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



Since then we have had the very exact measurements by PASCHEN* in the ultra-red 

 of Li, Na, and K, and the remarkable discovery by Woor>t of the absorption by its 

 own vapour of the NaP series, lines being observed down to TO = 48. 



A superficial examination of the series of the different alkali elements shows that 

 Li apparently occupies an anomalous position, and leads to the surmise that the series 

 are not homologous with those of the other elements. Thus, if the first lines of the 

 principal series in each be compared, viz.', 



Li. Na. K. Kb. Cs. 



6708 5890 7665 7805 8527 



it is seen that the wave-lengths continually decrease with decreasing atomic weight 

 until we arrive at Li, when the wave-length jumps back above Na. Further from 

 analogy with the others LiP(l) should be a doublet with v = 1'5 about, and the LiS 

 and LiD should be all doublets of the same v, but, as a fact, they are all single, with 

 the possible exception of 4602, which KONEN and HAGENBACH believe to be a double 

 line. For this reason in my first examination I left Li for separate consideration, 

 and as this was fully justified by the result, it may be well to proceed on the same 

 lines now. We shall discuss then firsi the Avell-known principal, sharp, and diffuse 

 series of the other alkalies, then the additional series which appear in the spectra, 

 and finally return to Li in the light of knowledge gained from the others. 



The first step is to justify the use of the form m+p + a/m for the denominator, by 

 showing how closely it reproduces the observed lines. This is done in Table I. For 

 comparison the results as calculated from RITZ' formula are also given. In each 

 series the first column (O) gives the possible observational error, the second (H) the 

 excess of observed over the calculated wave-length, and the third '(R) the corre- 

 sponding values from RITZ' formula. Whenever this is outside the observational 

 error it is printed in thick figures. For the present purpose the values for Li are 

 added. The values of p, a, &c., are given in Table II. The results for RITZ are 

 given from his paper, or, for those, lines not observed at the time, are calculated from 

 his constants. It should be remembered that the estimated possible errors are 

 considerably greater than the probable errors, and consequently we ought to look for 

 a closer agreement between calculated and observed values than the possible errors 

 show. It should further be noted that for high orders any formula giving rough 

 approximation only will give some information as to the excellence or otherwise of 

 the observations themselves. If the deviations show gradual change as m increases, 

 the formula is probably in fault ; if, however, a deviation makes a sudden change 

 and comes back to its former course as m increases, the observations are probably the 

 cause. 



Where two values are given under P they refer to the doublets. All the S have 



* 'Ann. d. Phys.,' 27, !p. 567. 

 t ',Astro. Jour.,' xxix., p. 97. 



