Page:On the Continuity of Effect of Light and Electric Radiation on Matter.djvu/14

1901.] 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''Loc. cit. 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 continued 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$3$O$4$ 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