Stoney — Cause of Double Lines in Spectra. 



595 



the spectrum may be single. This will happen whenever the corresponding 

 pendulous component of the dominant motion of the electron is circular. 



Of the thirty-five known lines, Professor Ry dberg arranges thirty-two in the three 

 following series,* and the three remaining lines he supposes to be satellites of the 

 thii'd and fourth terms of series D. We must, however, here be on our guard 

 against lines that are intruders, and owe their presence to impurities in the sodium. 

 We should also bear in mind that they or some of them may be sodium lines which 

 are members of a series, if such exists, the rest of which lies beyond the part of the 

 spectrum that has been explored. 



Using the language of acoustics, we may regard the whole spectrum as an optic 

 chord which is being played by the molecules of sodium. Each of Rydberg's 

 series will then be one of the notes of this chord, and the individual lines will be 

 the partials of these notes. The three optic notes are — 



Series P (the principal series) of lines that form a definite pattern, and in each 

 pair of which the more refrangible line is the stronger. 



Series P. 



* Professors Kayser and Eunge, using a different formula fi'om Professor Eydberg's, distribute them 

 into the same three series. 



TEANS. ftOT. DTJB. SOC, N.S. VOL. IV., PAET XI, 4 P 



