A Theory of the Action of Rays on Growing Cells. 263 



negative charge leaving the positively electrified ion behind it. This is the 

 sufficient explanation of many phenomena collected under the name of photo- 

 electricity. It has been ascertained that the velocity of the electron at the 

 moment of its expulsion is the greater the shorter the wave-length of the 

 radiation concerned. The swift-moving /3-rays represent the electronic dis- 

 charge excited by 7- and X-radiations. 



Some years ago I endeavoured to explain the nature of the events taking 

 place in the photographic film in terms of photo-electric activity. The theory 

 has recently been republished and amplified by Mr. H. Stanly Allen in his 

 book on photo-electricity. 



According to my view the latent image is formed of molecular systems 

 which have been subjected to loss of electrons and which remain as ions 

 positively charged in presence of these electrons, the nature of the medium 

 being responsible for the maintenance of the static attraction between 

 electron and ion. In development these ions and electrons are discharged, 

 and as a consequence of the chemical reaction thereby effected between the 

 developer and the ionised photo-system the metallic atom is liberated, 

 constituting the visible image. The phenomenon of the reversal of the latent 

 image under excessive light stimulus is well known. On the theory this 

 significant event is explained as the result of the increasing electrostatic 

 stress attending over-exposure, whereby ultimately the resistance to recom- 

 bination breaks down and the original molecular structure is restored. The 

 luminous stimulant will now begin to re-form the latent image. A succession 

 of such constructive and destructive effects is obviously possible according to 

 the theory, and is, in fact, matter of observation. The theory can be shown 

 to explain the facts respecting the different types of reversal as ascertained 

 by E. W. "Wood. Classifying the modes of formation of the latent image as 

 (1) by pressure, (2) by X-rays, (3) by light-shock (very brief flashes, as by 

 light from an electric spark), (4) by lamp light, Wood found that the latent 

 impression produced by any one of these can be reversed by subsequent 

 exposure to any other following it on the list but not by any one preceding 

 it. He found that Becquerel rays (7-rays) behaved like X-rays. For the 

 manner in which Mr. Allen applies the photo-electric theory to these obser- 

 vations I refer to his book. 



My object in referring to the photo-electric theory of photographic actions 

 is to show that on the assumption that growth in the cell, generally, is 

 attended and conditioned by ionic activity, there is sufficient resemblance 

 between the effects of stimuli on the plate and on the cell to lead to the 

 belief that there must be, physically, much in common between the actions 

 in each case. Prima facie the formation of the normal latent image by 



