40 THE MODERN llEFLECTING TELESCOPE. 



niiiTor being removed during this part of tlie adjustment. Next, the small diagonal 

 plane is adjusted for inclination, care being taken that the intersection of the lines 

 scratched in its film is placed in the axis of the paraboloid. Then the convex 

 mirror is adjusted for inclination, by reflection. Finally, with the illuminated pin- 

 hole in place, the large plane mirror is adjusted, as pi'eviously described. 



CHAPTER XV. 

 SILVERING. 



It is not my purpose to discuss the various processes of silvering. Several 

 methods have been admirably desciibed by Diaper (see ]). 2 of his book), by 

 lirasheai', and by Common (see p. 159 of his papei- On the C<>7ist ruction of a Mve- 

 Foot Refecting Telescope^. I have used almost exclusively tiie formula published 

 by Brashear in 1884, in which sugar is the reducing agent. After experience with 

 this process, and when the grades of chemicals specified below are used, silver 

 films are invariably obtained which take a perfectly black polish, and which are so 

 thick as to be nearly opaque even to the sun's disk. Small mirroi's ai'e usually 

 silvei'ed face down ; films which are satisfactory in all respects are obtained when 

 this is done. 



In the case of large mirrors it is more economical of silver, as well as safer and 

 more convenient in manipulation, to silver face up. Two difficulties occur, how- 

 ever, when this is done ; first, minute transparent spots are liable to occur in the 

 film ; these are so small, howevei', that they can be seen only when looking 

 through the film at a bright object; second, the refuse silvering solutions must be 

 poured off the mirroi', after the silver has been deposited, at exactly the right 

 stage of the ivaction ; if poured off too soon the film will l)e thin ; if too late, the 

 muddy-brown precipitate which settles u])on the film will slightly tai'nish the lat- 

 ter in such a manner that it will not take a i)ei'fect polish ; it is only by ex- 

 perience that the optician is able to determine the I'ight instant for pouring off the 

 refuse solutions. Mi'. Common encountered similar difficulties in silvering faceup, 

 and resorted to the use of solutions without caustic potash, and also to the use of 

 Draper's method of reducing with Rochelle salt; these methods, while subject to 

 their own special difficulties, do not give the objectionable precipitate. The 

 writer has adhered to the use of a slight modification of Brashear's formula already 

 mentioned, in part l)ecause no opportunity has occurred for compai'ing thoroughly 

 the merits of the various formula), and in part because the films obtained by this 

 method give entire satisfaction in use. 



TJie Rediiciiig tSolntion. This consists of distilled water, 200 parts; loaf- 

 sugar or pure I'ock-candy, 20 parts ; alcohol (pure) 20 parts ; nitric acid (c. p.) 1 

 part. The proportions given are by weight. This solution is greatly improved by 

 keeping, a solution which has been made for several months working more surely 

 than one newly made. A gallon of this solution is usually made at one time. 



