1871. ] Proceedings of the Asiatic Society. 241 
agents, that it made quite insoluble in water, though still capable 
of retaining a certain amount of water, without interfering with its 
property of attracting greasy ink in the parts exposed to light and 
repelling it in the unexposed parts, so that if such a film be spread 
upon asurface of glass, metal, or other suitable material and after 
exposure to light under a photographic negative, be washed till all 
the chromic salt is removed, .v obtain a printing surface possessing 
the properties of an ordinasy lithographic stone, that is to say, it is 
absorbent of water in some parts, and absorbent of greasy ink in 
others, but, as I have mentioned, it also has another most valuable 
property which is not possessed by the lithographic stone, and which 
has been most aptly termed “ a discriminating power of absorption,” 
so that when it is inked in with a roller, the ink will be thickest on 
the parts representing the deepest shadows of the picture, and 
which have received the most exposure to light, the middle tints 
will take less, the lighter tints still less, while the high lights will 
take none at all, and be represented by white paper. It will readily 
be seen that in this way an exact transcript of the original photo- 
graph may be obtained, shewing the most delicate delineation of 
detail with as perfect gradation of tone as in a proof produced by 
the ordinary process of silver printing, but possessing the great ad- 
vantages of a lithograph or engraving over a silver print in respect 
of undoubted permanence, cheapness and rapidity of production. 
The above is the principle upon which these processes depend— 
the practice though presenting some difficulties of manipulation is 
very simple. A mixture of gelatine and bichromate of potash, 
with one of the hardening or oxidising substances I have mention- 
ed, and also a little glycerine, sugar or other substance, capable of 
preventing the gelatine film from being too brittle is poured upon 
the surface of a perfectly level finely ground glass plate, and care- 
fully dried in the dark in such a manner as to preserve a 
very even surface. When dry the plate is ready to be exposed 
under a reversed negative in the usual manner. After the 
surface has received sufficient exposure the plate is turned and 
its under surface is exposed to the full power of the light for 
a short time to render it thoroughly hard and insoluble and prevent 
it fromswelling too much in the after washing. The plate is now 
