44 



DISCOVERY 



under Ramesses IX, the modern authorities forced the 

 thieves to disclose the place. Thus, nearly twenty- 

 nine centuries after the first interment of the earliest 

 among them, the faces of Egypt's kings and emperors 

 were disclosed to the modern world. ' ' ' 



Lord Carnarvon and Mr. Howard Carter's discovery 

 would in any circumstances have been regarded as of 

 immense importance, both from the historical and the 

 artistic standpoint ; but to one who keeps before his 

 mind's eye the history of the Theban necropolis — 

 one long tale of plunder and destruction — to such an 

 one the value of the discovery and the good-fortune 

 of the discoverers seem things that it is scarcely 

 possible to over-estimate. 



Artificial Light — its 



Production and 



Application 



By J. S. Dow 



.\mong the advances of the past century few are more 

 remarkable than those associated with the production 

 and application of artificial light. In these days of 

 abundant illumination we can hardly picture the 

 limitations imposed by inadequate lighting in the past. 

 Not only is light far more abundant ; it is also much 

 cheaper than in the days when only oil-lamps and 

 candles were available. We are still accustomed to 

 think of the wa.x-candlc as the poor man's illuminant. 

 We forget how feeble is the light it yields. It has been 

 calculated that the cost of i,ooo candle-hours yielded 

 by a wax-candle is appro.ximately 12s. 6d., whereas 

 with modern electric lamps we can obtain the same 

 output of light at less than the cost of one unit of 

 electricity, say about 6d. Some of the principles 

 underlying the production of light, and our hopes of 

 future progress in this respect, may therefore interest 

 readers of Discovery. 



Incandescent Lights 



Hitherto illuminants have been based mainly on 

 incandescence. We heat up a solid substance until 

 forced vibrations of the crowded electrons take place, 

 and amidst the jumble of radiations emitted a small 

 portion is perceived to be luminous. Even the most 

 efficient incandescent illuminants only emit in the form 

 of visible light a very small proportion (probably not 

 exceeding about 5 per cent.) of the energy supplied 

 to them. The remainder is chiefly useless heat-energy. 

 1 Breasted, A History of Egypt (London, 1906), pp. 552 foil. 



Now it was shown long ago by Planck, Lummer 

 and Pringsheim,- and others that as the temperature 

 of incandescence rises, so the proportion of visible 

 radiation emitted by a " black body " increases. It 

 has been calculated that at the temperature of 

 the sun as much as 50 per cent, of the radiation 

 might be emitted in a luminous form. Therefore 

 attempts have been continually made to increase 

 the temperature of the incandescent illuminants. 

 The higher temperature attained in the tungsten 

 filament nearly trebled the efficiency of the carbon 

 filament lamp ; in the gas-filled (" half-watt ") lamp 

 it was again found possible to double the efficiency 

 by running the filament at a still higher temperature. 

 Similar recent advances in the efficiency of incandescent 

 gas burners are based mainly on securing more com- 

 plete combustion, a hotter flame, and higher luminosity 

 of the mantle. In ordinary artificial illuminants we 

 have not yet been able to attain the temperature of the 



Fu;. I.— THE ,\RE.^S OF THESE CONCENTRIC CIRCLES REPRESENT 

 THE .\PPROXIM.\TE LUMINOUS EFFICIENCY OF V.^RIOUS 

 SOURCES OF LIGHT, AS FOLLOWS : 



Source. 

 " Ideal " Yellow-Green Light 

 ■■ Ideal " White Light . . 



Flame Arc I^amp 



Gas-liUed Tungsten Filament Lamp 

 Vacuum Tuncjsten Filament Lamp 

 Carbon Filament Lamp 



Caudle-power per Watt. 

 65 

 16 



4-5 

 1-5-2 

 ■ 0-75-I 

 . 0-25-0-35 



sun. But Lummer,' working with an arc between carbon 

 electrodes — the same principle as that employed in 

 arc lamps for street and shop lightmg — but under very 

 high pressure, is stated to have obtained a crater- 



- For a summary of these and other researches see 

 Praktische Photometrie, by Dr. E. I.iebenthal, pp. 43-55. 

 3 Lichtiechnik, by L. Bloch, p. 113. 



