Stoney — Cause of Doulle Lines in Spectra. 



573 



Fig. 3 (a). — Simple elliptic motion as before. 



Fig. 3 {I). — The double line when the apsidal motion is in the opposite 

 direction to the elliptic motion. Here the less refrangible line is the stronger. 

 This case is represented by changing the sign of t|/ in equations (3 a) and (3 5\ 



Fig. 4 (a). — Spectrum of a simple circular partial. This case is represented by- 

 making h = a. 



Fig. 4 (5). — Position to which this line is shifted when there is apsidal motion 

 in the same direction. 



Fig. 5 [a). — Spectrum of a circular partial as before. 



Fig. 5 {h). — Position to which the line is shifted when there is apsidal motion 

 in the opposite direction. 



Fia. 6 (a). — Spectrum of a pendulous vibration in a straight line. This case is 

 represented by making J = in equations (1). 



Fig. 6 {h). — The spectrum of this vibration subjected to apsidal motion. Here 

 the constituents of the double line are equally strong. This case is represented by 

 putting 5 = in equations (3 a) and (3 V). 



171 



TTV 



a) 



(a) 

 (6) 



(a) 



ii) 



(a) 



ih) 



% 

 I 



Fig. 3. 



+ 

 S 



Fig. 4. 



I 



8 



Fig. 5. 



+ I 



Fig. 6. 



Precessional Motion. — Both the revolution of P in the elliptic partial and the 

 apsidal rotation of the ellipse, if not subjected to further disturbance, take place in 

 a fixed plane ; but unless special conditions are fulfilled within the molecules the 

 perturbations will be such that this plane will shift its position in relation to the 

 "invariable plane." To represent this motion let us conceive an axis per- 

 pendicular to the invariable plane and passing through the centre of the ellipse. 

 This axis is called the invariable line. It will in general be oblique to the plane 

 of the ellipse, and we are to suppose the plane of the ellipse to rotate round it 

 while maintaining its inclination to it unchanged. Hence arises — 



Problem III. — What change of the spectrum will result from a precessional 

 rotation round the invariable line, of the plane in which the elliptic and apsidal 

 motions take place ? 



Let us speak of the moving plane (the plane in which the elliptic and apsidal 

 motions take place) as plane B) and let the invariable plane be called plane A. 



4M2 



