378 SCIENTIFIC AMUSEMENTS. 



to produce a successful result in this way ; but when properly 

 effected with lampblack as pigment the "carbon prints" thus 

 obtained are practically unalterable ; if kept in a dry place, the 

 hardened gelatin undergoes no change, and the picture is as 

 indestructible as the paper on which it is formed, the influences 

 tending to destroy ordinary silver prints (Expt. 371) having no 

 influence on carbon prints. 



Expt. 399. Printing Photographs. Woodbury type. Gelatin 

 pictures thus obtained possess some peculiar properties which 

 lead to their being utilised for the reproduction of photographs by 

 ordinary printing processes in more than one way. When a 

 gelatin picture has been formed upon or transferred to a steel plate, 

 the film when dry will be more or less in relief according to the 

 thickness of the altered gelatin at each part of the surface ; by 

 pressing the picture against a plate of softer metal, or a sheet of 

 tinfoil, in a powerful hydraulic press a similar picture is transferred 

 to the metal, but in intaglio, or sunken in, instead of in relief. If 

 the hollow mould thus obtained be treated with a warm solution 

 of gelatin tinted with soluble colouring matter or insoluble finely 

 divided pigment, the liquid being poured over it, and a truly flat 

 scraper be then passed over the surface, all superfluous gelatin will 

 be removed ; where the hollo ws are least marked only an infini- 

 tesimal film will remain, but where they are deeper, a thicker and 

 consequently a deeper tinted layer of coloured gelatin is left. In a 

 short time the gelatin sets, and by then applying paper and passing 

 through a press the gelatin adheres to the paper, and quits the 

 metal mould, so that the paper becomes impressed with the picture 

 thus printed off from the soft metal plate as a negative (in refer- 

 ence to the object from which the gelatin photograph was taken) ; 

 the lights of the object corresponding with the highest relief 

 (greatest insolubility of gelatin) in the first gelatin picture ; with 

 the deepest depressions in the soft transfer metal plate ; and, con- 

 sequently, with the thickest layer of coloured ink in the final print. 

 If, therefore, the gelatin photograph first taken be a print from a 

 silver negative, the final print will be a positive with reference to 

 the object originally photographed ; the light parts of the original 

 yield the thickest silver film in the silver negative, and, conse- 

 quently, the portions least in relief in the gelatin picture ; these 

 correspond with the least depressed portions of the metal transfer 

 plate, and, consequently, with the thinnest layers of coloured ink, 

 or lightest portions of the final print. This method of reproducing 

 photographs by ordinary printing processes is called the Woodbury- 

 type, having been invented by Mr Woodbury. 



Expt. 400. Photolithography and Allied Processes. In the 



