Stoney — Analysis of the Spectrum of Sodium. 



213 



Accordingly all the observed points lie nearly on this parabola. 

 The parabola is in the position shown in fig. 3, from which it 

 follows that the primary curve (which represents the relation 

 between n and l/;«) is a curve which is nearly a hyperbola. (See 

 PI. XVII., figs. 4 and 5.) 



Application to Series D. 



Series D is best represented by a straight line for its derived 

 curve, and accordingly by a parabola for its primary curve. 

 Putting as before x = n and s = 1000/m-, and taking 



X = a - az 



as the equation of the derived curve, in which 



a = 244-93 and log a = -04357 



and computing the successive terms, we find 



results which show that the primary curve of Series D approxi- 

 mates very closely indeed to a parabola. (See PL xvii., figs. 4 

 and 5.) 



