1876.] Photographs of the Solar Spectrum. 187 



dye *, and exposed to diffused daylight for a moment before being placed 

 in the camera to receive the image of the spectrum. I also send another 

 plate, 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 

 that 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, Eizeau, 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. 



The apparatus used for obtaining two of these photographs consisted 

 of a Browning's chemical spectroscope, fitted with a very fine flint prism 

 of 60°, and used in conjunction with one of Dallmeyer's rapid rectilinear 

 lenses of 30" focus. The rays of the sun were collected from a 9" helio- 

 trope, worked by hand, and thrown on the slit by means of a condensing- 

 lens of 3" diameter and 25" focus. The aperture of the slit varied from 

 •003 to -005, as marked on the plate. 



The other single photograph was taken with a five-prism direct-vision 

 spectroscope, by the same maker, also attached to the rapid rectilinear 

 lens, and with the same arrangement of heliotrope and condenser. 



The plates were prepared with a bromized collodion made by Bouch, 

 sensitized in a bath of pure nitrate of silver at 16 per cent., then well 

 washed in several changes of water and treated with a watery solution 

 of the blue dye, again washed to remove the superfluous colour, and then 

 dried. The colour of the prepared plates when dry was a light lavender. 

 The development was by the alkaline method, followed by an intensifica- 

 tion with acid, silver, and pyrogallic acid. Before being placed in the 

 dark slide, the plates on which the extreme red rays are shown received 

 a momentary exposure to diffused daylight by quickly opening and 

 shutting the window of the dark room. 



It will be observed that the best results have been obtained with long 

 exposures of 30 or 40 minutes ; and it is also necessary to work with 



* I enclose a specimen of the dye. I do not know its chemical name, but it appears 

 to belong to the aniline or some analogous series, and was obtained in the native mar- 

 ket here. Dissolved in alcohol it is of a fine deep blue, but the addition of water 

 changes the tint to a rich violet or purple. 



[I have compared the specimen, as to its absorption-spectrum and the action of 

 reagents, with authentic aniline blue, and the two appear to be identical. — d. Gr. S-] 



