586 LIFE MOVEMENTS IN PLANTS 



indicator for the varying intensity of light during the 

 course of the day. 



THE PHOTOMETRIC RECORDER. 



This diflBculty I have been able to overcome by the 

 automatic device for continuous record of the variation 

 of light. The electric resistance of a selenium cell under- 

 goes diminution with the intensity of light that falls on 

 it. The photo-sensitive cell was made the fourth arm of a 

 Whefatstone bridge, the resistance of the cell being exactly 

 balanced when the shutter of the sensitive cell was closed. 

 The selenium receiver was pointed upwards against the 

 sky. Precaution was taken that it was protected from the 

 direct action of sunlight. On opening the shutter a deflec- 

 tion of the index of a sensitive galvanometer was produced, 



Fig 213 — Photometric record showing variation of intensity of^light from morning 

 to evening. Successive dots are at intervals of 30 minutes. 



and the deflection increased with increasing intensity of 

 diffuse skylight. The special difficulty was iu securing 

 automatic record of the galvanometer deflections, This 



