M. C, Lea on the Invisible Photographic Image. 109 
Art. XIIL—On the nature of the Invisible Photographic Image ; 
by M. Carry Lexa, Philadelphia. In a letter to the Editors. 
that iodid of silver “is never sensitive unless there is a body 
present capable of taking iodine from it under the influence of 
light. And Russel believes that the developed image is chiefly 
produced at the expense of the silver haloid in the film. : 
The following experiments seem to me to decisively close this 
controversy in favor of the physical theory. 
Experiment 1.—If the iodid or bromid of silver in the film 
undergoes decomposition in the camera, and still more, if the 
developed image is formed at its expense, the film of iodo-bro- 
mid must necessarily be greatly consumed in the development 
under the dense portions of the negative, which it has contrib- 
uted to form. 
To settle this point, I exposed and developed an iodo-bro- 
mized plate in the ordinary manner. Then, instead of remov- 
ing the unchanged iodid and bromid by fixing in the ordinary 
manner, I took measures to remove the developed image without 
affecting the iodid and bromid. This I succeeded in doing with 
the aid of a very weak solution of acid per-nitrate of mercury. 
Now, if the iodid, or bromid, or both, had been in any way de- 
composed, to form, or aid in forming, the developed negative 
image, when this came to be removed, there should have been 
left a more or less distinct positive image, depending upon varyin 
thicknesses of iodid and bromid in the film, much like a fix 
negative that has been completely iodized. Nothing of this sort 
was visible, the film was perfectly uniform, just as dense where 
an intense sky had been, as in those parts which had scarcely 
received any actinic impression, and looking exactly as it did 
when it first left the camera, and before any developer had b 
applied. : = ; 
is experiment seems sufficiently decisive. But the follow- 
ing is far stronger, 3 
