July 27, 1905J 



NA TURE 



309 



We must first ask from what substances can light 

 liberate the electron. There are many — metals as well as 

 non-metals and liquids. It is a very general phenomenon, 

 and must operate widely throughout nature. But what 

 chiefly concerns the present consideration is the fact that 

 the haloid salts of silver are vigorously photo-electric, and, 

 it is suggested, possess, according to Schmidt, an activity 

 in the descending order bromide, chloride, iodide. This 

 is, in other words, their order of activity as ionisers (under 

 the proper conditions) when exposed to ultra-violet light. 

 Photographers will recognise that this is also the order of 

 their photographic sensitiveness. 



Another class of bodies also concerns our subject : — the 

 special sensitisers used by the photographer to modify the 

 spectral distribution of sensibility of the haloid salts, e.g. 

 cosine, fuchsine, cyanine. These again are electron-pro- 

 ducers under light stimulus. Now it has been shown by 

 Stoletow, Hallwachs, and Elster and Geitel that there is 

 an intimate connection between photo-electric activity and 

 the absorption of light by the suljstance, and, indeed, that 

 the particular wave-lengths absorbed by the substance are 

 those which are effective in liberating the electrons. Thus 

 we have strong reason for believing that the vigorous 

 photo-electric activity displayed by the special sensitisers 

 must be dependent upon their colour absorption. You will 

 recognise that this is just the connection between their 

 photographic effects and their behaviour towards light. 



There is yet another suggestive parallel. I referred to 

 the observation of Sir James Dewar as to the continued 

 sensitiveness of the photographic film at the lowest attained 

 extremes of temperature, and drew the inference that the 

 fundamental photographic action must be of intra-atomic 

 nature, and not dependent upon the vis viva of the molecule 

 or atom. In then seeking the origin of photographic 

 action in photo-electric phenomena we naturally ask. Are 

 these latter phenomena also traceable down to low tempera- 

 tures? If thev are, we are entitled to look upon this fact 

 as a qualifying characteristic or as another link in the 

 chain of evidence connecting photographic with photo- 

 electric activity. 



I have quite recently, with the aid of liquid air kindly 

 supplied to me by Mr. Moss, and made in the laboratory 

 of this societv, tested the photo-sensibilitv of aluminium 

 and also of siK-er bromide down to temperatures approach- 

 ing that of the liquid air. The mode of observation is 

 essentially that of Schmidt — what he terms his static 

 method. The substance undergoing observation is, how- 

 ever, contained at the bottom of a thin copper tube, 5 cm. 

 in diameter, which is immersed to a depth of about 10 cm. 

 in liquid air. The tube is closed above by a paraffin 

 stopper which carries a thin quartz window as w'ell as 

 the sulphur tubes through which the connections pass. 

 The air within is verv carefully dried by phosphorus 

 pentoxide before the experiment. The arc light was used 

 as source of illumination. It was found that a vigorous 

 photo-electric effect continued in the case of the clean 

 aluminium. In the case of the silver bromide a distinct 

 nhoto-electric effect was still observed. I have not had 

 leisure to make, as yet, any trustworthy estimate of the 

 percentage effect at this temperature in the case of either 

 substance. Nor have I determined the temperature 

 accuratelv. The latter mav be taken as roughly about 

 -1:^0" C. 



Sir James Dewar's actual measurements afforded twentv 

 per cent, of the normal photographic effect at — 180° C. and 

 ten per cent, at the temperature of — 252°-5 C. 



