CHAPTER XXI 



DETERMINATION OF THE ENERGY OF THE DIFFERENT 

 RAYS OF THE SOLAR SPECTRUM 



Securing a pure solar spectrum— Sliding of the spectrum on to the plant 

 by a reflecting mirror — The Magnetic Radiometer — Distribution of 

 energy in the spectrum. 



The investigation of the relative photosynthetic effective- 

 ness of different wave-lengths of light involves the securing 

 of a pure solar spectrum, and the determination of the 

 energy of the different rays. I first describe the method of 

 obtaining a pure spectrum. 



The Pure Solar Spectrum 



For photosynthetic investigation the spectrum has to be 

 fairly enlarged so that the plant may be exposed to rays 

 which are approximately monochromatic. It is quite easy 

 to do this by placing the plant at a considerable distance 

 from the prism, but the enfeebled light is then ineffective 

 for photosynthesis. A compromise has therefore to be 

 made as regards the breadth of the spectrum, by the employ- 

 ment of a carbon disulphide prism. I secured a spectrum 

 the visible size of which was 18 cm. from the extreme 

 red at one end to the violet at the other ; it was also found 

 to be continued into the invisible infra-red region for a 

 distance of about 6 cm. 



The plant-vessel with the attached Bubbler had to be as 

 narrow as possible : a test-tube having a diameter of i cm. 

 was used for the purpose. 



The employment of the carbon disulphide prism accounts 



