Page:Collected Physical Papers.djvu/201

Rh

(a) Effect of Sub-minimal Intensity of Stimulus.—Another very curious phenomenon met with is the opposite effects of radiation below and above the critical intensity. I have in a previous paper shown that, whereas the effect of radiation of moderate intensity on As is to induce an increase of resistance, the effect of feeble intensity of stimulus is to produce a diminution. Positive classes of substances exhibit parallel results. The opposite effects of feeble and strong stimulation are exhibited not only under electric radiation, but also under mechanical stimulus (cf. p. 204). This result is certainly very curious, but I will show later on that exactly similar effects are produced under mechanical stimulation.

Possibly connected with the above is the following: A receiver subjected to radiation of moderate intensity, often exhibits 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, and thus gives the preliminary twitch of opposite sign to the normal which occurs later by the absorption of a larger amount of radiation. The response-curve thus exhibits a negative twitch which precedes the normal.

(b) Reversal due to Overstrain.—I have also shown that overstrain due to the continued action of radiation gives rise to a reversal of response, the reversal being partial or complete. I give below a curve (fig. 38) for Fe3O4 under continuous radiation, where, after the