on the Nature of the Light emitted by a Substance. 231 



the centre of mass, of the molecule, revolving in all kinds of 

 orbits, suppose for simplicity circles; then the period, or, what 

 comes to the same, the time of describing the circumference 

 of these circles, will be determined by the forces acting be- 

 tween the atoms, and then deviations of the period to both 

 sides will occur through the influence of the perturbing forces 

 between sether and atoms. The sign of the deviation of course 

 will be determined by the direction of motion, as seen from 

 along the lines of force. The deviation will be the greater the 

 nearer the plane of the circle approximates to a position per- 

 pendicular to the lines of force. 



16. Somewhat later I elucidated the subject by representing 

 to myself the influence exercised on the period of a vibrating 

 system if this is linked together with another in rapid rotatory 

 motion. Lord Kelvin (now 40 years ago*) gave the solution 

 of the following problem : — Let the two ends of a cord of any 

 length he attached to two points at the ends of a horizontal 

 arm made to rotate round a vertical axis through its middle 

 point at a constant angular velocity, and let a second cord 

 bearing a material point be attached to the middle of the first 

 cord. The motion now is investigated in the case when the 

 point is infinitely little disturbed from its position of equili- 

 brium. With great angular velocity the solution becomes 

 rather simple. Circular vibrations of the point in contrary 

 directions have slightly different periods. If for the double 

 pendulum we substitute a luminiferous atom, and for the 

 rotating arm the rotational motion about the magnetic lines 

 of force, the relation of the mechanical problem to our case 

 will be clear. 



It need not be proved that the above-mentioned considera- 

 tions are at most of any value as indications of somewhat 

 analogous cases. I communicate them, however, because 

 they were the first motive of my experiments. 



17.' A' real explanation of the magnetic change of the 

 period seemed to me to follow from Prof. Lorentz's theory f . 

 In this theory it is assumed that in all bodies small elec- 

 trically charged particles with a definite mass are present, 

 that all electric phenomena are dependent upon the confio-ura- 

 tion and motion of these "ions," and that light- vibrations 

 are vibrations of these ions. Then the charge, configuration, 

 and motion of the ions completely determine the state of the 

 sether. The said ion, moving in a magnetic field, experiences 

 mechanical forces of the kind above mentioned, and these must 



* Proc. Roy. Spc. 1856. 



t Lorentz, La Theorie electromagnetiqiie de Maxwell. Leyde, 1892 • 

 and Versuch einer Theorie der electi'ischen und optischen Erschehuaigen in 

 bewegten Korpern, Leiden, 1895. 



