THE SELENIUM CELL 99 



experiment the variation of light as perceived by the eye 

 is very slight, yet the effect on assimilation is very pro- 

 nounced, for the vigorous photosynthesis near the window 

 becomes slowed down and arrested. 



In utilising sunlight and its dependent light from the 

 sky in experiments on photosynthesis, it is therefore 

 necessary to find the rate at which daylight undergoes 

 variation from hour to hour. In doing this we discover a 

 turning-point in the intensity of light, where the variation 

 is from positive to negative, from an increase to a decrease. 

 At this turning-point the variation of intensity of light is 

 very slight ; hence this is the most suitable period for 

 photosynthetic observations. 



For the determination of the hourly variations of day- 

 light, and of the turning-point beyond the maximum in- 

 tensity of light, I have devised a special instrument, the 

 Radiograph. 



The Selenium Cell 



Any method for obtaining a record of the intensity of 

 light and its variations depends on utilising the property of 

 some substance sensitive to light. Selenium is well known for 

 the characteristic diminution of its electric resistance under 

 illumination. A definite deflection is produced in a galvano- 

 meter when the selenium cell is placed in the dark in series 

 with a battery of voltaic cells. Exposure to light, causing 

 a diminution of resistance, gives rise to an increased deflec- 

 tion. The variation in the deflection of the galvanometer 

 thus indicates the variation in the intensity of light. The 

 selenium cell is relatively more sensitive to the less refran- 

 gible rays of the spectrum. I will show in a later chapter 

 that the less refrangible rays are likewise more effective 

 in photosynthesis. Hence the selenium cell is quite suit- 

 able for the measurement of the effective rays. 



Several difficulties are encountered in practice in obtain- 

 ing a continuous record of the intensity of light during the 

 day. The resistance of selenium undergoes a change under 



