4 26 SCIENCE PROGRESS. 



the lunar orbit although the amplitude of the radiation such 

 a rotation might emit might be directly proportional to this 

 eccentricity. The whole question is very complicated. It 

 practically assumes that the fundamental motions in the 

 molecules are immensely more rapid than any of those we 

 deal with in light vibrations, and that these latter are merely 

 perturbations of the fundamental motions. It is well to keep 

 these things in mind, although we are to all appearance so 

 very far from any satisfactory explanation of it all, because 

 it points out the direction in which to look for an advance 

 in our knowledge. That we know so little and see so little 

 ahead in these fundamental matters may be disheartening, 

 but it shows how very important any advance in our know- 

 ledge of molecular motions is and should encourage us to a 

 study of even minute effects such as Zeeman has observed. 



In the simple case referred to above, the radiation 

 emitted under the action of magnetic force, when studied 

 by a spectroscope, has the following characteristics : the 

 radiation emitted along the lines of magnetic force consists 

 of two circularly polarised beams, one right- and the other 

 left-handed, vibrating at slightly different rates. This is 

 exactly what might be expected to be emitted from an elec- 

 tron moving in the way already described. The component 

 of the vibration along the lines of force would not give rise 

 to any sensible disturbance at a distance along that line, and 

 the two circular components of its vibration in the plane at 

 rio-ht angles to the force would produce the observed cir- 

 cularly polarised beams. The effect of the magnetic force 

 on a spectral line which is thus simply affected is, when 

 viewed along the lines of force, to double the line, one of 

 the components being right-handed and the other left-handed 

 circularly polarised light. 



The radiation emitted in directions at right angles to 

 the magnetic force is most simply described as consisting of 

 three beams. One due to the component along the lines 

 of force whose frequency is unchanged, and which produces 

 a plane polarised beam and resulting line in the spectrum in 

 the same place as when there is no magnetic force acting ; 

 and two other beams due to the two circular components 



