Page:Collected Physical Papers.djvu/209

Rh 12 inches. Previous maintenance of the plates in the dark enhances the sensitiveness; the cell then responds to feeble light emitted by a candle. The similarity of responses to light and electric radiation is very remarkable (cf. fig. 35). The recovery curve is also convex to the abscissa. The time allowed for recovery was not sufficient to bring the substance 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.

Just as in the case of electric radiation, light produces a maximum effect, corresponding to a given intensity. Fig. 40 (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 source of light 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.

With electric radiation it was found that the effect of a flash was sometimes a preliminary negative twitch immediately followed by the normal response. With light, too, it is frequently found, as has also been