Page:Collected Physical Papers.djvu/365

Rh An important point arises as regards the diurnal variation of light and of temperature, and determination of their periods of maximum and minimum. For this purpose records of diurnal variations of temperature and of light were taken on the same day in summer with the

thermograph and the radiograph. The two curves are given in figure 108; it will be seen that while the maximum intensity of light is at 12 noon, the thermal maximum is at about 2 p.m. The thermal noon is thus two hours later than the light-noon. Light disappears at night from 6 p.m. to 6 a.m., that is to say, the period of minimum or zero light is prolonged for 12 hours. But the fall of temperature is gradual, and the minimum is attained at about 5 a.m. which is the thermal dawn. The characteristic variations of these two important factors should be borne in mind, since the diurnal movements of plants are modified by the