CHAPTER XIII 



THE AUTOMATIC RADIOGRAPH . 



The Selenium Cell — -Elimination of effect of polarisation — The Wheatstone 

 Bridge for balance — The automatic keys for periodic closure of circuit 

 — The Galvanograph — Automatic record of hourly variation of day- 

 light — Record on a cloudy day — Simultaneous records of variation of 

 daylight and temperature — Light-noon and thermal noon. 



The most important requirements in the quantitative 

 investigation of photosynthesis are, as previously stated, 

 the maintenance of uniform intensity of light, and a means 

 of effecting the quantitative increase of the intensity. 



The illumination to which plants are accustomed is 

 that of direct sunlight and of diffused light from the sky ; 

 we shall in this chapter confine our attention to these 

 natural sources. Both sunlight and sky light are liable to 

 variation, and there is no means at present available by 

 which we may be assured that the light remains constant 

 during the period of an experiment. This difficulty is 

 specially great when the period of observation is prolonged. 

 Our power of judging variations in the intensity of light is, 

 unfortunately, very defective. In making observations on 

 photosynthesis under the light of the sky it frequently 

 happens that the bubbling-rate undergoes variation though 

 the light appears to be unchanged. By means of the 

 Electric Photometer, which will be presently described, it 

 can, however, be shown that the variation in photosynthesis 

 in such a case is really due to a change in the intensity of 

 light, caused by the passage of impalpable mist undetected 

 by the eye. That a slight variation of light may cause 

 considerable change in' photosynthesis is shown by slowly 

 moving the plant through a short distance from a position 

 near the window to the interior of the room. In such an 



