DEGLI SPETTROSCOPISTI ITALIANI 119 



From the Proceedings, Asiatic Society of Bengal, for Noveniber 1875. 



Spettro solare fotografato dal Gap, Waterlioitse. 



Capt. Waterhonse exhibited photograplis on glnss of the solar spectrum showing 

 the extreme red rays, and read the following note regarding them. 



While recently makiug some investigations ou Dr. Vogel's interestiiig and va- 

 luable 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 piate acted on by the red rays, so that the Fraunhofer lines in that 

 region, which shonld appear light on a dark ground, carne out dark on a light 

 ground , and on some few occasi ons 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, obtaiued in the ba- 

 zar, but was also perceptible on plates stained with orange, red and green dyes, 

 These plates were ali slightly fogged, but as the reversed lines were uot obser- 

 ved on plates prepared with uneoloured bromide , I attribnted the cause to the 

 action of the dyes alone. 



On repeating the experimeuts on this action of the blue dye with another se- 

 ries of plates, prepared after the nitrate of Silver bath had been puriiìed so as 

 to give plates eutircly free from fog, I was rather snrprised to tìnd that the re- 

 versed action was no longer obtained. On thiuking over the subject it carne to 

 my recollection that in the early days of photograpliy, when the daguerreotype 

 was in vogue, Sir J. Herschel, Dr. Dreper, Messrs. Clandet, Pizeau, Becquerel and 

 other eminent dagnerreotypists had observed that if a sensitive piate were expo- 

 sed 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 

 nentralised, and in this uiauner photograplis had been obtained in which not only 

 the rays in the extreme red just below the ordinarily visible spectrum coniti be 

 distinctly seen, but also traces obtained of three or tour groups of rays in the beat 

 spectrum where it was ordinarily supposed no photographic action existed. 



1 therefore tried the effect of a short preliminary exposure to daylight with one 

 of the blue-stained bromide plates prepared at the sanie time and in the sanie 

 man nei* as the one on which no reversed action was apparent. On developing, the 



