II. On reversed photographs of the solar spectrum beyond the red, obtained on a collodion plate, in a letter to Prof. Stokes. By Captain J. Waterhouse, Assistant Surveyor General of India. Communicated by Professor Stokes

Mv deae Sir,—I have the pleasure to send you, by Overland Parcel Post, for presentation to the Royal Society, two photographs on glass of the solar spectrum, showing the extreme red rays below A, obtained on a dry collodion plate prepared with bromide of silver stained with a blue and exposed to diffused daylight for a moment before being placed a the camera to receive the image of the spectrum. I also send another late, also a dry bromide plate, stained with the same blue dye, and prepared at the same time and in the same manner as the other plates, but not exposed to light and quite free from fog; and you will observe hat on this there is no trace of the reversed action in the red rays, and that the direct action only extends slightly below C. This power of the red rays of the spectrum to neutralize the action of white light on sensitive daguerreotype plates was frequently noticed by Sir J. Herschel, Draper, Fizeau, Claudet, and other daguerreotypists about thirty years ago, but, so far as I can ascertain, it has never been observed on collodion plates. As collodion has so many advantages over the daguerreotype, it seems probable that this new extension of an old principle may have an important practical application in spectroscopic photography, particularly for the mapping of a part of the spectrum in which eye-observations can only be made with difficulty and under favourable circumstances.