SPECTRAL LINES 439 



and Adams, 1 and others, who have shown that the phenomenon 

 is a very complicated one. There are, in reality, two separate 

 effects involved : as the density of the substance in the arc is 

 increased the spectral lines are broadened, in some cases 

 symmetrically, in others very unsymmetrically towards the red ; 

 and superposed upon this broadening is a progressive displace- 

 ment of the lines towards the region of longer wave lengths. 

 Gale and Adams 2 divide the various spectral lines into five 

 main classes according to their behaviour under pressure : 



i. Lines which are symmetrically reversed. These lines are 

 the ones which are most readily and most accurately measur- 

 able, and in general are amongst the strongest lines in the 

 spectrum. 



2. Lines which are unsymmetrically reversed. These lines 

 are not so numerous as those belonging to the first class, and 

 the reversals are as a rule fainter. Most of the enhanced lines 3 

 belong to this class. 



3. Lines which remain bright and fairly narrow under 

 pressure. 



4. Lines which remain bright and symmetrical, but become 

 wide and diffuse under pressure. Most of the lines in the 

 metallic spectra belong to these two classes, whose distinction is 

 more or less arbitrary. 



5. Lines which remain bright and are widened very un- 

 symmetrically towards the red. These lines are almost all in 

 the yellow-red portion of the spectrum and are all enormously 

 displaced, but owing to the lack of symmetry and the extent of 

 the widening, it is difficult to measure the displacement with 

 any great degree of accuracy. 



They also found that most of the characteristics of the lines 

 in the arc spectra under pressure were retained in the spark 

 spectra, but that in the latter case the lines were much more 

 diffuse, and the accuracy of measurement was accordingly 

 correspondingly reduced. 



Experiment has shown that the amount of the displacement 

 is practically independent of whether the line is or is not 



1 Astroph. Journ. 35, p. 10, 1912. 



' Gale and Adams, ibid. p. 15. 



3 The enhanced lines are lines which appear in the spectrum when strong 

 spark discharges are used. Their presence indicates the characteristic difference 

 between the spark and the arc. 

 29 



