Beversah in Spectn())i Pliotojraplis. 583 



fouml that as the duration o£ the sliock was increascMl the 

 reversals bf'canie weaker, tlie images finally tailing to appear 

 at all on the })late. notwiih-tandino- the longer duration of 

 the flash. On still further increasing the duration the images 

 came out not reversed. There may be some connexion be- 

 tween the condition in which the light shocks fail to develop 

 at all, and the zero condition of the plate described by 

 Nijdier. 



The experiment in which the fogging of the plate was 

 effected by exposure to ultra-violet light furnished by a quartz 

 spectrograph shows that the time factor plays a role in the 

 ioo^gino-lioht as well as in the lioht-slnck. 



It was f 'und that if the fot^ging lio-ht was rather intense 

 but of short duration, the image of the light-shock did not 

 reverse ; if the light was less intense, but of a little longer 

 duration, no trace of the shock appeared, while if the light 

 was still less intense, and of somewhat longer duration, the 

 image came out reversed. With a suitable ratio of intensities 

 and durations of time, it is possible to superpose two impres- 

 sions on a photographic plate, only one of which appears on 

 development. 



This effect is shown in fig. 3, Plate XXY T. A series of spark- 

 images of equal intensity was impressed on the plate, which 

 was then fogged in sections, the lower strip being exposed 

 to the light of one spark at a distance of a metre, the next to 

 the light of four sparks at a distance of two metres, the next 

 to nine sparks at three metres, and so on. The total amount 

 of fogging-light was thus approximately the same in each 

 case, though it was found that considerably greater action 

 was produced by a large number of sparks at a cjnsiderable 

 distance, than by a single spark close to the plate. It will 

 be noticed that on the third strip from the bottom there is 

 scarcely a trace of the spark-images. The ratio of the times 

 of duration of the shock and the fogging-light was in this 

 case about 1 : 9. On the two strips below this one, the sparks 

 appear not reversed, while on all of the strips above reversal 

 has taken place. 



A more careful quantitative investigation of these effects 

 is much to be desired, with apparatus of such design that the 

 duration and intensitv of both the light-shock and fogo-ino-- 

 light can be accurately controlled. 



I am of the opinion that the result of such an investigation 

 would be the establishment of the fact that with very l)rie£ 

 and intense light-shocks, comparatively intense fogging-light 

 of short duration will yield reversals, while in the case of 

 shocks of say 1/1000 sec. duration, the fogging-light must be 



202 



