502 Scientific Proceedings, Royal Dublin Society. 



silver. The process is, then, one of reduction on the whole. But it is 

 effected in two stages. First the photo-electric action ; then the chemical 

 action of the developer or reducing agent on the latent image. 



The view that the process of formation of the latent image is founded in 

 the expulsion of the electron under the photo-electric force is supported by 

 many circumstances.' The halides of silver are independently known to be 

 strongly photo-electric. The special sensitizers (certain aniline dyes) which 

 are used to increase the sensitiveness of the film for particular spectral 

 regions are also energetically photo-electric, and, moreover, they are specially 

 active towards those rays wliicli they absorb, and for which they sensitize 

 the film. Again, the latent image formation by radiation is taken out of 

 the category of chemical actions by the fact that it takes place at the lowest 

 extremes of temperature, at which all chemical activity ceases.^ 



The fact last referred to supports the view that translatory molecular 

 movements are not involved in the initial formation of the latent image. 

 What may be described as a state of static ionisation is set up, the discharged 

 electrons creating negative ions by attachment to the surrounding gelatine. 

 The electrons, emanating cliiefly from the electro-negative chlorine of the 

 photo-electric molecule, collect around the gelatiue-silver-bromide systems, 

 forming an electro-negative region wliich may be regarded chemically as 

 comparable to an increase in negative ionic concentration. This view is 

 supported by tlie chemistry of development so far as this is known. 



The developer acting at this stage — i.e. when tlie latent image has been 

 formed — finds the process of reduction facilitated by these conditions. 

 For the developer is essentially a reducing agent possessing generally a 

 concentration of negative hydroxyl ions ; that is, it is alkaline in cliaracter : 

 and the latent image represents a temporary release of the positive silver ion 

 from attachment to the chlorine ion, the latter having lost its charge. 



The latent image is, in short, one stage in tlje reduction of the complex 

 silver bromide molecule. But it is a stage reached by physical means, and 

 owes its stable character to the solid nature of the medium in whicli it is 

 immersed. Nevertheless, it runs down in course of time and disappears ; tlie 

 negative electron gradually being attracted back to the central positive 

 system, and recombining with the chlorine whence it came. The process 

 of destruction of the latent image may be accelerated by over-exposure to 

 light, X or 7 rays. This is the phenomenon of reversal or solarisation. 

 The probable explanation is tliat by continued exposure the electrostatic 



' Joly: "Nature," vol. Ixxii, July 27, 1905, p. 308. 

 » Joly : Pioc. Eoy. Dublin Soc, toI. viii, 1894, p. 222. 



