296 Actinism and Magnetism. [July, 



the organic matter, operates in the latter a great change, 

 rendering gelatine, albumen, and such like substances inso- 

 luble. It was reserved, however, {or a succession of other 

 labourers in the field to develope this discovery into the 

 method of printing photographs in gelatine, impregnated 

 with carbon and other pigments. Under the action of light, 

 gelatine, charged with the bichromate of potash or ammonia, 

 becomes insoluble by warm water in exact proportion to the 

 degree in which it has been affected by the light. Hence, 

 by spreading gelatine in plates of some degree of thickness 

 on films of collodion, exposing these with their collodion 

 side next the negative, and subsequently dissolving away 

 the portions more or less unaffected by the light, pictures 

 are obtained in relief. Of this property Woodbury availed 

 himself to take metallic casts from those pictures in relief, 

 and from these metallic plates to take impressions on paper 

 in pigmented gelatine. 



The utilisation of the original discovery has been recently 

 brought to still higher perfection by its having been found 

 that the plates of gelatine, thus impressed by light, may them- 

 selves be rendered directly available for obtaining impres- 

 sions on paper in engraver's ink. Yet how simple the 

 matter appears now that it is known ! When on a plate of 

 glass, previously coated with white wax dissolved in ether, 

 there is spread a plate of gelatine charged with bichromate 

 of potash and chrome alum, and when, after being allowed 

 thoroughly to dry in the dark, this gelatine plate is removed 

 from the glass, and placed under a negative photograph, 

 wherever the light penetrates, the gelatine becomes, in a 

 greater or less degree, not only insoluble in warm water, 

 but incapable of imbibing moisture. But the parts thus 

 acted on by light can, with greater or less degrees of readi- 

 ness, receive engravers' ink. Those portions which have 

 been most hardened by the light will receive the stiffest 

 ink ; those which have been but partially hardened will 

 take on the ink only when it is more or less diluted ; while 

 those portions which have escaped the action of the light, 

 and have become moist (but only very slightly swollen) from 

 imbibing water, refuse the ink altogether. In this manner 

 every gradation of shade may be given to the impression 

 produced from the gelatine plate, and it is said that as many 

 as 1500 impressions may be taken from the same plate, 

 direct pressure being employed. This last, which is the 

 most perfect application of the double salts of chromic acid 

 to photographic purposes, is due to the laborious industry 

 and skill of Ernest Edwards. 



