M. C. Lea—Dr. Vogel's Color Theory. 49 
nied by proof that some of these colorless substances showed a 
power of absorbing the rays to which they increased the sensi- 
tiveness of silver bromide. No such proof wasgiven. Although 
scarcely called upon to prove a negative, yet desiring to leave 
no side of this question unexamined, I have recently subjected 
to careful spectroscopic examination those colorless substances 
which I have described as distinctly increasing the sensitive- 
was first filled with water and interposed, then the 
argand burners used were moved to such distances as to make 
the two spectra exactly equal in intensity. In the examination, 
the size of the slit was varied from that which barely gave a 
visible spectrum, up to such as gave a powerful illumixation, 
and the comparison was made with all intensities, though of 
course the faint illuminations gave the most critical tests. e 
substances examined were potassic arsenite, codeia, salicine and 
morphia acetate. All of these substances, as I have elsewhere 
shown, exhibit a marked power of increasing the sensitiveness of 
silver bromide to the green rays. 
_ No elective absorption could be detected in any of them. It 
is therefore certain that their capacity to increase the action of the 
green ray is independent of any power to absorb that ray. 
Certainly if a law such as enunciated by Dr. Vogel existed, 
there ought to be found, without difficulty, very many sub- 
Stances which would exemplify it. On the contrary Dr. Vogel 
named very few cases in which he has recorded results con- 
forming to his hypothesis. Indeed his hypothesis has seem 
to rest chiefly upon three substances, coralline, chlorophyll, 
and naphthaline red. 
T have very carefully examined the action of all three of the 
substances with the ehowing results: ig 
ralline, as 1 have before said, enhances the sensitiveness 
more to the color which it chiefly transmits, red, than to those 
Which it absorbs. Moreover its power of increasing sensitive- 
hess (at least as far as the green rays are concern 2 may 
the con : 
Am. Jour, a Serres, Vou. XII, No. 67.—JuLy, 1876. 
