1875.] J. "Waterhouse — Photographs of the Bed rays of the Spectrum. 199 



While recently making some investigations on Dr. Vogel's interesting 

 and valuable discovery of the effect of certain dyes in increasing the 

 sensibility to the less refrangible rays of the spectrum, of dry collodion 

 plates prepared with pure bromide of silver, I observed on several of the 

 plates a reversed action in the part of the plate acted on by the red rays, 

 so that the Fraunhofer lines in that region, which should appear light on 

 a dark ground, came out dark on a light ground, and on some few occa- 

 sions distinct traces of lines in. the extreme red below A were also visible. 

 This effect was most marked on the plates stained with a blue dye, one of 

 the aniline or an analogous series, obtained in the bazar, but was also 

 perceptible on plates stained with orange, red and green dyes. These 

 plates were all slightly fogged, but as the reversed lines were not observed 

 on plates prepared with uncoloured bromide, I attributed the cause to the 

 action of the dyes alone. 



On repeating the experiments on this action of the blue dye with another 

 series of plates, prepared after the nitrate of silver bath had been purified 

 so as to give plates entirely free from fog, I was rather surprised to find that 

 the reversed action was no longer obtained. On thinking over the subject 

 it came to my recollection that in the early days of photography, when 

 the daguerreotype was in vogue, Sir J. Herschel, Dr. Draper, Messrs. 

 Claudet, Fizeau, Becquerel and other eminent daguerreotypists had observed 

 that if a sensitive plate were exposed to the spectrum after a short preliminary 

 exposure to diffused daylight, the red rays exercised a negative effect by 

 which the action of the white light was neutralised, and in this manner 

 photographs had been obtained in which not only the rays in the extreme 

 red just below the ordinarily visible spectrum could be distinctly seen, but 

 also traces obtained of three or four groups of rays in the heat spectrum 

 where it was ordinarily supposed no photographic action existed. 



I therefore tried the effect of a short preliminary exposure to daylight 

 "with one of the blue-stained bromide plates prepared at the same time and 

 in the same manner as the one on which no reversed action was apparent. 

 On developing, the reversed action was well marked, as I had expected, and 

 not only were the A and other lines in the red and yellow clearly to be seen 

 as dark lines on a transparent ground, but the bands and lines in the ex- 

 treme red below A were also distinctly visihle and reversed, as will be seen in 

 the two negatives I have the pleasure to exhibit this evening. One of 

 tip in was taken with a very fine single-prism spectroscope kindly lent me 

 by Mr. Pedler of the Presidency College and shows several images of spectra 

 in which the lines in the extreme red are fairly distinct though faint. 

 The other was taken with a Browning's direct-vision spectroscope, and on 

 this plate the bleaching action is particularly well shown, not a trace of 

 deposit remaining on a great part of the plate acted on by the red rays, 



