120 MEMORIE DELLA SOCIETÀ* 



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 tran- 

 spareut ground, but the bands and lines in the extreme red below A were also 

 distinctly visible and reversed, as will be seen in the two negatives I nave the 

 pleasure to exhibit this evening. One of them was taken with a very fine single- 

 prisin 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 spec- 

 troscope, and on this piate the bleaching action is particularly well shown, not 

 a trace of deposit remaini ug on a great part of the piate acted on by the red 

 rays, Avhile the lines stand out clearly, though not in very good definition owing 

 to my haviug used a rather large opening of the slit. 



It will be observed also that the part of the spectrum acted upon by the blue 

 aud violet rays is also reversed, so that the whole spectium is reversed from a 

 little above H 2 to below A. This action I have noticed on ali dry bromide plates, 

 both staiued and unstained, and have frequently obtained very beautiful images 

 of the lines between H 2 and P on a perfectly clear ground. The effect appears 

 to be chiefly caused by overexposure and may perhaps be partly due to the ac- 

 tion of the alkaline developer. 



As above stated, the reversing action of the red rays has been observed with 

 daguerreotype plates aud therefore I cannot claim to have macie a discovery, but 

 so far as I ara aware the application of the principle to collodion photography 

 is entirely new, and as it appears capable of an important practical application 

 in spectroscopic photography, particularly in extending our knowledge ofa part 

 of the spectrum in which eye-observations can only be made with difficulty and 

 under very favourable circumstaiices, 1 have thought it worth bringing to the no- 

 tice of the Society and hope to be able to continue the investigations with stili 

 better results. 



Oapt. Waterhouse further remarked that these photographs had an additional 

 interest from the fact that the lines in the extreme red had hitherto only been 

 observed about sunrise and sunset and that according to Mr. J. B. N. Hennessey, 

 who had made a very careful study of them at Mussooree in 1870, complete dark- 

 ness prevailed at sun-high in the part of the spectrum iu which they are silua- 

 ted. The photograpu, however, which shows the lines with most dislinctness was 

 taken between 1 and 2 p. u. and the spectrum extends to a distance below A eqnal 

 to that of the line C above it, or very considerably beyond the limits of Mr. Hen- 

 nessey's map and of the earlier oncs by Brewster and Gladstone. 



