230 Intelligence and Miscellaneous Articles. 
It is still incomplete, and the leading facts only are mentioned here 
to take date. 
Not all colouring-matters are capable of uniting with the silver- 
salts, but the number of those that do so unite is considerable. 
What is curious and tends to show that the combination is intimate, 
is that the colour assumed by the silver-salt is not always that of 
the dye, but may differ from it considerably. Also the three silver- 
salts may be differently coloured by one and the same colouring- 
matter. 
More frequently, however, colouring-matters impart their own 
shade or something approaching to it. Thus, silver bromide preci- 
pitated in presence of excess of silver nitrate takes from aniline- 
purple a strong purple colour; from cardinal-red a bright flesh or 
salmon-colour ; from naphthaline-yellow a light yellow colour ; from 
eosin a brilliant pinkish or salmon; and so on. 
Different specimens of the same colour gave sometimes quite 
different results: thus, silver bromide precipitated in presence of 
silver nitrate was dyed by one specimen of methyl-green to a biuish 
ereen. Another specimen of the same colour obtained from a dif- 
ferent source coloured the same silver-salt a deep purplish shade. 
Silver iodide showed the same difference. 
Sixteen years ago I proposed to colour or stain the photographic 
film in order to modify its behaviour towards light, principally to 
prevent blurring or irradiation™. 
Of many colouring-matters then tried, the best proved to be 
litmus coloured red by acetic acid. ‘This was very effectual for the 
purpose, and was long used by others as well as myself. So far as 
T have been able to ascertain, it was the first suggestion made of 
this mode of acting on the sensitive plate. Since then, staining 
the film has been found to have other applications; and many others 
have experimented in this direction, in most cases with a view to 
alter its sensitiveness relatively to the different colours of the 
spectrum. Major Waterhouse was, I believe, the first to recognize 
this effect. 
Dr. John W. Draper appears to have first advanced the view 
that substances sensitive to light are affected by the rays which 
they absorb. There is much to support this theory, although it 
cannot be considered as definitely established. 
Some years since Dr. H. W. Vogel expressed the opinion that, 
when sensitive films were washed over with solutions of colouring- 
matters, the films gained sensitiveness to those rays of the spectrum 
which the colouring-matters absorb, with this condition—that the 
colouring-matter in question must be capable of combining with Cl, 
Br, or I, as the case might be. My own results were different. I 
found that the action of the rays was profoundly modified by 
colouring the film; but the result did not seem to conform to any 
law, and as often contradicted Vogel’s view as agreed with it. 
Vogel’s theory necessitates the assumption that the colour 
* British Journal of Photography, 1868, pp. 210, 506; 1870, p. 145 &e, 
