circumstances, too, minute fissures will spread all o\rv tlu^ surface of the sih-(>r. and 

 it will appar(>ntly lose its adhesion to the glass. This ph(>nomenon seems to be 

 couu(>cted witb a continued exposure to dampness, and is axoided by grinding the 

 od'^v of the coiic,i\e mirror flat, and kee])ing it covennl wben not in use witli a sbeet 

 of tiat plate glass. Heat seems to liaxc no prejudicinl etfect, though it might liave 

 been supposed that the difference in expansibility would have overcome the nnitual 

 adhesion. 



Generallv silvered mirrors are xm-} enduring, and will 1)ear polishing rejunitedly, 

 if previously dried bv lieat. 1 have some wbieli have been used as diagonal re- 

 Hectors in tlie Newtonian, and have l)een exposed during a larg(> part of the day 

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

 are never covered, and yet the one now in the telescope has been ther<> a year, and 

 has had the dusty film — like that which accumulates 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 unnec(>ssary. A partial con- 

 version of the silver film into a golden one, \\hen it will resist sulphr.retted hydrogen. 



