ENGRAVING WITH A SUNBEAM. 
147 
photography, Mr. Talbot availed himself of the discovery of 
the photogenic properties of bichromate of potash which had 
been made a short time before by Mr. Mungo Ponton. From 
the apparently trivial discovery of this gentleman, that paper 
which had been washed with a solution of this salt became 
darker in colour when exposed to light — a discovery followed 
by some researches by M. Becquerel into the nature and cause 
of this action — the active and practical mind of Mr. Talbot at 
once led him to see how this discovery might be turned to a 
valuable and practical issue. Accordingly the scientific world 
■was startled and gratified by the announcement in the Athenaeum, 
in 1853, that the problem of permanent photographic printing 
had been solved by this gentlemam’s discovery of a method 
by which photographs could be printed from an engraved 
steel plate in the usual carbonaceous ink of the copperplate 
printer. Some of the specimens shown as the result of this 
discovery possessed great delicacy and beauty; and we have 
scientific journals which have been illustrated b y engraved 
photographs of natural scenery effected by the process in 
question, which is based on the fact that bichromatised 
gelatine, gum, and other organic bodies become, after ex- 
posure to light, insoluble in water, and that an etching 
ground thus composed may be dissolved away in all those 
parts from which the light has been debarred access. 
This was the original discovery, but who can estimate 
the magnitude of its results ? For, arising out of it, and 
based on its simple principles, are the numerous varieties of 
photo-lithography, photo-zincography, photo-galvanography, 
photographic engraving in its now numerous phases, carbon 
printing, vitrified or enamelled photographs, surface block- 
printing, and, lastly, the process of relief-printing, now more 
immediately under consideration. 
Gelatine is the principal agent in relief-printing ; and several 
previously unknown properties possessed by this substance 
have been brought to light through the agency of its photo- 
graphic application. But before entering on the subject in 
detail, a synopsis of the process had better here be given. 
Woodbury's relief-printing is based on the fact that, if a 
layer of any dark-coloured transparent material be placed upon 
a white sheet of paper, the colour transmitted to the eye will 
be light or dark in proportion to the thickness of the material; 
if extremely thin, then the paper will appear white or almost 
so, every increase in the thickness causing the colour to appear 
deeper. If now a mould be prepared in intaglio, and it be 
filled with a coloured transparent body, such as gelatine, con- 
taining a dark pigment mixed with it, a sheet of paper pressed 
on this mould by means of a flat plate of metal would cause 
