Page:Collected Physical Papers.djvu/366

346 algebraical summation of the effects of light and of temperature.

It is sometimes desirable to carry out researches during a period. when the intensity of light remains approximately constant. This period is found to be between 11 a.m. and 1 p.m. for the variation is then only ±5 per cent. of the mean.

The record given of the diurnal variation of light is true of days when the sky is clear. But the passage of clouds causes change in the intensity which is accurately recorded by the Radiograph. A record of such irregular variation in a stormy day is given in the lower record of figure 107. (Life Movements in Plants, Vol. IV, 1923.)