x.] THE "SERIES" EVIDENCE. 89 



Series Nomenclature. 

 Intensity. Kayser and Runge. Rydberg. 



The strongest lines which they observed at the temperatures they 

 worked with, they put into what they call a " principal series," and 

 then the weaker lines were distributed among other two series. 

 Kayser and Eunge called them the " first- " and " second-subordinate " 

 series ; Rydberg calls them the " nebulous-series " and the " sharp- 

 series." The lines of the principal series almost always reverse 

 themselves very easily indeed that is to say, that the absorption is 

 indicated by them more readily than it is by the other lines. When 

 we come to the second subordinate or sharp series, it is found that 

 these sometimes broaden out towards the red end of the spectrum. 



This work, of course, has required considerable investigation ; 

 the first attempts were not quite satisfactory, because the observations 

 on which they were based had not been of sufficient accuracy. With 

 greater dispersion it has been found that some of the lines which were 

 supposed at first to be single are really double ; so that it is quite usual 

 now when we consider this question of series to suppose that in some 

 cases the series are composed of single lines, in other cases of doubles, 

 and in other cases of triplets ; and it was at first, indeed, imagined that 

 in these differences we were face to face with a very important physical 

 difference between the various elements, but Rydberg has suggested 

 that possibly after all it may be a difference merely in the seeing. 



He says :* 



" The difference between the doubles and triplets is only relative. 

 This opinion is confirmed by the fact that the triplets appear often 

 in the form of doubles, the most refrangible component not having 

 sufficient intensity to become visible. Further, the relative intensity 

 of the components of the doubles seems equal to that of the two less 

 refrangible components of the triplets. 



" For these reasons I have dared to propose the hypothesis that 

 the two kinds of component rays are of the same order, or that 

 the doubles are only triplets of which the most refrangible com- 

 ponent is too feeble to be seen, or has perhaps the absolute value of 

 zero. . . ." 



* Kon. Sv. Vet. Ale. Hand., vol. xxiii, ii, p. 135. 



