DISCOVERY 



45 



brightness exceeding 1,000,000 candles per sq. in., and 

 a temperature of 7,600° abs. — probably the highest 

 temperature yet attained by artificial means. 



However, even assuming that such a temperature 

 could be obtained in a commercial illuminant, we 

 should still, apparently', only usefully employ about 

 half the energy as light. But there is another possi- 

 bility of improvement. These data are calculated for 

 conditions where the substance heated to incan- 

 descence is black in colour. But it is beUeved 

 that white or grej- materials may e.xercise " selective 

 radiation," i.e. emit a higher proportion of radiation 

 in a luminous form than that derived from a black 

 body at the same temperature. The incandescent 

 mantle and the glower of the Xernst lamp exhibit 



spectrum, but only light of selected wave lengths. 

 Since the colour of light is determined bj' the length 

 of the ether-waves which cause the light, it may be 

 of peculiar colour — illustrated in the complete absence 

 of red in the light of the mercury lamp, and the de- 

 ficiency of green, blue, and violet in that of neon lamps. 

 The colours of objects illuminated by such sources are 

 naturally much distorted. 



Can more Efficient Illuminants be Produced ? 



At present, therefore, we are only feeling our way 

 towards complete success in light-production. For 

 most purposes we require an approximately white 

 light giving to objects around us their true colours. 



Fig. j.— showing some EXPERIilEXT.Ai NIGHT-LIGHTING AT THE CROYDON AERODROME 

 One of Uie special lighting units, introduced by Major J. P. .\shley Waller, is to be seen on tbe left. This is mounted on a tripod and utUises a i,ooo- 

 watt gas-filled electric lamp, equipped with a dioptric refractive lens to concentrate light below the horizontal and evenly illuminate the ground 

 surface. The photograph was taken entirely by the light of two of these units, and the buildings are seen to be brightly illuminated. 



selective radiation in some degree, but, generally 

 speaking, the effect is not very pronounced in solid 

 illuminants. 



Luminescence 



In order to obtain more definite selective radiation, 

 and so a\oid the energy-waste of heat, we may excite 

 metallic vapours or gases by an electric discharge — a 

 process which is termed ' ' luminescence. ' ' The mercury 

 vapour lamp and the flame arcs cored with suitable 

 metallic salts are instances of luminescence, and are 

 comparatively efficient sources of light. The new 

 " Osglim " lamps, utilising the orange glow of rarefied 

 neon gas when subjected to electric discharge, again 

 illustrate the same effect. But this method of pro- 

 ducing -light, while conducive to higher efficienc\', 

 has its drawbacks. One does not obtain a continuous 



If we could devise an ideal source yielding white light, 

 and no useless invisible radiation such as heat, it might 

 yield as much as 26 candle-power per watt, or unit 

 of energj' — a value about six times as high as that of 

 the most efficient illuminant at present available. 



This, however, by no means exhausts the possi- 

 bOities of research. There are cases (for instance in 

 lighthouse work) where w'e are indifferent to colour and 

 only wish to produce maximum brightness. Now the 

 radiation in the extreme red and violet of the spectrum 

 are barely visible, and the great impression of bright- 

 ness is produced by the yellow-green, in the middle of 

 the spectrum. If our source of light yielded only light 

 of this wave-length, it might give us 55 or even 

 65 candles per watt. This would apparently be the 

 ideal from the pure standpoint of brightness. The 

 goal may be reached by research from some unexpected 



