% 
50 M. C. Lea—The Latent Photographie Image. 
appropriate conditions is more powerful than any yet known, 
in which there is no free alkali present. 
4, It has been held that ferrous salts act only in the presence 
of a soluble salt of silver, and scarcely attack the silver haloid 
in the film, to produce an image in the absence of silver nitrate 
or other soluble silver salt. This also I think can be disproved. 
As the development of the latent image depends (in the class 
of cases which shall here consider,) upon a reduction of the 
portions of the sensitive substance which have been acted upon 
by light, it is evident that it is amongst reducing agents that 
we must look for bodies possessing this power. But here we 
are at once met by the fact that it is not the reducing power 
alone that is involved. We find that many substances which 
reduce silver salts energetically, have no power to evoke the 
latent image, and also that the developing power, where it exists, 
stands in no sort of relation to the energy of the reducing power. 
It is an elective power that is required, a tendency to reduce, not 
the whole surface, but only those parts that have been acted 
upon by light, and to spare the others. For example, it is easy 
to prepare alkaline solutions of ferrous salts, by having present 
a sufficient quantity of neutral potassium tartrate. But such 
solutions, applied to a sensitive surface that has received suffi- 
cient exposure under a negative, or in the camera, for ordinary 
development, seem to attack equally those parts that have been 
acted upon by light and those that have not. So that the sen- 
sitive surface darkens uniformly without showing the vestige 
of an image. Other similar cases will appear farther on. 
n the following series of investigations a pure photographic 
paper was selected as the material for containing the sensitive 
substances. In the selection of the sensitive material itself, 
tassium bromide, two of potassium iodide, and one and a third 
ammonium chloride, to the ounce. This proportion was that 
which gave the best results and was maintained in the experi- 
ments throughout. Facility of development is always increased 
by the presence in the sensitive film of some suitable organic 
substance. In these investigations a decoction of cocculus indt- 
cus was employed : this was applied together with the haloids, 
