54 M. C. Lea—Silver Salts and Coloring Matters. 
then staining the film has been found to have other applica- 
tions and many others have experimented in this direction, im 
most cases with a view to alter its sensitiveness relatively to 
the different colors of the spectrum. Major Waterhouse was, 
I believe, the first to recognize this effect. 
Dr. John 
. Draper appears to have first advanced the 
view that substances sensitive to light are affected by the rays 
which they absorb. There is much to support this theory, 
although it cannot be considered as definitely established. 
Some years since Dr. H. W. Vogel expressed the opinion 
that when sensitive films were washed over with solutions of 
coloring matters, the films gained sensitiveness to those rays. 
of the spectrum which the coloring matters absorb, with this 
condition, that the coloring matter in question must be capable 
of combining wit , Br, or I, as the case might be. 
own results were different. I found that the action of the rays 
was profoundly modified by coloring the film, but the result 
did not seem to conform to any law, and as often contradicted 
Vogel’s view as agreed with it. 
Vogel's theory necessitates the assumption that the color 
imparted to the silver salt is identical with that of the dye 
used, and as has been shown above, that by no means follows. 
He supposed that the capacity of any given color to influ- 
ence the silver salt depended upon its tendency to combine 
with Cl, Br, and I, whereas, as we have above seen, its action 
most probably depends upon its ability or inability to combine 
with the silver haloid. 
ut the principal source of error has arisen from the fact 
that when the film is stained the effect is necessarily a confused 
Whether with these sources of error eliminated Draper's — 
view, that a sensitive substance is influenced by those rays 
which it absorbs, can be applied to these new combinations 
which I have here dese , is a matter on which I am no 
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