LIGHT AND VEGETATION 



25 



for each wave-length a luminosity (or visibility) factor. Thus, 

 the wave-length of 5,500 A has the highest luminosity factor 

 and is conventionally given the value of 1; to produce a 

 similar luminous impression (although of a different colour) 

 with the wave-length of 4,200 A, 250 times more energy is 

 required. The luminosity factor for this wave-length is there- 

 fore 1/250 or 0-004. 



I 



OA 



Fig. I, 6. 



0'7M^ 



Mean curve of relative visibility factors for the 

 human eye 



The curve in Fig. I, 6 gives the luminosity factors for the 

 different radiations of the spectrum ; these factors have been 

 adopted by photometry laboratories all over the world and 

 have been ratified by international agreements. They are, as a 

 matter of fact, rather arbitrary, for they depend on individuals, 

 conditions of observation, etc., and it has been necessary, in 

 order to reach agreement, to fix a mean conventional curve. 



Composition of Solar and other Radiations 



The study of radiations and of their quantitative com- 

 position, which is obviously of fundamental importance for all 



