476 On the Iron Lines present in the Hottest /Stars. 



Defining the hottest stars as those in which the ultra-violet spec- 

 trum is most extended, it is known that absorption is indicated by 

 few lines only. In these stars iron is practically represented by the 

 enhanced lines alone ; those which build up, for the most part, the 

 arc spectrum are almost or entirely absent. 



The intensities of the enhanced lines in some of the hottest stars 

 are shown in the appended diagram, and, for the sake of comparison, 

 the behaviour of a group of three lines which are among the most 

 marked at lower temperatures, is also indicated. In addition, the 

 diagram shows the inversion in intensities of the spark and arc 

 lines in the spectrum of a relatively cool star such as a-Orionis. 



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The facts illustrated by the diagram indicate that the enhanced 

 lines may be absent from the spectrum of a star, either on account of 

 too low or too high a temperature. In the case of low temperature, 

 however, iron is represented among the lines in the spectrum, but at 

 the highest temperature all visible indications of its presence seem 

 to have vanished. 



This result affords a valuable confirmation of my view, that the 

 arc spectrum of the metallic elements is produced by molecules of 

 different complexities, and it also indicates that the temperature of 

 the hottest stars is sufficient to produce simplifications beyond those 

 which have so far been produced in our laboratories. 



