366 



NA TURE 



[August 19, 1897 



•of the lines under different conditions, as observed by 

 Kayser and Runge (K. and R.) and myself (L.) in the 

 arc, and by Thal^n (T.) and myself in sparks : — 



Lilies of Iron which are enhanced in Spark. 



Wave- 

 length. 



4233-3 

 4508-5 



4515-5 



4520-4 



4522-8 I 



4549-6 



4584-0 



4924-1 



5018-6 



Intensity 



in 



flame. 



Intensity j Length in 

 '" °-^'^ ' arc r LI 



Intensity 

 in spark (T). 

 Max. =: 10. 



Intensity 

 in hot spark 



(L). 

 Max. = 10. 



hottest stars are shown in the diagram on p, 368, 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 Ononis (Fig. 16). 



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 tempera- 

 ture. 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 pro- 

 duced 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. 



Fig. 14. — The spectroscope observatory at Baikal in 1871. 



Combining this with former results, we seem justified 

 in concluding that, in a space heated to the temperature 

 of the hottest spark, and shielded from a lower temper- 

 ature, these lines would constitute the spectrum of iron. 



To enforce what I have previously written concerning 

 the value of the solar work in relation to the study of 

 the physics and chemistry of the stars, it is worth while 

 to consider for a moment the behaviour of these lines 

 of iron which are found to brighten as the temperature 

 is increased and which play such an important part in 

 the chromosphere spectrum, in stellar spectra (Fig. 15). 



Defining the hottest stars as those in which the ultra- 

 violet spectrum is most extended, it is known that absorp- 

 tion 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 

 NO. 145 1, VOL. 56] 



i We may say broadly that the stars Bellatrix, a Cygni, 

 and Arcturus represent three very different stages of 



! star life from the point of view we are considering. 



In Bellatrix the metallic lines, both enhanced and cool, 



I are almost entirely absent. In a Cygni we get the 

 enhanced metallic lines alone ; in Arcturus they are 

 generally absent ; this statement is true for the sun, 

 the spectrum of which is almost identical with that 

 of Arcturus. 



Now it has been found from the study of the photo- 

 graphs of the chromosphere obtained in 1893 and 1896, 

 that among the bright lines recorded the enhanced lines 

 hold a most important place. I have already given 

 copies of two of the photographs obtained in 1893. I 

 can now add untouched copies of an enlargement of one 

 of the photographs obtained in 1896, which has quite 

 recently been published by the Royal Society. The 

 photograph was obtained by Mr. Shackleton, attached 



