504 Scientific Proceedings, Royal Dublin Society. 



a gelatine dry plate in the dark and, after a few minutes, is again washed 

 ofF, it will be found that a developable latent image has been formed. Strong 

 pyrogallic acid solution powerfully affects tlie plate in tlie same manner, even 

 without the addition of alkali. Alkaline tannin gives the same result, but as 

 it renders the gelatine almost impermeable, the effect is superficial and feeble, 

 sometimes inappreciable. It is an active absorbent of the halogen. Tobacco 

 smoke bubbled through water gives a solution which is intensely active in 

 forming the invisible image. 



As opposed to the concentration of the negative hydroxyl ion in promoting 

 development, the action of the positive hydrogen ion upon the latent image is 

 to inhibit its growth, or reverse it if already formed by light or otherwise. 

 This result is made apparent by introducing a very weak solution of a 

 mineral acid. The oxidation of the reduction product, or the rehalogenisation 

 of the partially reduced silver, may be involved, according to the nature of 

 the acid used. The feature common to all acid intervention is the increased 

 concentration of the positive hydrogen ion. 



The photo-sensitive molecular system of the film can exist in different 

 states of sensitiveness, ranging from a highly sensitive to a relatively insen- 

 sitive state. The behaviour of the " ripened " emulsion of the fast plate (i.e. 

 of an emulsion wliich has been subjected to a process of prolonged heating) 

 is similar in kind to that of the " unripened " emulsion of the slow plate, 

 but in the former all the phenomena are relatively advanced. The latent 

 image is sooner formed under a given exposure, and much more readily 

 reversed. Chemical effects are correspondingly accelerated. The grain of the 

 sensitive or ripened film is much coarser than that of the slow or unripened 

 film. 



We now turn to the living cell. 



We find that radiation may, if carefully modulated, stimulate, and, if too 

 intense, retard its growth and ultimately destroy the molecular structure 

 required for mitosis. In so far the effects on the growth of the cell — 

 superficially, at least — resemble those on the formation of the latent image. 



It is also found — and as already stated the whole efficacy of radioactive 

 treatment turns upon this — that in the case of the pathological cell these 

 plienomena appear all in advance of the like effect in the normal cell. There 

 exist then states of the cell differing in sensitiveness towards radiation just 

 as there exist differing states of the film. 



The accelerated mitosis and growth of the pathological cell appear in 

 some cases to be traceable to repeated mechanical stimuli. This is parallel 

 with the formation of the latent image by similar stimuli. 



Finally, the destruction of the pathological cell is said to be brought about 



