50 LIGHT, VEGETATION AND CHLOROPHYLL 



This measured illumination is proportional to the flux 

 entering into the enclosed chamber, whatever its distribution 

 in direction may be, but the inner walls must be covered with 

 a coat of paint, perfectly mat and diffusive, very white and 

 having a high reflecting power. 



Some laboratories are equipped with apparatus for making 

 these measurements, adapted either to the visible or to the 

 ultra-violet wave-lengths. 



For the infra-red, the low sensitivity of the thermocouple, 



Monochromatic light 



Photoelectric cell 



Leaf 



Fig. I, U . Diagram of a method of measuring the reflection 

 factor of a leaf for monochromatic light and a certain angle 



of incidence 



the receiver generally used, renders the problem almost 

 insoluble. 



We can see, therefore, that the problem of making photo- 

 metric measurements on the leaf is clearly posed and that the 

 most eff"ective methods are known. On the other hand, the 

 monochromator and the diff'usive sphere are great wasters of 

 hght and the illuminations to be measured are so small that 

 it is difficult to evaluate them correctly. 



That is why results have sometimes been obtained for a 

 few isolated wave-lengths that can be produced with sufficient 

 intensity like those of the spectral Unes of mercury; some- 

 times it has been decided to abandon the use of purely mono- 

 chromatic light or to measure the diff"used light with less 



