1901.] Effect of Light and Electric Radiation on Matter. 173 



back to its original condition. The successive starting-points are 

 therefore slightly ascending. In the last curve in the series (a) 

 sufficiently long time was allowed, and the substance completely 

 recovered in about 37 seconds, (b) shows the effect of light of about 

 four times the intensity, the source being brought nearer to a distance 

 of 6 inches. The effect is stronger, but not quite equal to four times 

 the effect produced in the last case. 



10. Effect of continuous Action of Light. 



Just as in the case of electric radiation, light produces a maximum 

 effect, corresponding to a given intensity. Fig. 9, (c), shows the effect 

 of continuous light of 30 seconds duration, the source of light being 

 at a distance of 18 inches. Observe the tendency of the curve to 

 become horizontal when reaching the maximum, (d) shows the effect 

 of four times the intensity, the light source being at a reduced distance 

 of 9 inches. Here, too, the intensity of effect is not quite four times 

 the effect of light at a distance of 18 inches. 



11. Reversal Effects. 



With electric radiation it was found that the effect of a flash was 

 sometimes a transitory negative twitch immediately followed by the 

 normal response. With light, too, it is frequently found, as has been 

 observed by Minchin, that the immediate result is a transitory nega- 

 tive effect followed by the normal action. Under molecular modifica- 

 tion there may sometimes be seen a reversed response. 



If the action of light is continued, after the maximum effect is 

 reached, there is produced a tendency towards, or actual reversal. 



In fig. 10 is given an interesting series of results showing the growth 

 of fatigue, the different phases culminating in actual reversal. (Compare 

 with fig. 6.) It will be seen that in the second response was feebler, and 









































































































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Pig. 10. — Fatigue and reversal in AgBr cell. 

 VOL. LXX. n 



