Page:Collected Physical Papers.djvu/348

328 of light multiplied by duration. The effect of increasing intensity of light has already been shown (cƒ. fig. 95). When the intensity is maintained constant, the amplitude of response undergoes an increase with the increased duration of exposure (fig. 100). But this increase does not go on indefinitely, for the continuous action of light causes a maximum negative response beyond which a decline sets in. It is probable that this is due to an opposing element which tends to neutralise the normal excitatory D-effect. The existence of this opposing A-reaction has already been seen in the transient after-effect in figure 97. Other results which I obtained, show that stimulus induces, in general, both the D- and A-effects; in excitable specimens the D-effect is predominant and therefore, masks the A-effect; the positive A-effect is, however, exhibited under feeble or even as an after-effect of strong stimulation.

Certain conditions, moreover, are specially favourable for the exhibition of the A-effect. When the green leaf has an abundant supply of chlorophyll, the photo-synthetic process of building up becomes specially marked. I have thus obtained under the action of light, a positive response with the green leaf of Luctuca