1866. | Physics. | 121 
has been opposed by two distinguished photographers, Dr. Vogel 
and Major Russel. ‘The former affirms that iodide of silver is never 
sensitive unless there is a body present capable of taking iodine 
from it under the influence of light. And the latter believes that 
the developed image is chiefly produced at the expense of the silver 
haloid in the film. 
The following experiments seem to Mr. Lea to decisively close 
this controversy in favour of the physical theory :— 
Experiment 1.—If the iodide or bromide of silver in the film 
undergoes decomposition in the camera, and, still more, if the deve- 
loped image is formed at its expense, the film of iodide-bromide 
must necessarily be greatly consumed in the development under the 
dense portions of the negative which it has contributed to form. 
To settle this point, an iodo-bromized plate was exposed and 
developed in the ordinary manner. Then, instead of removing the 
unchanged iodide and bromide by fixing in the ordinary manner, 
the developed image was removed, without affecting the iodide and 
bromide, with the aid of a very weak solution of acid pernitrate of 
mercury. Now, if the iodide or bromide, or both, had been in 
any way decomposed, 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 
varying thicknesses of iodide and bromide in the film, like a fixed 
negative that had 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 been applied. 
Experiment 2.—A plate was treated in all respects as in No. 1, 
except that the application of the nitrate of mercury for removing 
the developed image was made by yellow light. The plate, now 
showing nothing but a uniform yellow film, was carefully washed, 
and an iron developer, to which nitrate of silver and citric acid had 
been added, was applied. In this way the original image was repro- 
duced, and came out quite clearly with all its details. 
Now, as every trace of a picture and all reduced silver had been 
removed by the nitrate of mercury, it is by this experiment abso- 
lutely demonstrated that the image is a purely physical one, and 
that after having served to produce one picture, that picture may be 
dissolved off, and the same physical impression may be made to pro- 
duce a second picture by a simple application of a developing agent. 
Mr. F. H. Wenham, a gentleman well known in all scientific 
circles by his admirable device for securing binocular vision in the 
microscope, has communicated to the Microscopical Society some 
interesting notes on the Fracture of Polished Glass Surface. 
It is a fact known to the philosophical instrument makers that 
