395 
RECENT DISCOVERIES IN PHOTOGRAPHY. 
By J. TRAILL TAYLOR. 
MONG recent discoveries in photography there are two 
which appear to exercise an important influence upon the 
future practice of the art-science. One of these refers to the 
production of negatives, the other to the printing of positives, 
or proofs upon paper. 
As hitherto practised, the production of a negative involves 
several manipulations of a somewhat messy nature. The argentic 
halogens, iodide and bromide of silver, which constitute the 
sensitive body in the collodion film, have been formed by double 
decomposition, a collodion containing a haloid salt being im- 
mersed in a bath of nitrate of silver, by which iodide of silver is 
precipitated in the film, together with the nitrate of the base of 
the salt, which, being soluble, remains in the bath to contaminate 
the silver. If the plate is to be dried and stored away for future 
use, it is now subjected to a series of washings to eliminate the 
soluble salts, after which it is 44 preserved ” by the application of 
an organic solution such as gelatine, tannin, gum, and other 
bodies. 
With a view to simplify these complex processes it had long 
been considered desirable that the haloid salts of silver should 
be mixed with the collodion, so as to ensure by a single opera- 
tion the coating of the plate with a sensitive layer. But it was 
found that the large atoms of iodide of silver would not emulsify 
with collodion ; and all attempts at producing a sensitive collo- 
dion failed, until about ten years ago, when two Liverpool 
amateurs, Sayce and Bolton, solved the problem of producing a 
sensitive collodion emulsion by discarding entirely the iodide of 
silver, and substituting for it bromide of silver. The best con- 
ditions under which this bromide emulsifies has for several 
years formed a theme for the investigations of scientific pho- 
tographers, and these researches have greatly conduced to the 
high state in which the art exists at the present time. Such is 
the state of perfection to which the system of preparing sensitive 
