HISTORY OB^ PHOTOGRAPHY. 189 



quickly dried in the dark. In this state the i:)apers may be kept for 

 use. When they are requir-ed the above solution of silver is to be 

 plentifully applied and the paper i)laced iret in the camera, the 

 greatest care being taken that no daylight, not even the faintest 

 gleam, falls upon it, until the moment wlien we are prepared, by re- 

 moving the screen, to permit the light radiated from the objects we 

 wish to copy to act in i)roducing the picture. After a few seconds 

 the light nnist Iw again shut ott' and the camera removed into a dark 

 room. It will be found on taking the paper from the box that there 

 is but a very slight outline, if any, as yet visible. Place it aside in 

 perfect darkness until quite dry, then fix it in a mercurial vapor box 

 and apply a very gentle heat to the mercury. The moment the mer- 

 cury vaporizes the picture will begin to develop itself. The spirit 

 lamp must now be removed for a short time, and wdien the action of 

 the mercury appears to cease it is to be very carefully applied agaiu 

 until a well-defined picture is visible. The vaporization must now 

 be suddenly stop})ed and the photograph removed from the box. 

 The drawing will then be very beautiful and distinct, l)ut much de- 

 tail is still clouded, for the development of which it is only necessary 

 <o place it cautiously in the dark and allow it to remain undisturbed 

 for some hours. There is now an inexpressible charm about the pic- 

 ture, equaling the delicate l)eauty of the daguerreotypes, but being 

 still very susceptible of change it must be viewed by the light of a 

 taper only. The nitrate of silver must now be removed from the 

 Itaper by well Avashing in soft water. AVhen the picture has been 

 dried wash it quickh^ over with a soft brush dipped in a warm solu- 

 tion of the hyposulphite of soda and then well wash it for some time 

 in the manner directed for the ordinary photographs in order that 

 all the hyposulphite may be removed. The drawing is now fixed, 

 and we may use it to jirocure positive pictures, many of which may 



be taken from one original. 



* * * * * * * 



III. Photographs on Glass Plates and IJecent Improvements. 



To Sir John Herschel we are indebted for the first use of glass 

 plates to receive sensitive photographic films. 



precipitates or silver salts. 



The interest wdiicli attaches to this is so great and there appear to 

 be in the process recommended by the English experimentalist so 

 many suggestive points from which future photographists may start 

 that the passages are given in Sir John Herschel 's own words: 



With ii view to ascertain how far organic matter is indispensable to the rapid 

 discoloration of argentine coniponnds. a process was tried wliieli it may not be 



