LIGHT AND VEGETATION 39 



this phenomenon as it occurs in pure, dry air, containing 

 neither dust nor water vapour. 



It is caused by the molecules of nitrogen and oxygen in 

 the atmosphere. The weakening of the direct rays by diffusion 

 can be calculated exactly; it is quite neghgible for the infra- 

 red but it increases rapidly as the radiations become shorter. 



In their vertical passage through a pure and dry atmosphere, 

 the red rays would lose one-tenth of their intensity, the blue 

 rays of wave-length 0-45 /x two-tenths, the ultra-violet rays 

 of wave-length 0-3 /x nearly seven-tenths. 



If the thickness of air traversed is greater, the quantity 

 transmitted becomes smaller. Thus, the blue radiation, which 

 is reduced to 0-8 of its initial value after the vertical passage 

 through the atmosphere or, in other words, through an air 

 mass m=l, is further reduced in the same ratio if it has to 

 traverse an equal thickness again, so that its intensity becomes 

 (0-8)2=0-64 for m=2; it falls to (0-8)3=0-512 for m=3; and, 

 in general, it becomes (0-8)™ times its initial value. 



When the sun is very low on the horizon, m assumes large 

 values and the blue is almost completely diffused; since only 

 radiations of long wave-length are transmitted, the sun 

 appears red. 



For the ultra-violet the diffusion is still greater; at a high 

 altitude, the reduction of the thickness of air to be traversed 

 enables the ultra-violet rays to be felt with more intensity. 

 On the other hand, the sinking of the sun on the horizon and 

 the consequent increase of the air mass m rapidly reduces the 

 quantity of ultra-violet reaching the ground. If for m=l the 

 transmission is 0-3, for m=2 it will be (0-3)2=0-09, for m=3 

 it will be (0-3)3=0-027, and so on. 



Absorption by Gases 



Molecular diffusion disperses the radiation but does not 

 absorb it; only a part of the diffused Hght, which returns to 

 the space beyond the atmosphere, is lost. The rest reaches the 

 ground after diffusion ; this is the blue hght of the clear sky, 

 light whose spectrum includes all the solar radiations although 



