Page:Collected Physical Papers.djvu/354

334 quickened. The plant is such a sensitive detector of light that it may be employed as a photometer for indicating the slightest variations in the intensity of skylight.

For this determination I took successive records from 7-30 a.m. until 5 p.m. for five minutes at a time. When the sun rose at 6-45 a.m. the light was too feeble to be effective. At 7-30 a.m. assimilation began, and the plant evolved four bubbles of oxygen in the course of five minutes; with the progress of the day it became