356 M. C. Lea—Influence of Color upon Reduction of Light. 
position of AgBr, and which substance must absorb the rays 
in question, and not other rays.* 
My own conclusion was that the power of heightening sensi- 
tiveness to particular rays was one In no way connecte 
the color of the sensitizing body; indeed, that perfectly color- 
less bodies, or bodies having very pale neutral colors might 
also exercise the se of heightening seidiibeenenn to par- 
ticular colored ra 
ince the piiblication of my former paper, I have made 
further investigations, and Dr. Vogel has 5 sro i a second 
paper. -In this ee 89 paper Dr. Vogel, from whom I have 
regretted to differ, has to a maddiabie extent adopted my 
own views. Her a the same conclusion, as to the power 
of substances destitute of color to exalt the sensitiveness of 
AgBr to particular rays. As this was the main result of a long 
sarees and the demonstration of it the principal object 
of m er (from which paper and its conclusions Dr. Vogel 
abe ae 5 issented), it would have been more agreeable to me if 
Dr. Vogel had acknowledged it as originating with me. This, 
however, is a matter of small importance. Dr. Vogel does not 
seem to have adverted to the fact that this conclusion is scarcel 
compatible with his theory, formulated in the foot-note; wit 
the alleged function of color as modifying sensitiveness to par- 
ticular rays. 
If this theory were true, it must be easy to find very numer- 
ous instances capable of as Y aeers it, as the number of organic 
coloring matters capable of taking up bromine is very large. 
Nevertheless, three only have eth Siromahe forward by Dr. 
Vogel in the two papers referred to, out of which three, two at 
least certainly do not seem to afford the desired proo 
Coralline and naphthaline red are substances which transmit 
strong red light, and consequently the sum of their absorption 
spectra must in each case be green: to confirm Dr. Vogel’s 
theory, they should enhance the sensitiveness of AgBr to Gite 
light. But Dr. Vogel finds an increased sensibility to yellow 
rays only. Obviously yellow cannot represent the absorption 
by a red color. If Dr. Vogel’s law be true, a red substance 
may heighten sensitiveness to yellow, but it se heighten it 
reen. 
Now with the substance principally experimented on by 
Vogel, coralline, I have lately made a number of trials, and 
ree succeeded in definitely fixing its action with respect 
AgBr. 
* Wir im Stande sind Bromsilber fiir jede beliebige Farbe sree zu 
machen, respectiv die bereits vorhandene Empfindlichkeit fir gewisse Farbe zu 
steigern; es ist nur néthig einen die chemische Zersetzbarkeit des Bromsilbers 
beférdernde Stoffe zuzusetzen, welcher die betreffenden Farbe absorbirt, die 
andern nicht.—Ber. Deutsch. Chem. Ges., 1873, p. 1305. 
