THE MODERN REFLECTING TELESCOPE. 41 



The operation of silvering a 2-foot mirror face up will now be described. 

 It will be assumed for the present that the back of the mirror is unsilvered. A 

 silvering table is used, which is a strong structure of oak wood having a tilting 

 fi-arae carried on two trunnions, so that the mirror can be quickly turned from a 

 horizontal to a veiiical position, for the purpose of pouring off the cleaning and 

 silvering solutions; a strong narrow edge- band of flexible steel prevents the miri'or 

 from sliding off; the tilting frame is heavily weighted below so that it cannot 

 turn down accidentally. Thus all handling of the mirror while silvering is 

 avoided. 



The old silver film, if one exists, is removed with strong nitric acid on a 

 bunch of absoi'bent cotton tied to a srlass rod. The face and ed^'e of the mirror 

 are tlien quickly washed with distilled water. A band of strong brown drawing 

 paper, which has been dipped in melted paraffin, is drawn around the edge of the 

 glass and tightly bound to it by means of a thin band of coppei- with tightening 

 screws; the paper should project about three inches above the glass; the joints 

 should all be made water-tight by means of more pai-aflin and a warm iron. A 

 dish about three inches deep is thus formed, with the mirror as its bottom. 



A 10 per cent solution of pure caustic potash in distilled water is now used 

 foi' thoroughly washing the face of the glass and the inside of the paraffin band ; 

 this is done with a large bunch of absorbent cotton tied to a glass rod. This 

 solution is then poured ont and the glass is similarly washed several times with 

 fresh supplies of distilled water, to get rid of all traces of potash. Enough dis- 

 tilled water is now poured on the glass to entirely cover it while the silvering 

 solutions are being mixed. 



All of the vessels, graduates, etc., used for mixing the silvering solutions, 

 must be thoroughly \vashed, first with nitric acid, then with caustic potash, and 

 linsed with distilled water, just as the mirror is cleaned. 



For silverincr the face of a 2-foot mirror, 2 ounces of sil\-er nitrate 

 (Powers & Weightman) are dissolved in 20 ounces of distilleil water. One and 

 one-third ounces of caustic potash, pure by alcohol (Merck), are dissolved in 

 20 ounces of water in a separate vessel, and the solution is cooled. Strong aqua 

 ammonia (pure) is added, drop by drop, to the nitrate solution, while the liquid is 

 thorouo-hly stirred ; the mixture turns light-brown, then dark-brown ; the am- 

 monia is slowly added until the liquid becomes clear. The caustic potash solution 

 is now added slowly, with thorough stirring; the mixture now becomes very dark- 

 brown or black. Ammonia is again added, with thorough stirring, until the 

 liquid again just clears. A solution of one-fourth ounce silver nitrate in 16 ounces 

 of distilled water having been prepared, this is added to the mixture, a few drops at 

 a time, with thorough stirring, until the entire solution has a decided straw color, 

 while remaining transparent. This straw color is the test for the condition of in- 

 stability which is absolutely necessary in order that the metallic silver shall be 

 thrown out of combination when the reducing solution is added later. The 

 solution is now thoroughly filtei'ed thi'ough absorbent cotton. 



A quantity of reducing solution is taken containing an amount of sugar equal 



