Stoney — Cause of Double Lines in Spectra. 



577 



Variety 3. — The apsidal motion opposite the other two. Therefore m is 

 positive, n negative, and Ic positive. 



Fig 9 (c) represents the spectrum : a pair, with satellites outside, the less 

 refrangible group the brighter. 



Variety 4. — The fourth variety is when both the apsidal and precessional 

 motions are in the opposite direction to the elliptic. Here m is positive, and n 

 and k are negative, and the spectrum is represented by fig. 10 (c) : a pair, with 

 satellites inside, the less refrangible group the brighter. 



m 



5s ^ S 



I 1 1 



+ I 



+ + + 

 + I 



■> ^ ?i 

 I I I 



I + 



+ + + 

 I + 



3^ JS- 



Fio. 9. 



Fig. 10. 



These diagrams represent what occurs when the apsidal and precessional 

 perturbations are slow compared with the original orbital motion aroused by the 

 last encounter of the molecule with another molecule. In this case the satellites 

 lie, as in the diagrams, on opposite sides of their primaries, and the primaries 

 themselves have been displaced in opposite directions by the precessional motion. 



If, however, the apsidal motion be swift, the orbital motion must be slow to 

 account for the close double Knes that are seen in the spectrum. Such relative 

 swiftness of the apsidal motion seems unlikely, and accordingly I will not pursue 

 the supposition further than to remark that if it prevails in any gas the satellites 

 of both components of a double line will lie on the same side of their primaries, 

 i. e. either all to the right or all to the left ; and the primaries themselves will be 

 displaced in the same, instead of in the opposite directions, by precession. 



Corollary. — If there be precessional motion of an elliptic partial without apsidal 

 motion, there will be three equidistant lines in its spectrum, of which the inten- 

 sities could be computed if a, 5, a and /8 wei'e known, jS being the angle between 

 the axis major of the ellipse, and the line in which the plane of the ellipse 

 intersects the invariable plane. For the converse problem, we can observe the 

 intensities of the three lines, and their interval. These will determine the value 

 of k^ and will furnish three equations between a, ^, a and ^, but will not fully 



