LIGHT AND VEGETATION 29 



limited sense and meaning "capable of making an impression 

 on the human eye and giving it the sensation of light") of a 

 monochromatic radiation by the product of two factors, one 

 being its energy and the other its visibility factor. 



When the radiation is complex, like that of an incandescent 

 lamp, the products thus obtained for each wave-length are 

 added together. The sum calculated in this way is, by definition, 

 the luminous intensity of the source and the units created 

 specially for this purpose are photometric units. 



These photometric units are used almost exclusively in the 

 measurement of radiation because for a long time only 

 hghting appHcations were considered. 



The usual method of describing the radiation of a lamp 

 by the number of lumens is therefore particularly incomplete 

 and inadequate. It obviously has no value for those who 

 wish to use infra-red or ultra-violet. 



Long-wave Terrestrial and Atmospheric Radiation 



Our planet receives a large quantity of power from the 

 sun. Its emission contains visible and infra-red radiations of 

 short wave-length and almost the whole of its spectrum is 

 below 5 jLt. This is radiation, or light in the broadest sense, 

 emitted by a star whose temperature is of the order of 6,000°C. 



The property of emitting radiation is not pecuUar to the 

 sun; on the contrary, it is absolutely general. Every material 

 body radiates round itself. We are bathed in the radiation of 

 the objects which surround us and we send out our own 

 radiation to them. 



The intensity of the radiations increases with temperature; 

 we feel it as our hand approaches a warmer object than our 

 surroundings. We feel also, as a sensation of cold, the 

 reduction of the radiation in the neighbourhood of a cooler 

 body which is substituted for and masks the warmer bodies 

 in front of which it is placed. 



The ground therefore also radiates and grows cooler as it 

 loses the energy that it emits. The pure atmosphere and the 

 clouds behave in the same way; they send radiant energy 



