654 
CAPTAIN W. DE W. ABNEY ON THE PHOTOGRAPHIC METHOD OP 
formed by joining the ordinates fairly represents tire relative sensitiveness of the 
compound to the action of white light. 
With silver bromide the curve falls gradually towards B, and similarly towards the 
ultra-violet. With silver iodide and bromo-iodide the curve falls suddenly near G, and 
the chemical effect from the bottom of this descent to E is but very feeble ; whilst for 
the more refrangible end above G it follows pretty nearly the curve of the bromide. 
When collodion films containing silver bromide are dyed, by flowing over them an 
alcoholic or aqueous solution of certain dyes, and exposed to the spectrum, the resulting 
curves are modified in a marked manner. They almost exactly correspond to curves 
compounded of the absorption curve of the dye used and of the absorption curve of 
the unreduced silver bromide. For example, if we examine a simple bromide film 
with the spectroscope, it will be seen that an absorption takes place along the whole 
visible spectrum corresponding with the density curve of this compound. An exami¬ 
nation of the spectrum of eosine, supposing we are going to employ that dye to stain 
the film, gives an absorption band in the green, together with a less marked region of 
absorption in the blue and violet. If these two absorption curves be combined, we 
shall find that in the density curve obtained by exposing a stained film to the action 
of the spectrum we get a reproduction of this compound curve. Vogel explained 
this remarkable action by a theory which seemed to be contrary to our present idea 
of molecular physics ; and in my experiments conducted in this direction I had the 
good fortune to arrive at a more acceptable solution, involving no new laws of the 
action of radiation on matter. Some of the dyes employed (and most, if not all, of 
these belonged to the aniline series) could form compounds with silver, and when 
brought in contact with silver nitrate the action of light on the compounds was per¬ 
fectly intelligible. Vogel, however, pointed out that if a bromide film were perfectly 
freed from all silver nitrate by immersion in potassium bromide, then washed, next 
treated with tannin, and dyed, a similar density curve was obtained. I need scarcely 
recount the numerous experiments which I undertook; one of the final ones will be 
sufficient to show how a reasonable explanation could be offered. 
A few granules of the dye were taken and dissolved in normal collodion, a glass 
plate was coated in the ordinary manner, and the film dried and exposed to the 
spectrum. It was found that in those regions where the spectrum was absorbed 
a bleaching of the dyed film was evident. Thus, with eosine a bleaching took place 
in the green rays corresponding with its absorption band, and to a lesser degree in the 
yellow and blue rays. If after such an exposure the dyed collodion were coated in 
the dark room with collodion containing silver bromide in suspension, and an alkaline 
developing solution applied, it was found that the silver bromide was reduced to 
metallic silver on those parts of the plate which had been bleached by the action of 
the spectrum, and the density curve followed the curve of absorption of the dye. 
Photographers have been long aware of the fact that some sorts of organic matter 
on a glass plate will cause the reduction of silver on those portions of the plate on 
