PROF. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. Ill 



called the Principal series is formed on the D sequence, and what has been called 

 the D on the F sequence. The set belongs to what has been here called the negative 

 type. The first, the so-called Principal, is the only one in which the formula does 

 not reproduce all the lines. It is possible that it requires a larger N, as is required 

 for the P lines in the other singlet series discussed, viz., He'. 



9. As we pass down through the alkali metals we find a change of type occurring, 

 first in Na, and then in Li. In Na, what is called the D series, has a /JL much closer 

 to the F type than to the D type. There seems to be a certain instability, and a 

 large number of extra lines appear whose development appears to depend on narrow 

 limits of temperature, amongst these are possibly three lines which show the D type 

 normal to the other elements. In Li all the normal type have disappeared, and we 

 get a singlet system depending on a new kind of D sequence for the Principal series, 

 and the F sequence for the D series. In addition, we get lines conforming to the D 

 type based on the new D sequence. Also there are indications of the normal D 

 series in a doublet spectrum, with the doublet separation to be expected in the 

 normal type. But no P or S lines of the normal type have been seen. 



10. In He the singlet sets of series are of the P type, with the associated D based 

 on the F sequence. The doublet sets are of the D type with the associated D also 

 based on the F type. These spectra have been used to determine accurate values of 

 N by the method of least squares. The important result conies out that the value 

 of N is not the same for each series. It is practically 109(575 for the 1)', D", and P" 

 sets, 109725 for S' and S" (probably higher if the lines S (2) had been employed in 

 the determination), and 109810 for P'. 



The relations found strengthen the evidence in favour of Lord HAYLEIGH'S theory 

 as to the kinematical origin of these spectra. A general consideration of questions 

 relating to origin is, however, postponed until the spectra of the other elements are 

 discussed in a future communication. 



