THE AURORA BOREALIS 211 



was noticed. Different observers gave : Angstrom, 5568 ; 

 Vogel, 5572; Vijkander, 5573; Lemstrom, 5570; Huggins, 

 5570-4; Copeland, 5573; Gyllenskjold, 5569; Campbell, 

 5571-6; Sykera, 5570; the Danish Mission to Iceland in 

 1899-1900, 5570. Many other lines have been photo- 

 graphed, of which more hereafter ; but this line is extra- 

 ordinarily intense, and, indeed, can often be seen when 

 there is no visible aurora by simply directing a pocket 

 spectroscope towards the north. The line when first 

 observed was not known to be characteristic of the 

 spectrum of any element. 



In 1898 I had the honour to announce to the Royal 

 Society the discovery, in conjunction with my assistant, 

 Dr. M. W. Travers, of the existence of three new elemen- 

 tary substances in the atmosphere, to which we gave the 

 names neon, or ' the new one ' ; krypton, or ' the hidden 

 one ' ; and xenon, or ' the stranger.' 



The spectrum of neon is characterised by many lines 

 in the red, orange, and yellow ; while that of xenon 

 shows many green and blue lines. The light evolved 

 from tubes containing these gases under low pressure 

 when an electric 'current of high tension is passed through 

 them, is of a corresponding hue ; thus neon sends out a 

 splendid rose or flame-coloured light ; and xenon, a sky- 

 blue ; while the light of krypton is nearly white, although 

 seen by some of a pale lilac, and by others of a pale-green 

 colour. 



The densities of the elements proved to be as had been 

 expected : that of neon, compared with hydrogen taken as 

 2, is 20 ; of krypton, 82 ; and of xenon, 128. 



Shortly after the discovery of krypton, my assistant, 

 Mr. Baly, measured carefully the wave-lengths of its more 

 important lines ; and one of these, a very brilliant green 

 line, had the wave-length 5570*5. The day after this 

 was published, Sir William Huggins wrote me privately 



