174 CHAP. XX. EFFICIENCY OF DIFFERENT COLOURS 



piece we use two such pieces of red glass superposed one 

 over the other, we obtain the same red light, but there is a 

 considerable diminution of intensity. The result is that 

 the blue light under these circumstances is more effective 

 than the red. 



In order to avoid the indefiniteness in the result intro- 

 duced by the use of coloured screens, we have to employ 

 the solar spectrum itself, in which we obtain pure mono- 

 chromatic light in the different regions, the wave-length of 

 which is definitely known. A complication, however, arises 

 on account of unequal dispersion ; it is therefore necessary 

 to measure the energy of the different rays in the spectrum. 

 Both important elements in photosynthesis, namely the 

 wave-length and the energy of the particular ray, have there- 

 fore to be determined. An account of the successful solution 

 of this problem will be given in the next two chapters. 



Summary 



The results of previous researches on the photosynthetic 

 activity of the different rays are not concordant. Engel- 

 mann regards the curve of photosynthesis as exhibiting 

 two maxima, one in the red and the other in the blue ; 

 this conclusion is controverted by others. 



The important conditions in this research are : (i) a 

 pure monochromatic light ; (2) a sufficiently sensitive device 

 for the quantitative determination of photosynthesis ; and 

 (3) the simultaneous measurement of the energy of the 

 different rays. 



Coloured screens do not, generally speaking, give pure 

 monochromatic light. 



A new method of measuring photosynthesis is described : 

 that of recording the rate of the successive flotations of a 

 piece of a water-plant suspended in C0 2 -solution from one 

 arm of a balance. The photosynthetic production of a 

 bubble of oxygen causes flotation of the specimen, followed 

 by sinking on the bursting of the bubble at the surface. The 



