LIGHT AND VEGETATION 27 



when the sun is at the zenith and the atmosphere is pure and 

 dry. 



It is represented by the lower curve of Fig. I, 7. Note that the 

 apex of this curve occurs in the middle of the visible region. 

 On the ultra-violet side we find the limit imposed by the 

 absorption of ozone, at 2,880 A, which has already been 

 mentioned in connection with the transparency of the air. 

 The minima situated in the infra-red (the magnitudes of which 

 vary with the angle of the sun's rays and therefore with the 

 thickness of air traversed) are also due to atmospheric 

 absorption. The absorption bands of carbon dioxide and of 

 water vapour can be identified. 



Taking this absorption into account, it has been possible 

 to calculate the spectrum of sunlight before it has passed 

 through the atmosphere. This spectrum is represented by the 

 upper curve of Fig. I, 7. As we move towards the infra-red, the 

 power of the solar radiation diminishes, but the radiation 

 extends further than can be shown by the scale of the curve. 



Although the apex of the curve is in the visible region, a 

 httle more than half of the solar emission is composed of 

 infra-red radiations; the rest is radiated in the visible band 

 except a little more than a hundredth part in the ultra-violet. 



These results have enabled us to calculate the temperature 

 of the sun's surface, which is about 6,000° C. 



The electric arc used to be favoured for street lighting; 

 it is a source of artificial light of high eSiciency, that is to say, 

 it converts into radiation a large part of the electrical energy 

 which is used to supply it. The specially luminous part is the 

 extremity of the carbon rod connected to the positive pole of 

 the electricity supply, for it is there that the temperature is 

 highest— about 3,400° C. 



The curve giving the composition of the radiation from the 

 arc resembles that for the sun, but it is displaced towards the 

 long wave-lengths ; the infra-red emission is proportionately 

 greater, to the detriment of the visible and especially of the 

 ultra-violet. Nevertheless, the arc is one of the sources which 

 most nearly approaches sunlight and the possibiUty of making 



