Electroluminescence 257 



In brief Whiston considered the auroral display as " a kind of im- 

 perfect unripe Lightning." 



In general the idea of a " thin Nitro-sulphureous Vapor," which 

 " by Fermentation takes fire " was the current opinion well into 

 the eighteenth century, as expressed by John Rowning in " A Com- 

 pendious System of Natural Philosophy " (1735) . Rowning held that 

 the great arch of an aurora was the center of burning exhalations, 

 the streamers of light were " Streams of Fire," the trembling was 

 due to " the Quickness wherewith the Flashes succeed one another," 

 the great height to " the exceeding Lightness of the Effluvia," and 

 the various spatial configurations to " motions of the air and to 

 perspective." " When the matter of the aurora is so far spent, as 

 to emit no more Streams, it appears only as a bright steady Light in 

 the North, which gradually dies away, for want of fresh Fuel to 

 support it." 



EDMUND HALLEY 



A quite different and a novel point of view from the above was 

 taken by Edmund Halley (1656-1724) , the great English astronomer 

 and mathematician, popularly known for his prediction of the re- 

 turn of " Halley's comet " of 1680. Stimulated by a request of the 

 Royal Society to look into the " Etiology " of the " lights in the air," 

 the great auroral display of March 6, 1716, Halley (1716) rejected 

 as the " Material Cause " the then current idea that they were " the 

 Vapour of Water rarified exceedingly by subterranean Fire, and 

 tinged with sulphureous Streams. ..." He proposed that they re- 

 sulted from " Effluvia of a much more subtile Nature . . . the 

 Magnetical Effluvia, because the distribution of light was along the 

 lines of magnetic force from a spherical magnet," a " Terrella," such 

 as the earth." These magnetic effluvia might " be capable of pro- 

 ducing a small Degree of Light . . . after the same manner as we 

 see the Effluvia of Electrick Bodies by a strong and quick Friction 

 emit Light in the Dark: to which sort of Light this seems to have 

 a great Affinity." 



Halley's view is remarkably modern. Although he compared the 

 corona to the field of a spherical magnet, he did not emphasize in 

 his 1716 paper the fact that the center of the corona corresponded 

 in direction to the dipping needle, a point later stressed by Halley 

 in 1726 " and by de Mairan in 1747.^^ His most important contribu- 

 tion is the comparison of the aurora to magnetic and electric effluvia. 



^' Halley had previously postulated that the earth was a double terella, a globe- 

 magnet inside an outer magnet " as the Shell includes the Kernel." 

 ^*In the Journal of the Royal Society, Nov. 10, 1726. 

 ^^ In an article in the Memoires of the French Academy for 1747. 



