23 



nitrate. The effect of this double decomposition was there- 

 fore to cause an insoluble bromide of silver to be intimately 

 blended with every particle of the collodion in which the 

 original bromide had been dissolved, while the new nitrate 

 (which was fortunately readily soluble) remained to be elimi- 

 nated by either washing the film in water after the plate 

 had been coated, or by a more complicated process of 

 evaporating the emulsion to dryness and then washing the 

 pellicle to which it had thus been reduced, and re-emulsifying 

 it by the addition of fresh ether and alcohol. I need only 

 advert further to the development of the latent picture on a 

 collodion emulsion plate, when produced by exposure in the 

 camera. 



The theory of development is no doubt the key to all 

 photographic processes. I will not venture to do more than 

 refer briefly to the methods by which the extension and 

 stimulation of the invisible action of light on silver bromide 

 are attained. I must observe in passing that although I 

 have only spoken of bromide I must not imply that it alone 

 possesses the properties which I have described. The other 

 haloid elements, viz., chlorine and iodine, give also much 

 the same results, but from various causes bromine has taken 

 and seems likely to keep the lead. To return, — the de- 

 velopment of what has been called the latent image, (though 

 Capt. Abney, the leading authority on the subject objects to 

 this term,) has been effected in two ways, called respectively 

 physical and chemical development. The former of these 

 worked somewhat crudely though with exquisite results on 

 wet plates, by abstracting the oxygen from the free silver 

 nitrate which was always present on those plates, and 

 depositing the metallic silver on the film. The reason for 

 this deposition following with such exact fidelity the required 

 lights and shadows of the picture, was the formation, by the 

 action of the light in the camera, of an unstable compound 

 termed asu6-bromide of silver, containing two atoms of silver 

 to one of bromine, instead of equal proportions of each as 

 in the original bromide. This new compound necessarily 



