CHAPTER XX 



DETERMINATION OF THE PHOTOSYNTHETIC EFFICIENCY OF 

 LIGHT OF DIFFERENT COLOURS 



Contradictory views regarding the rays effective in photosynthesis — In- 

 vestigation by the Method of Flotation and the Torsion Balance — New 

 method of recording photosynthetic activity under light of different 

 colours — Characteristic differences in response under different colours 

 — Effect of intensity of radiation. 



Having in a previous chapter determined the effect of in- 

 creasing intensity of light on photosynthesis, there remains 

 the question of the relative effectiveness of light of different 

 colours. For the solution of this problem we require (i) 

 monochromatic light of sufficient intensity to be physio- 

 logically effective ; (2) a sensitive method of measuring the 

 photosynthetic activity under a nairow beam of mono- 

 chromatic light in a pure spectrum ; and (3) the accurate 

 measurement of the energy of the different rays. To obtain 

 monochromatic light, liquid screens have been frequently 

 employed, notably solutions of bichromate of potash for the 

 yellow, and the ammoniated solution of copper sulphate 

 for the blue rays. But the light transmitted through the 

 bichromate solution is not pure red, but contains green also ; 

 similarly, copper sulphate solution allows the transmission 

 not only of the blue but also of green rays. Additional 

 screens may be used to cut off the impurities in the mono- 

 chromatic light, but this procedure causes so great a 

 diminution in the intensity of the light that it fails to 

 produce any photosynthesis. 



The spectrum of sunlight obtained by a prism has been 

 employed for similar investigations, but its use is attended 

 by various complications. The dispersion produced is 



