154 



Mr. J. Norman Loekyer. 



indicated by the fact that its spectrum shows the whole series of 

 hydrogen lines, but by the fact that there is bright continuous radiation 

 far in the ultra-violet." 



We shall not go far wrong in supposing that the star with the 

 most intense continuous radiation in the ultra-violet is the hottest, 

 independently of absorbing conditions, which, in the absence of 

 evidence to the contrary, we must assume to follow the same law 

 in all. 



An inquiry into the facts placed at our disposal by the stellar 

 photographs taken at Kensington, shows that there is a considerable 

 variation in the distance to which the radiation extends in the ultra- 

 violet, and that the stars can be arranged in order of temperature on 

 this basis. 



Judged by this criterion alone, some of the hottest stars so far 

 observed are 7 Orionis, Orionis, a Virginis, 7 Pegasi, q Ursae 

 Majoris, and X Tauri. Of stars of lower, but not much lower, tem- 

 perature than the above, may be named Rigel, £ Tauri, a Andromeda?, 

 /3 Persei, cc Pegasi, and /3 Tauri. 



Comparison of Terrestrial and Stellar Line Spectra. 



Having, then, this independent determination of relative tempera- 

 ture, we are in a position to study the results of the comparison of 

 terrestrial and celestial line spectra. 



As the variations of spectra with temperature are very striking in 

 the case of iron, calcium, and magnesium, the spectra of these sub- 

 stances afford a ready means of checking the relative temperature of 

 the absorbing region of stars as determined by a consideration of 

 the extension of the continuous radiation into the violet and ultra- 

 violet. 



But, before the results of such inquiries can be properly used, some 

 general considerations have to be passed under review. 



The Loci of Absorption in Stellar Atmospheres. 



Assuming that the most valid absorbing vapours in any particular 

 star are all near one temperature, we can proceed to investigate the 

 origins of the spectrum lines by first getting a clue as to the probable 

 temperature from the extent of continuous spectrum, and then 

 inquiring into the presence or absence of the lines which are longest 

 in the spectra of various substances at that temperature. If, how- 

 ever, the absorptions take place at different levels in the atmosphere 

 of a star, the proper spectrum of each substance to be thus investi- 

 gated can only be determined by a comparison of the stellar with 

 the terrestrial lines of the substance under varying temperature con- 

 ditions. 



