586 Prof. II. W. Wood on FhotograjMc 



I next ascertained that i£ the p^ate he exposed simul- 

 taneously to light and X-rays, the latter inhibit the action of 

 the former. A candle and an X-ray tube were set up at 

 some little distance apart in front of a plate, the latter being 

 much nearer the plate however, owing to its less energetic 

 action. An iron rod mounted in front of the pLite cast two 

 shadows upon the sensitive film, one a light shadow, the oiher 

 an X-ray shadow. After the double exposure the p'ate was 

 developed, with the curious result that one shadow was darker 

 than the background, the other lighter, showing that the 

 light was more energetic in its action on the area screened 

 from the X-radiation. 



It may be worthy of mention that both this result and the 

 preceding one were predicted before the actual experiments 

 were tried. The prediction was the result of an attempt to 

 apply Bose^s strain theory of photographic acion* to the 

 phenomena in question. This theory seemed rather promising 

 at first, especially as it enabled me to predict new phenomena, 

 but it failed to account for so many things that I was finally 

 forced to abandon it. 



The action of other stimuli was next investigated. It has 

 long been known that pressure-marks on the film can be 

 developed. I found that if the plate was fogged by lamp- 

 light before developing, the pressure-marks came out reversed. 

 It then occurred to me try the effect of light shocks on 

 pressure-marks, and I found to my surprise that the fiash of 

 a single spark was as effective in reversing the pressure- 

 mark as the exposure to the lamp. The pressure-marks can 

 also be reversed by exposure to X-rays. 



As a result of numerous other experiments, I finally found 

 that if we arrange the stimuli in the following order, 

 pressure-marks, X-rays, light-shock, and lamp-light, an im- 

 pression of any one of them can be reversed by subsequent 

 exposure to any other following it in the list, but under no 

 circumstances by any one preceding it. For example, 

 pressure-marks can be reversed by any of the other three 

 stimuli, while X-ray images are only reversed by light-shock 

 and lamp-light. 



Experiments with Becquerel rays have given rather un- 

 certain results. Pressure-marks can be reversed by them, 

 and they in turn can be reversed by lamp-light, but these 

 were the only two cases in which reversals were obtained, 

 which makes it difficult to fit the rays into the series and still 

 have the rule hold. 



These experiments show that the effects of the different 

 kinds of stimuli on the sensitive film are quite different. 

 * J. (J. Bose, Proc. Roy. Soc. Jane 19, 1902.' 



