Photographic Processes. 239 



Photolithography. The stone is covered with a varnish 

 consisting of bichromate of ammonia, water, and albumen, and 

 when dry is exposed to light, under the engraving, etc., which 

 is to be copied. Nothing is visible until the surface of the 

 stone is washed with Marseilles soap, which removes the solu- 

 ble portions — those where no insoluble oxide of chrome has been 

 formed, and which, being allowed to act for a sufficient time, 

 slightly hollows the stone wherever its surface has been laid bare. 

 If it is then wetted and inked, as for lithography, the ink enters 

 the hollows, but it is repelled from the parts in relief, which 

 are to form the lights. The engraving, etc., is not reproduced 

 in reverse, nor is it injured by the process of preparation. 



Barreswil's method consists in covering the stone with a 

 solution of bitumen of Judea in ether, which forms, not a var- 

 nish, but a granulation. A negative is laid on this ; and the 

 portions of the bitumen rendered soluble by exposure are washed 

 off in the usual way. Impressions may then be taken from 

 the stone ; and, for some time, each is better than the preced- 

 ing one. 



Zinco-photography . Paper is prepared with bichromate o 

 potash and gelatine, and, after having been exposed under a 

 negative, is covered uniformly all over with ordinary litho- 

 graphic ink ; it is then washed with gum water, which removes 

 the unaltered gelatine, and leaves a well-inked positive picture. 

 This is transferred to a properly grained zinc plate, by pres- 

 sure ; after which the process is that ordinarily used with zinc. 



Photographic Engraving. Dilute nitric acid dissolves the 

 silver from a Daguerreotype, without acting on the portions 

 covered with mercury. In this way may be obtained a plate 

 which will afford a few tolerable impressions. A great im- 

 provement is effected by rubbing grease into the cavities formed 

 by the acid, gilding the prominent parts by the electrotype 

 process, and then deepening the hollows with acid. The plate 

 must then be finished with the burine, which of course injures 

 its truth as a photographic product. 



Photography in Relief. A sheet of gutta-percha is coated 

 with a mixture of gum arable and bichromate of potash, and 

 when dry is exposed in the camera. The parts of the gum 

 which have thus been rendered soluble are then washed away 

 with water ; after which the sheet is dried. It is next held 

 horizontally, the gummed side being under, and, the corners 

 being pinched up so as to form a kind of rectangular trough, 

 hot water is poured upon it. This causes the gutta-percha to 

 become prominent wherever the gum has been removed ; and 

 thus the lights appear in a relief, which is unfortunately too 

 great. 



General Coloration of Photographs. Besides the care usually 



