1901.] On the Strain Theory of Photographic Action. 



191 



cell by interrupted and continuous illuminations of the same total 

 duration. (See fig. 19.) Though light acted for the same length of 

 time in both cases, yet in that of interrupted illumination the mole- 

 cular effect as measured by the galvanometer deflection was only 

 seven divisions, whereas with continuous illumination the deflection 

 was 11 '5, or one and a-haif times as great. 





a> 





fifty 











pa 









Fig-. 19. — Effect of (a) intermittent and (b) continu. us illumination. In (a) there 

 are four interrupted illuminations of 15" each, the total duration being 1'. In 

 (b) there was continuous illumination for 1'. 



It is thus seen that owing to self-recovery, the effect of light with 

 intermittent illumination is less. It is also evident that the greater the 

 period allowed for self-recovery (during the interval of darkness) the less 

 will be the resultant effect. In connection with this, the experiments 

 of Abney are very interesting. In experimenting on the difference 

 between the effects on photographic emulsion of a continuous exposure 

 and a series of intermittent illuminations, he finds that in the latter 

 case the effect produced was always less, and that the longer the 

 interval between the exposures, the smaller was the effect. 



8. Photographic Effect Modified by Time-rate. 



It will thus be seen that the photographic effect is not solely 

 governed by the total amount of radiation, but by the time-rate also. 

 The influence of this factor appears to be exhibited in the three 

 following cases. Cases (2) and (3) derive an additional interest from 

 the fact that the effects are probably due not to absorption of 

 radiation, as is usually the case in photography, but to the emission of 

 radiation. 



(1.) In photographs of lightning, the line of discharge often comes 

 out dark (the so-called dark lightning). It has been shown that 

 reversals are produced by intense radiation; we may thus have 

 reversals of the first, second, and succeeding higher orders. Now it 

 is possible that the reversal, or the dark-lightning effect, may be 

 obtained, not only by a subsequent diffuse illumination (Clayden 

 effect), but also by the action of lightning itself, provided that the 

 intensity of illumination is sufficiently great and sudden to produce the 

 reversal. The luminous intensity of lightning discharge is incom- 



