94 



PKOF. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 



It was my first impression that this would be the same for all the alkalies and the 

 987 of the earlier discussion. It would be the basis of a fundamental series for each 

 element, and the letter F which has been attached to it had its origin in this idea. 

 But RITZ, who has discussed these series very fully and compared with those in other 

 elements, proposes to represent the denominators by the difference of the denomi- 

 nators of the P! and P 2 series. At first sight this may appear a fanciful relation, but 

 if it be remembered that this difference is merely the atomic weight term, whose 

 subtraction from the atomic volume term in P 1 gives the D of P 2 , the supposition is 

 quite reasonable. His proposal, then, comes to this : to put //, = 1 2W where 2W is 

 the atomic weight term. Now, the values of 2W are about '000744, '002933, '012887, 

 '032435, their differences from unity giving (to four figures) '9993, '9971, '9871, 

 '9670, which march closely with the values of /A obtained from the lines as observed. 

 There is something to be said, therefore, in favour of the view that the value of p. for 

 this series is 1 atomic weight term, though it cannot yet be regarded as certain, 

 against the supposition of constancy. The following table is interesting as throwing 

 light on this point. It gives the differences of wave-numbers between successive 

 lines for the elements indicated : 



TABLE III. 



What indication of change there is, is that there is an increase with increasing 

 atomic weight. It is just possible, however, that they might be the same as the 

 possible limits of error inserted below each number shown. Where no number is 

 inserted (H and He excepted) the limits are very large, but no definite values can be 

 assigned. At the same time the evidence is in favour of RITZ' theory. In 

 accordance with what has been learnt as to the formation of the P and D sequences, 

 I was inclined to think that the true denominators of the F sequence were 



m+ I 2W( I- -j, the alkali P and D series depending on its second term, m = 2, 



m 



giving 1-W for the basis of the P and D. Such a form agrees well with the 

 observed lines for Rb and Cs where the measures are not exact. When, however, it 

 is applied to PASCHEN'S observation of the two first lines (m = 3, 4) in Na and K, it 



