572 Stoney — Cause of Double Lines in Spectra. 



their frequencies are m + n and m — n, and their radii are {a + b) / 2 and 

 {a-b)/2. 



If the molecules of the gas be immersed in an sether such as we have assumed, 

 viz. one susceptible of transverse vibrations only, the foregoing motion will 

 produce two lines in the spectrum whose positions on a map of oscillation-frequen- 

 cies will he m + n and m — n. Moreover, the ratio of the intensities of the two 

 rays propagated in any one direction from the gas through the sether will be the 

 ratio of (« + by to (a — bf, whether we take into account the contribution from one 

 molecule only, or the combined effect of all the molecules. 



We thus find that the double lines which are a conspicuous feature of all gaseous 

 spectra, and of which the spectra of the monad elements appear wholly to consist, 

 are accounted for by supposing that an apsidal perturbation operating during the 

 jom-neys of the molecules between their encounters, affects the dominant motion 

 set up in them by the encounters. 



The equations hitherto given represent the motion when the apsidal motion is 

 in the same direction as the elliptic, and here the more refrangible line, whose 

 oscillation frequency is m + n, is the brighter. If, however, the perturbating 

 forces are such that the apsidal motion takes place in the opposite direction to the 

 revolution of P in its ellipse, we must change the sign of \jj in all the equations ; 

 from which it appears that it is now the less refrangible line which is the brighter. 

 If any of the elliptic partials should chance to be a circle, b = a, and one constituent 

 of the double line is of cypher intensity. Accordingly, the other alone will present 

 itself in the spectrum, and will have in it the position m + n when the circular 

 motion and the apsidal are in the same direction, and tlie position }n — n when they 

 are in opposite directions. And, finally, whenever the partial of the dominant 

 motion represented by equations (1) is a mere vibration in a straight line instead 

 of a revolution in an ellipse, b, the axis minor, vanishes, and the intensities of 

 the spectral lines (which are always to one another in the ratio of («-l-^)'^ to 

 (a — by) become equal. 



The following figures represent the several cases which have been considered. 

 All of them are met with in the actual spectra of gases. 



Fig. 2 (a). — Spectrum of one of the partials of the domi- ttl 



nant motion of P, viz. of a pendulous elliptic revolution of 



P in the molecules of the gas such as that represented by . 



equations (1). 



Fig. 2 (b). — The double line into which this resolves 



itself when the elliptic motion in the molecules is affected by ^ , 



an apsidal motion in the same direction as the ellijDtic motion. § § 



In this case the more refrangible line is the stronger. See p^^ 2. 



equations (3 a) and {S b). 



(^) 



(6) 



