1882. 177 Trans. N. Y. Ac. Sez. 
the object would be produced. But such a photograph would fade if 
brought out into the light ; besides, it would not be very intense. It is 
necessary, therefore, to devise some method by which the change pro- 
duced by the light can be strengthened, and then fixed. In practice, 
the sensitive compound is obtained upon a glass plate in one of two 
ways; either by coating the plate with a film of some substance that 
is permeable to fluids, such as collodion containing a bromide or an 
iodide, and then dipping the plate thus coated into a bath of silver ni- 
trate, whereby the silver bromide or iodide is formed within the pores 
of the collodion ; or else by precipitating the silver compound in a so- 
lution of collodion, gelatin or other medium, in such a manner that the 
particles will be held in suspension in a state of minute division. By 
the latter process a so-called emulsion is formed, which is poured over 
a plate and allowed to dry before use. The causes which affect the 
sensitiveness of the bromide are still very obscure. It is probable that 
this is controlled by the physical conditions under which it is formed ; 
in other words, the size of the particles of bromide in the emulsion. 
The smaller the particles, the more sensitive they are. Yet it seems 
possible to increase the sensitiveness of an emulsion, by causing the 
minute particles to become mechanically aggregated into larger masses. 
At least, this is the explanation suggested by the able experimentalist, 
Captain Abney, to account for the greater sensitiveness of a gelatin 
emulsion after boiling. This aggregation of the particles cannot in 
any wise affect'the sensitiveness of the constituent molecules ; but, if we 
can understand how it causes the film to be more sensitive, we will gain 
an insight into a molecular change which may be said to lie as the 
foundation of the process of strengthening the image, which is the pro- 
cess commonly known as development. 
It has been stated that a perfectly uniform layer of the sensitive silver 
salt upon a plate would show an image after exposure in the camera. 
This image results from the decomposition of the sensirive salt into a 
sub-bromide, sub-iodide or sub-chloride, as the case may be, the bromine, 
icdine or chlorine being set free; thus 2 (Ag Br)= AgeBr + Br. 
The change of color, resulting from a short exposure in the camera, is 
not great ; but it is possible to so prepare a plate that the chemical 
change produced by light in a fractional part of a second suffices to 
produce an image. Under such circumstances the chemical change 
must be very superficial, and the decomposition too slight for the eye 
to detect. Hence, after a plate is properly exposed in the camera, no 
image is visible upon it. To make the image visible, it must be 
strengthened or developed. 
Before describing the process of development, I will illustrate the 
process of making an emulsion of silver bromide for the dry-plate pro- 
