ENGRAVING WITH A SUNBEAM. 
149 
sensitive gelatine film. The possibility of obtaining lialf-tones 
is dependent upon the power of the light to penetrate the 
yellow coating of bichromate and gelatine by which it is 
rendered more or less insoluble, but the impermeability to 
light of this layer long stood in the way of the best results 
being obtainable, until Mr. Burnett, of Edinburgh, solved the 
difficulty by printing on the under instead of the upper sur- 
face, and since that time no more difficulties have intervened in 
the way of producing photographs in relief in solid gelatine. 
Bearing this principle in view, we shall see how it is applied 
by Mr. Woodbury to aid him in securing a mould. 
A sheet of talc of the size of the picture required is af- 
fixed to a plate of glass by means of a little gum or water, 
and after being placed on a levelling- stand, some of the 
bichromatised gelatine — prepared as previously intimated — is 
poured on to its surface so as to form an even coating. When 
it has become quite dry, the talc, by means of a sharp knife, 
is removed from the glass, and the exposed surface of the 
gelatinised talc carefully cleaned and placed in contact with 
the negative of the subject that is to be reproduced. The 
surface of the gelatine is protected by means of a sheet of 
blotting-paper, after which it is covered over with a glass to 
ensure uniform pressure and close contact between the talc 
and the negative. After exposure to the light of the sun for 
about an hour, the film must be placed face upward in a dish of 
hot water, by which means all the gelatine unacted on bythe light 
will be dissolved away, leaving a picture in relief the height of 
which depends upon the penetrating power of the light through 
the negative, the parts most acted upon standing in highest 
relief. When no more gelatine will dissolve from the film, it 
is dried by a gentle heat up to a certain stage, after which 
the drying is allowed to be completed spontaneously. This 
precaution serves to prevent the gelatine film from splitting 
away from the talc. 
Beliefs obtained in the manner described may be kept in 
a portfolio for any length of time, and are always ready for 
the next operation, that of securing an intaglio impression in 
metal. To -obtain this impression, the electrotype process 
at once suggests itself as the most suitable one for the 
purpose, and in the early days of the process — if such a 
phrase be applicable to that which has not yet been a year in 
existence — the moulds were obtained by electrical deposition. 
This, however, was attended by a loss of time which it was 
desirable should be avoided ; accordingly, after some experi- 
ments, Mr. Woodbury found that when the gelatine u relief” 
had become thoroughly dried, it was hard enough to be 
impressed in soft metal, faithfully transmitting' its most 
