CONTRIBUTIONS TO PHOTOGRAPHY.
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being actually required for use. Attempts were accordingly made to
prepare sensitive dry plates soon after the introduction of the collo-
dion process, the first important step in this direction having been
made by Prof. Taupenot, in 1855. This investigator coated his
plates with ordinary iodized collodion, sensitized in a silver bath,
washed out the excess of silver nitrate, and then coated with iodized
albumen, and dried. Before use the plates were sensitized by immer-
sion in a silver acetate bath and then washed, and again allowed to
dry. Taupenot's dry plates were the first successfully used by pho-
tographers, but they were very insensitive and uncertain in their
action. An improvement was introduced in 1858 by Fothergill, who
used ordinary albumen instead of iodized albumen for coating the
sensitized collodion film, thus doing away with the second sensitizing
operation. Passing over certain minor modifications in the dry plate
processes introduced by subsequent workers, we come to the year
1861, when Russell put the finishing touch to the method of collodion
dry plate photography. The plate was first of all coated with gela-
tine as a substratum, and then received the film of bromo-iodized
collodion which was sensitized in a silver bath in the usual way.
After washing out the silver nitrate the plate was coated with a solu-
tion of tannin and then dried. The tannin, known technically as a
"preservative," restored the sensitiveness lost by the removal of the
silver nitrate, and thus placed in the hands of photographers a really
dependable dry plate possessed of greater sensitiveness and durability
than any other dry plate known up to that time. Russell's tannin
dry plates soon came into general use; his work on the process was
published in 1861, and reached a second edition in 1866, both
editions having been translated into German.
In these days of rapid photography we should consider a tannin
dry plate a very insensitive thing to work with, being at the best less
sensitive than a good wet plate. But the other advantages of the
dry plate outweighed this one disadvantage, and the favour with
which the tannin process was received showed how eager photo-
graphers were to dispense with the cumbrous appliances of the wet
plate method. From this discovery of Russell's to modern dry plate
photography it is but a step—that step being the precipitation of the
silver iodide or bromide in the collodion itself in the form of an
emulsion instead of coating the plate first with the salted collodion
and then forming the sensitive silver salt by immersion in a silver
nitrate bath. Emulsion photography is par excellence the photography