218 PROCEEDINGS OP THE AMERICAN ACADEMY 



A year or two ayo, Vogel, as has already been referred to by me, in 

 a previous communication to this Academy, observed that tlie addition 

 of certain pigments, cliiefly aniline, to the above-named silver bromide 

 salt, would cause a partial reduction of the silver salt, when these were 

 exposed to rays of less refrangibility than the blue. He explained 

 this phenomenon by supposing that the pigment corallin or fuchsin, 

 which has a red color, absorbed and stored up latent energy, and so 

 re-enforced the primary action on the silver bromide. Captain Abney, 

 as has also been stated, informed me last year that the addition of gum 

 benzoin increased likewise the sensitive action on the silver bromide, 

 so that by its addition he could photograph the less refrangible rays of 

 the solar spectrum. He has publicly, in his South Kensington ad- 

 dress, explained this increased action on the principle that the mole- 

 cules of silver bromide rotating and vibrating at a given rate were 

 weighted down and moved at a slower rate, so that the inteiference of 

 rays of less refrangibility than what constitutes blue light would re- 

 duce the strained bromide silver salt, and so give an image from green 

 and yellow light. This he explained on the piinciple that these silver 

 bromide molecules consequently could be made to swing in discord 

 with waves of greater amplitude. 



If Vogel's explanation is correct, we should suppose that any pig- 

 ment which absorbs the green or yellow rays, and does not prevent the 

 chemical process of reduction, would likewise increase the sensitiveness 

 of the silver salt to these rays. Now, the same aniline blue that I 

 mentioned at the beginning of this communication absorbs green and 

 yellow rays, and does not prevent the reduction of silver bromide to 

 rays of blue light. Tliis blue-stained silver bromide was exposed to 

 the solar spectrum from about line G to line A, and yet I could obtain 

 no image below line F in the blue. Again, if a silver bromide emul- 

 sion (so called) be stained with the same aniline colors which Vogel 

 himself used, — viz., fuchsin or corallin, — we ought to have the sil- 

 ver salt reduced on exposure to the green rays of light. Unless there 

 be free nitrate of silver in the emulsion, we get no such effect. These 

 two experiments conflict with Vogel's explanation. Captain Al)ney 

 states that, if there is an excess of silver bromide, the addition of the 

 aniline does not increase its sensitiveness to the less refrangible rays. 

 Now, if there is an excess of silver bromide, there can be no free silver 

 nitrate ; and, unless there be free nitrate, there is no action from the 

 pigment and silver bromide. The explanation of this photographical 

 action of the green and yellow rays of light must be sought out in some 

 other way. • 



