28 LIGHT, VEGETATION AND CHLOROPHYLL 



it function in the free air, without a glass globe, allows its 

 ultra-violet emission to pass unhindered. 



The light from incandescent lamps is emitted by a 

 tungsten filament, raised to a high temperature by the passage 

 of the current, and enclosed in a bulb containing an inert 

 gas. The filament is designed to remain at a lower temperature 

 than its melting point, which is 3,117° C. Its temperature is 

 therefore lower than that of the electric arc ; generally it lies 

 between 2,600° and 2,800° C. The distribution in the spec- 

 trum of the energy radiated differs from the preceding by a 

 displacement still nearer to the long wave-lengths; the energy 

 emitted in the ultra-violet and the visible violet is very small. 

 On the other hand, the greater part of the energy is radiated 

 in the red and especially in the near infra-red. 



Photometric Units 



We have seen that all radiation can be decomposed, by 

 means of a monochromator, into a series of pure radiations 

 of known wave-lengths. In giving the power of the radiation 

 of each wave-length, we can describe its physical nature in a 

 complete and unambiguous fashion. A curve, continuous or 

 discontinuous, is therefore necessary to characterize it and 

 we have thus described the radiation of the sun. 



When, for a definite purpose, only a small part of the 

 spectrum is usable, it is not essential to know the other parts 

 and a knowledge of the energy radiated in the useful part is 

 then sufficient. 



An example of considerable importance is that of the 

 visible radiation appUed to artificial fighting. The part of the 

 usable spectrum lies between the wave-lengths of 3,900 and 

 7,500 A approximately, but in this rather extensive band the 

 same energy emitted in radiations of different wave-lengths 

 has a fighting efficacy proportional to the very variable 

 luminosity factor of these wave-lengths; the values of these 

 factors are given in Fig. I, 6. 



It has therefore been agreed to represent the ^'luminous" 

 intensity ("luminous" this time being taken in the strictly 