With this much to go upon, and the important additional 

 fact that the electronic discharge — as from the X-rav tube 

 or from radium — generates the latent image, I think we 

 are fully entitled to suggest as a legitimate lead to experi- 

 ment the hypothesis that the beginnings of photographic 

 action involve an electronic discharge from the light 

 sensitive molecule ; in other words that the latent image is 

 built up of ionised atoms or molecules the result of the 

 photo-electric effect on the illuminated silver haloid, upon 

 which ionised atoms the chemical effects of the developer 

 are subsequently directed. It may be that the liberated 

 electrons ionise molecules not directly affected, or it mav be 

 that in their liberation they disrupt complex molecules built 

 up in the ripening of the emulsion. With the amount we 

 have to go upon we cannot venture to particularise. It 



will be said that such an action must be in part of the 

 nature of a chemical effect. This must be admitted, and, in 

 so far as the re-arrangement of molecular fabrics is in- 

 volved, the result will doubtless be controlled by tempera- 

 ture conditions. The facts observed by Sir James Dewar 

 support this. But there is involved a fundamental process — 

 the liberation of the electron by the electric force in the 

 light wave, which is a physical effect, and which, upon 

 the hypothesis of its reality as a factor in forming the 

 latent image, appears to explain completely the outstanding 

 photographic sensitiveness of the film at temperatures far 

 below those at which chemical actions in general cease. 



Again, we may assume that the electron-producing power 

 of the special sensitiser or dye for the particular ray it 

 absorbs is responsible, or responsible in part, for the 

 special sensitiveness it confers upon the film. Sir Wni. 

 .Abney has shown that these sensitisers are active even if 

 laid on as a varnish on the sensitive surface and removed 

 before development. It must be remembered, however, 

 that at temperatures of about —50° these sensitisers lose 

 much of their influence on the film. [See a paper by me 

 read before the convention in 1894.] 



It appears to me that on these views the curious 

 phenomenon of recurrent reversals does not present a 

 problem hopeless of explanation. The process of photo- 

 ionisation constituting the latent image, where the ion is 

 probably not immediately neutralised by chemical com- 

 bination, presents features akin to the charging of a 

 capacity — say a leyden jar. There may be a rising potential 

 between the groups of ions until ultimately a point is 

 attained when there is a spontaneous neutralisation. I 

 may observe that the phenomena of reversal appear to 

 indicate that the change upon the silver bromide 

 molecule, whatever be its nature, is one of gradually in- 

 creasing intensity, and finally attains a maximum when a 

 return to the original condition occurs. The maximum is 

 the point of most intense developable image. It is probable 

 that the sensitiser — in this case the gelatin in wliich the 

 bromide of silver is immersed — plays a part in the con- 

 ditions of stability which are involved. 



Of great interest in all our considerations and theories 

 is the recent work of Prof. Wood on photographic reversal. 

 The result of this work is — as I take it — to show that the 

 stability of the latent image may be very various according 

 to the mode of its formation. Thus it appears that the 

 sort of latent effect which is produced by pressure or 

 friction is the least stable of any. This may be reversed 

 or wiped out by the application of any other known form 

 of photographic stimulus. Thus an exposure to X-rays 

 will obliterate it, or a very brief exposure to light. The 

 latent image arising from X-rays is next in ortler of in- 

 creasing stability. Light action will remove this. Third 

 in order is a verv brief light-shock or sudden flash. This 

 cannot be reversed by any of the foregoing modes of stimu- 

 lation, but a long-continued undulatory stimulus, as from 

 lamp-light, will reverse it. Last and most stable of all is 

 the gradually built-up configuration due to long-continued 

 light exposure. This can only be reversed by overdoing 

 it according to the known facts of recurrent reversal. 

 Prof. Wood takes occasion to remark that these pheno- 

 mena are in bad agreement with the strain theory of 

 Mr. Bose. We have, in fact, but the one resource — the 

 allotropic modification of the haloid — whereby to explain 

 all these orders of stability. It appears to me that the 

 elasticity of the electronic theory is greater. The state 

 of the ionised system may be very various according as it 

 arises from continued rhythmic effects or from unorganised 

 shocks. The ionisation due to X-rays or to friction will 

 probablv be quite unorganised, that due to light more or 

 less stable according to the gradual and gentle nature of 

 the forces at work. I think we are entitled to conclude 

 that on the whole there is nothing in Prof. Wood's beau- 

 tiful experiments opposed to the photo-electric origin of 

 photographic effects, but that they rather fall in with 

 what might be anticipated. 



When we look for further support to the views I have 

 laid before you we are confronted with many difficulties. 

 I have not as yet detected any electronic discharge from 

 the film under light stimulus. This may be due to my 

 defective experiments, or to a fact noted by Elster and 

 Geitel concerning the photo-electric properties of gelatin. 



NO. 1865, VOL. 72] 



