Page:On the Continuity of Effect of Light and Electric Radiation on Matter.djvu/20

1901.] 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 negative effect followed by the normal action. Under molecular modification 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