568 Stoney — Cause of Double Lines in Spectra. 



that the oscillations of the lines in these spectra are not quite synchronous 

 with the motions in the molecules that originate them, while in hydrogen, by 

 reason of the extreme closeness of the double lines, they are almost exactly syn- 

 chronous. It -will also be shown how the periodic times of the molecular motions 

 may be deduced from the observations. 



Professor Rydberg designates by the letters P, D, and S, the three series of 

 pairs of lines found in the spectrum of each of the foregoing elements : P being 

 what he calls the principal series of pairs of lines, D a series of pairs of diffuse or 

 nebulous lines, and S a series of jDairs of sharp lines. All the observations he has 

 been able to collect support the conclusion that the more refrangible line of each 

 pair of the series P is the stronger, while the reverse is the case in the two other 

 series. What this means will apjiear in the sequel. 



Professor Rydberg is of opinion that when the lines are plotted down on a 

 map of oscillation-frequencies, the distance between the two lines of each pair, 

 which we may call A, is the same throughout the whole of each series, and even in all 

 the three series. It may be doubted whether the observations he has collected 

 are as yet sufficient to give us confidence on this point. It will doubless be 

 settled by the great photographs that Professor Rowland has succeeded in 

 obtaining with his unrivalled apparatus, and which we may hope will soon be 

 published. It may, however, prove to be the case ; and we shall see in the 

 following chapters the important meaning which would attach to it. 



Finally, Professor Rydberg has ascertained that the value of A (the interval 

 between the lines of each pair), while it varies but little between the three series 

 of pairs of lines in each element, differs very much in passing from one element 

 to another : the pairs being closest in lithium, somewhat wider in sodium, wider 

 still in potassium, very wide in rubidium, and widest of all in C£esium. What 

 this means will also be explained. 



