172 



Mr. A. A. Common on Astronomical Telescopes. [May 30, 



in distilled water over niglit. Leave it to dry until next day, then 

 the slight yellowish " bloom " can be polished off by rubbing softly 

 with a pad of chamois leather and cotton wool. Carefully polish 

 afterwards with a little dry well-washed rouge on the leather pad. 

 The film should be opaque and brilliant, and with careful handling 

 will be very little changed with long use. 



Dishes. — Use porcelain, glass, or earthenware dishes whenever 

 possible ; but, if these are not available, a zinc dish, coated inside 

 with paraSSn or best beeswax. 



For small mirrors (up to 12 inches) the easiest method of 

 supporting them during silvering is to attach them to a wooden rod 

 by pitch, and arrange the dish thus 



WOODEN ROD 



WOODEN BLOCK PITCH 



: ' ; ' ''''' '' '' ; ' ' ' 1! ! lil'!lH ! 111 ll il l T !!ll l l l ll!!i lll l l ll i ll l ll!llllll!IIITi'i'l^;!^^ 



"W!l 



'^^ - '^''-^^' 



SUPPORTING BLOCK 



DISH CONTAINING SILVERING SOLUTION 



Temperature and Time. — Half an hour is the usual time taken in 

 silvering, but this is shortened by using warmer liquids. About 

 65° F. is best for silvering. In colder weather longer time must be 

 allowed for the film to be deposited. In very hot weather a smaller 

 quantity of sugar can be used, say 150 c.c. For a 12-inch mirror it is 

 a safe rule to allow four times the time required to get the first indi- 

 cations all over the mirror as the total time for the mirror to be in 

 the bath. 



In cases when it is necessary to silver face upwards, a band may 

 be put round the mirror, and the solutions poured on. It is necessary 

 in this case to leave out the potash solution, and allow a longer time 

 for the silver to deposit ; as much as two hours being sometimes 

 necessary. 



If a very thick film is required, two silvering baths can be used, 

 the mirror being left in the first for 15 minutes, then lifted out, rinsed 

 with distilled water, and at once immersed in the second bath, which 

 should be ready in a second dish. The film must not be allowed to 

 dry during the operation of changing from one bath to the other. 



