PHOTOGRAPHIC PRINTING. 125 



II. PHOTOGRAPHIC PRINTING PROCESSES. 



WHITE light, and certain of its coloured components, have the 

 power of splitting up into simpler forms the molecules of certain 

 kinds of matter, in a manner analogous to that possessed by heat. 



In some cases the change is only effected after very prolonged 

 exposure, as in the case of certain kinds of glass and of the 

 aniline dyes, whilst in others it is very rapid. The latter class of 

 bodies are impressed into the service of photography. 



The bodies most generally known to be sensitive to the action 

 of the luminous rays are compounds of silver, such as the iodide, 

 chloride, and its more complex organic salts. 



The light separates these into more elementary forms, and when 

 it is controlled by a lens, or by being passed through a photographic 

 negative, a picture can be obtained formed entirely of metallic silver. 



The earliest known photographs (by Wedgewood) were produced 

 on silver chloride. Sir John Herschell, amongst others, experi- 

 mented with silver chloride, allowing, in some instances, a fine 

 precipitate of this salt to be deposited by subsidence on glass. 

 When washed with a dilute solution of silver nitrate, and dried, he 

 exposed the sensitive layer in the camera. The picture of the 

 4o-foot telescope that is exhibited, is the earliest known photo- 

 graph on glass, having been obtained in 1839. At a later date, 

 Niecphore de Niepce discovered that the bitumen of Judaea, or 

 asphaltum, when exposed to light, became insoluble in the usual 

 menstrua employed for dissolving it. An early picture, produced 

 in the camera, of Kew Church, of the date of 1824, is still pre- 

 served in the British Museum. This discovery of Niepce's is 

 still utilised in certain photo-engraving processes, and although 

 the method is slow, the resulting images have certain qualities of 

 the highest importance. Of late years what is erroneously termed 

 carbon printing has been much adopted by photographers. If a 

 solution of a dichromate of an alkali be mixed with certain organic 



