= 
54 M. C. Lea—The Latent Photographie Image. 
action on the image. When a salt of silver is present in solu- 
tion, to suffer reduction and to furnish silver for building up 
the i image to be developed, as in the case of the so-called O wet 
process,” ferrous sulphate is the most Se developing 
agent known, and is accordingly, always employed. But in 
the absence of silver in actual solution it is powerless. It is 
in connection with such sensitive surfaces (i. e., those from 
which silver nitrate or other soluble silver salt has been 
removed by washing), that the resent investigation is made, 
A surface of sensitive paper, om which all silver ‘compounds 
have been removed, may, after receiving a latent image, be 
left long in contact with ferrous sulphate without an image 
being develope 
Desirous of finding a means of applying ferrous salts to these 
developments, my first efforts were directed to the formation 
of alkaline solutions containing ferrous oxide in solution, but 
they did not appear to possess any power of development. I - 
then tried ferrous oxide in combination with organic acids, and 
here at once most interesting results followed. Severa Sree 
salts furnished beautiful developments, among which may 
especially mentioned /errous succinate, lactate and eatin 
he succinate is best used by forming a slightly acid solution 
i i ing to it a solution of ferrous 
sulphate in quantity not quite sufficient to form a precipitate. 
any other ferrous salts, among whith may be mentioned 
citrate, formate and tartrate, give images, but much inferior to 
errou. 
manner ated described, echibiis that powers af a 
ment, the ‘other with the wy alkaline pyrogallic develop- 
ment, — the following differences : 
“The sa e exposure which with the alkaline pyrogallol gives 
a weak or sunk-in image after a protracted development, 
gives with ferrous oxalate a bright, bold image, and this in a 
* much less time. The development is particularly clear and 
clean. Rotate unexposed parts are not attacked: the developer 
possesses a great deal of that elective power previously spoken 
of, whick causes it to react strongly on those parts which have 
received the influence of light, and spare those which have not. 
Ferrous oxalate is easily obtained by adding a strong warm 
solution of oxalic acid to one of ferrous sulpbat A bright 
