1890.] on Astronomical Telescopes. 171 



Note on a Method op Silvering Glass Mirrors. 



Solutions. — Make up 10 per cent, solutions of pure recrystallised 

 nitrate of silver, pure caustic potash, and loaf sugar. To the sugar 

 solution add ^ per cent, of pure nitric acid and 10 per cent, of alcohol. 

 The sugar solution is very much improved by keeping, its action 

 being more rapid and the film cleaner when the sugar solution has 

 been made for a long time. Make up also a weak solution, say 1 per 

 cent, of nitrate of silver and a 10 per cent, solution of ammonia. 

 (90 per cent, distilled water, 10 per cent, ammonia, '880 specific 

 gravity). Distilled water must be used for all the solutions. 



Cleaning the Mirror. — Thoroughly clean the mirror. To do this 

 pour on a strong solution of caustic potash, rub well with cotton wool, 

 rinse with ordinary water, wash again with absolute alcohol, and rinse ; 

 finally pour on strong nitric acid, and rub with a piece of cotton 

 wool, inserted in the open end of a test tube. Rinse again thoroughly 

 with ordinary water, and then place the mirror face downwards in 

 distilled water in a dish sufiiciently large to leave two inches margin 

 round the edge of the mirror, and to keep the face of the mirror one 

 inch from the bottom of the dish. The liquid should stand half an 

 inch above the face of the mirror which should not be completely 

 submerged, and care should be taken to exclude all air-bubbles. 



For Silvering a 12-incJi Mirror. — Take 400 c.c. of the nitrate of 

 silver solution and add strong ammonia until the brown precipitate 

 first formed is nearly dissolved, then use the diluted ammonia until 

 the solution is just clear. Then add 200 c.c. of the caustic potash 

 solution. A brown precipitate is again formed, which must be dis- 

 solved in ammonia exactly as before, the ammonia being added until 

 the liquid is just clear. Now add the 1 per cent, solution of silver 

 nitrate until the liquid becomes a light brown colour, about equal in 

 density of colour to sherry. This colour is important, and can 

 only be properly obtained by adding the weak solution. Dilute the 

 liquids to 1500 c.c. with distilled water. 



All being ready add 200 c.c. of the sugar solution to 500 c.c. of 

 water. Then lift the mirror out of the dish, taking care to keep its 

 face downwards during the time it is out of the water, pour the 

 washing water away, add the sugar solution to the silver potash 

 solution, taking care they are thoroughly mixed, and pour them into 

 the dish. Place the mirror face downwards in this solution, taking 

 care to exclude all air-bubbles. 



The liquid will turn light brown, dark brown, and finally black. 

 In four or five minutes, often sooner, a thin film of silver will com- 

 mence to form on the mirror, and this will thicken until in about 

 twenty minutes the whole liquid has acquired a yellowish brown 

 colour, with a thin film of metallic silver floating on the surface. 



Lift the mirror out, thoroughly wash with distilled water, and stand 

 the mirror on its edge, or rest it in an inclined position until it is 

 dry ; if time can be allowed, the silvered mirror may be left to soak 



