PEOF. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 99 



of the typical form. In Li all the negative group come in, and we only have one 

 indication of lines of the positive group. It is apparent, not that the formulae have 

 ceased to follow regular laws, but that the energy has passed from one set to another. 

 It is necessary to consider whether the identification of PASCHEN'S doublet with 

 the first line of the D series will hold if we regard the S and D forms to be the same 

 for both positive and negative groups. The difficulty is, there is no appearance of 

 the P form. It is possible, however, to determine VP(l) so as to make the 

 identification correct, and see whether it leads to a likely value. Now 



VD(1) = 28580. 

 Hence 



VP(1) ~ 28580 = 3718, 

 or 



VP(1) = 32298 or 24862, 



giving values of the denominator of P respectively 1'8427 or 2'1003. The former is 

 quite out of character, but the latter is more in character with analogy. It makes 

 the D of P (I) only '016 less than that of Na. and the fractional part greater than the 

 fj. '5 of the S set, which for m = I is '089. This fractional part is greater in all 

 the previous cases. The identification, although possible, does not commend itself. 



Hydrogen. 



For our present purposes the spectrum of hydrogen is chiefly important, because 

 on it is based the determination of the constant N. The value was determined by 

 RYDBERG on the supposition that BALMER'S formula was correct, by applying the 

 method of least squares to AMES'* measurements of the first 13 lines. I have 

 already given reasons why, in deducing N, RYDBERG'S formula should be used in 

 preference to BALMER'S the probability being that p. 1 would be so small that the 

 term in oLJin would be quite negligible. Also, since AMES' results, EVERSHED! has 

 given measurements from solar prominences down to m = 31. For these reasons it 

 seemed desirable to recalculate the value of N with RYDBERG'S formula. ROWLAND'S 

 measurements were used for HL> (3.4). For HD (5) the readings of various observers 

 differ greatly. The work was done twice first for AMES' value of HD(5), weighting 

 it one-half the others ; and second, leaving out the line altogether. HD (6.7) have 

 been very carefully and specially measured by EVERSHEDJ. The remaining 24 lines 

 used were from EVERSHED'S measurements, all of course corrected to vacuo. In 

 absence of any indication to the contrary, all observations were treated as of equal 

 value, although as they are prismatic measurements it might have been better to 



* ' Phil. Mag.,' xxx., p. 33 (1890). 

 t ' Phil. Trans.,' vol. 197, p. 413. 

 \ ' Astro. Journ.,' xxviii., p. 72. 

 o 2 



