24 



LIGHT, VEGETATION AND CHLOROPHYLL 



Before we can know the composition of the radiation trans- 

 mitted by a coloured filter, obviously we must know that of the 

 initial radiation; this is a fact which should not be overlooked. 



Sensibility of the Eye to Light 



For a rather small part of the radiations that we are 

 considering — but a part nevertheless which is important 

 because of its properties — the human eye is an extraordinarily 

 sensitive and flexible detector. A given flux of monochromatic 

 light entering the eye under given conditions produces a 

 sensation which depends quaUtatively and quantitatively on 

 the wave-length of the radiation. 



0-565 0'62 

 A. 0-4 0-44 0-49 \0-595/ 075 ^L 



Ultra- 

 violet 



Violet 



Blue 



Green 



>- 



0) 



no 



c 

 a 



o 



Red 



Infra- 

 red 



V 750 682 



612 / 504\ 400 



531 484 



Fig. I, 5. Scale of colours of the visible spectrum 

 A : wave-length in microns 

 v: frequency in lO^^ cycles per second 



The colour varies in a continuous fashion with the wave- 

 length, but we have become accustomed to distinguish the 

 following colours: violet, blue, green, yellow, orange and 

 red, with their boundaries marked as shown in Fig. I, 5. 



Wave-lengths shorter than 3,900 A are almost invisible; 

 when they are intense they produce a slight and diffuse 

 luminous impression of a bluish colour. 



Wave-lengths longer than 7,500 A are still visible, if they 

 are of sufficient intensity, but their luminosity rapidly 

 decreases as the wave-length increases. 



Two fluxes with equal energy but of different wave- 

 lengths cause diff'erent sensations, since the colour is not the 

 same. We can, however, more or less disregard the colour 

 and say that one of these fluxes is "more luminous" or "less 

 luminous" than the other. It is therefore possible to evaluate 



