184 CHAP. XXI. THE ENERGY OF THE SPECTRAL RAYS 



that the different rays could be thrown on to the stationary 

 plant-vessel and on to the adjoining sensitive strip of the 

 Radiometer quickly, one after the other. It was thus 

 • possible to make a simultaneous determination of the 

 photosynthetic activity and of the energy of any particular 

 ray of light. 



The energy of radiation was determined from the elonga- 

 tion of a blackened strip of zinc, the expansion being 

 magnified some 50,000,000 times by the Magnetic Radio- 

 meter. The Radiometer is capable of detecting a rise of 

 temperature of T q o!oTro° C. 



The maximum deflection of the Radiometer was attained 

 within a few seconds, and the stoppage of radiation was 

 followed by an exact return to zero in a dead-beat manner. 



Radiation was detected at some considerable distance 

 beyond the extreme red. The maximum energy was found 

 to be in the infra-red at 790 jxjx, after which, towards the 

 more refrangible blue and violet, the energy of the rays 

 exhibited a continuous decline. 



