TRANSACTIONS OF SECTION A. 575 



these. The wave-length of the line, when corrected for the refraction of the air, 

 jrives The periodic time of the motion of the electron in the corresponding partial, 

 and the brightness of the line gives a quantity proportional to cr + b', n and b 

 being the axes of the ellipse. 



But there is one case, and fortunately a case which at all events frequently 

 occurs, and that perhaps is universal, in which we receive a very interesting 

 addition to our knowledge ; explaining on the one hand the double lines that are 

 so frequent in spectra, and on the other telling us the actual forms of the elliptic 

 partials of the motion going on in the molecules. This important case occurs 

 whenever some of the forces which determine the motiDn of the electron are 

 feeble compared with the others, and are such as to produce that familiar form of 

 perturbation which consists in an apsidal motion of the elliptic partials. When 

 this perturbation prevails, the lines are rendered double by it, and an examination 

 of the positions and intensities of the two constituents of a double line enables us 

 to determine (a) the form of the elliptic partial to which they are due, {b) the 

 time which the electron takes to travel round it, and (c) both the direction and 

 speed of the apsidal perturbation. Thus the principal double line of sodium is 

 found to have its source in a long elliptic partial, the ratio of the axes of which 

 lies somewhere between 11:1 and 13:1. Round this partial the electron travels 

 abaut 1,984: times during one revolution of its slow apsidal perturbation, and there 

 is time for about 36 of these slow apsidal revolutions to take place during each 

 flight of the molecule. Moreover in this case the apsidal motion takes place in 

 the same direction as the motion of the electron round the ellipse. An equal 

 amount of information can be obtained in the case of every other double line that 

 can be adequately observed. 



The author thought he had reason to suspect from observation that almost all 

 spectral lines are double, and that they appear single only when our spectroscopes 

 have insufficient revolving power, or when each of the constituents has so widened 

 out as to obliterate the interval between them, or in the rare cases when the 

 partial from which they arise being circular one of the two constituents of the 

 double line is of cypher intensity.' If this shall turn out to be the case, there 

 must be some common cause for the apsidal perturbation, and the author ventured 

 to suggest as the most probable cause the feeble reaction which the fether exerts 

 on the electron, consequent on the energy which the molecule imparts to the 

 aether when producing the electromagnetic waves. A fuller account of the in- 

 vestigation is being printed by the Royal Dublin Society in its Scientific Trans- 

 actions. 



5. Seventh Report of the Committee on Solar Radiation, 

 See Eeports, p. 160. 



6. Report of the Committee on Meteorological Photography. 

 See Reports, p. 130. 



7. Report of the Comviiltee on the Meteorological Ol)servations on Ben Xevis. 

 See Reports, p. 140. 



8. Report of the Committee on the Eeduction of Magnetic Observations. 

 See Reports, p. 149. 



' It is shown in the inyestigation that each of the two constituents of a double 

 line arises from a circular motion. Accordingly they would not suffer further 

 duplication if an additional apsidal perturbation were introduced. 



