16 THE ROYAL SOCIETY OF CANADA 



that A = 2833 -17 A°U was the first member of the series n = (l-5, S)- 

 (2,p2) and A = 2170-50 A°U the first member of the real principal 

 series n = (1 -5, S) — (m, P). On trying out this idea it was found that 

 a number of strong wavelengths in the emission spectrum of lead 

 could be fitted into a series of this type. These are given in Table V. 



Table V 



As regards this series the first two members are absorbed by non- 

 luminous lead vapour, the wavelength A = 1821 -7 A°U is a prominent 

 one in the Schumann region and A = 1790 A°U is very close to the wave- 

 length A =1796-5 A°U, which also comes out strongly in the emission 

 spectrum of lead. The limit of the series, it will be seen, also approxi- 

 mates to that demanded on the quantum theory by Mohler, Foote 

 and Stimson's value of the ionisation potential. One objection to 

 the series is that the intensity of the line A = 2053 A°U is weaker than 

 that of the other numbers of the series in the spark and vacuum arc 

 spectra of lead. As against this, however, it may be pointed out that 

 it is very strongly absorbed by non-luminous lead vapour. 



Enhanced Series 



Reference has been made above to the fact that the wavelength 



A=2203-57 A°U is strongly enhanced in the spark spectrum of lead. 



The work of Fowler on the series spectrum of magnesium suggests 



that this wavelength may possibly belong to a series of the 4 N 



type. A search was therefore made for other wavelengths which 



exhibited enhancement in the spark. Two others were found, and 



these are given in Table VI . It will be seen that these wavelengths 



4 N 



fit in very well with a formula of the type n = A — — 



(m+p)2 



