PHOTOGRAPHY. 25 



being absorbed by another portion of the sub-chloride, with 

 which it combines to form the white chloride. The minute 

 atoms of reduced silver are shrouded by its whiteness, and 

 we have the consequent appearance of the bleaching of the 

 brownish-coloured film. 



The cause of the colours in the paper-print and in the 

 plate requires explanation. The fact that when the unaltered 

 compounds on either of them are dissolved away, the colour 

 vanishes, leaving only that due to silver itself throws a light 

 on the subject. 



In a soap bubble the beautiful colours which overspread 

 its surface are caused by the interference of the light re- 

 flected from the outer and inner surfaces which are micro- 

 scopically near each other, and it may be that the colours pro- 

 duced by the spectrum are the results of the interference of 

 the light reflected from the surface of the reduced particles 

 which are held apart by intervening silver chloride. 1 After 

 dissolving out the latter, the particles are brought in contact 

 and the colour disappears. 



In the Exhibition we have photographs in colour, re- 

 presenting the solar spectrum, by Becquerel, and one of 

 my objects in leading up so far as I have was to try and 

 give you an explanation of the method of their production. 

 On a bright silver surface silver sub-chloride was formed by 

 voltaic or other means, and a spectrum was caused to fall on 

 a plate so prepared imprinting itself in all its colours. If 

 exposed to the light these spectra fade away and leave nothing 

 behind but a bluish brown plate. Hence it is that they are 

 preserved in closed light-tight cases, and can only be rarely 

 exhibited. 



The cause of an action taking place by the impact of the 

 red and yellow rays has already been pointed out ; and the 

 vividness of the colours can also be readily accounted for 

 by the same explanation as given to account for those on 

 our paper and collodion film, when it is remembered that 

 there is the reflecting surface of the silver plate itself to aid 

 the interference. 



Hitherto we have only spoken about silver compounds 

 being sensitive to light, but nearly all matter is sensitive 

 in one respect or another. Most probably the first action 



1 Silver chloride is really a white transparent substance, as may be 

 proved by fuzing it in a crucible. 



