1901.] Effect of Light and Electric Radiation on Matter. 167 



tion of radiation, now there was a short period of hesitation and then 

 it began to recover somewhat slowly. The extent of recovery also 

 grew less and less, and at last the receiver suddenly exhibited the 

 reversal effect, by showing a diminution of resistance. 



A parallel instance under the continued action of light will be noticed 

 later on. 



6. Phenomena of Reversal. 



(a.) Reversal due to Sub-normal Intensity of Stimulus. — Another very 

 curious phenomenon met with is the opposite effects of radiation below 

 and above the critical intensity. Thus I have shown* that, whereas 

 under certain conditions the effect of radiation of moderate intensity 

 on As is to produce an increase of resistance, the effect of feeble intensity 

 of stimulus is to produce a diminution. Exactly parallel, though 

 opposite, effects are sometimes seen produced in the positive class of 

 substances. This result is certainly very curious, but I will show 

 later on that exactly similar effects are produced under mechanical 

 stimulus. 



Possibly connected with the above is the following : When a receiver 

 is subjected to radiation of moderately strong intensity, I have often 

 noticed a short-lived negative twitch immediately followed by the 

 normal response. This is probably due to the fact that it takes some 

 time for the sensitive substance to absorb the whole amount of incident 

 radiation. The first moiety absorbed may thus fall below the critical 

 intensity, hence the preliminary negative twitch, while a little later, on 

 the absorption of the whole amount, we get the normal response. 

 Thus under the continued action of radiation, the response curve 

 exhibits a negative twitch at the beginning followed by the normal 

 positive effect (see also fig. 17). 



(b.) Reversal due to Overstrain. — In addition to the above, I have also 

 shown* that reversal effects are produced by overstrain due to the con- 

 tinned effects of radiation ; and these reversals may be partial or 

 complete. This depends on the nature of the substance, and also on 

 the adjustments. I give below a curve (see fig. 7) for Fe 3 4 under 

 continuous radiation, where, after the maximum effect was reached, 

 there was a distinct trend towards reversal. 



Under certain conditions, we may thus have in a positive substance 

 an increase of resistance or negative effect under feeble radiation ; this 

 is specially seen when the receiver had been undisturbed for a long 

 time, and the substance had undergone certain unknown molecular 

 modification. Annealing makes the responses normal ; or under 

 moderately strong intensity, the abnormal negative response becomes 

 changed into positive, to be again reversed (or tend to be reversed) 

 under strong and continued action of radiation. All these peculiarities 



* Loc. cit. 



