1899.] on Magnetic Perturbations of the Spectral Lines. 157 



mena, I have had this elliptic frame constructed [model shown],, 

 which I ask you for the present to cousider as the orbit described by 

 one of those elements of matter which by their motions set up waves 

 in the ether, and thereby emit what we call light. This white ball, 

 which slides on the elliptic frame, is supposed to represent the 

 element of matter. It is sometimes called an ion, which name is 

 used to imply that the element of matter carries an electric charge 

 inherently associated with it. 



Now, under ordinary circumstances this ion revolving in its orbit 

 with very great rapidity will continue to do so peacefully, unless 

 external forces come into play to disturb it. When external forces 

 come into action the orbit ceases in general to be the same as before. 

 The orbit becomes perturbed, and the external forces are termed per- 

 turbing forces. But you now ask, What is the character of the forces 

 introduced by the magnetic field when the ion is moving through it ? 

 In answering this, we are to remember that the ion is supposed to be 

 an element of matter charged with an electric charge — or, if you like, 

 an electric charge possessing inertia. Now, if a charged body moves 

 through a magnetic field, it is an experimental fact that it experiences 

 a force arising from the action of the magnetic field on the moving 

 electric charge. The direction of this force is at right angles both 

 to the direction of motion of the charged body and to the direction of 

 the magnetic force in the field. The effect of this force in our case' 

 is to cause the elliptic orbits of the ions to rotate round the lines of 

 magnetic force ; or to cause them to have a precessional motion 

 [illustrated by model] instead of staying fixed in space, just as the 

 perturbing forces of the planets in the solar system cause the earth's 

 orbit to have a precessional motion. The angular velocity of this 1 

 precessional motion is proportional to the strength of the magnetic 

 field, and depends also, as you would expect, on the electric charge 

 and the inertia associated with the ion. 



This precessional motion of the orbit, combined with the motion 

 of the ion around the orbit, gives the whole motion of the ion in 

 space, and the result of this combined movement, of these two super- 

 posed frequencies — viz. the frequency of revolution of the ion in its 

 orbit, and the frequency of rotation of the orbit around the lines of 

 force — is that, in the case of the light radiated across the lines of 

 force, each period becomes associated with two new periods, or, in 

 other words, each spectral line becomes a triplet. A partial analogue 

 to this, which may to some extent help you to understand the intro- 

 duction of the two new periods, occurs in the case of sound, although 

 the two phenomena at basis are quite different. The analogue (or 

 quasi-analogue) is this. When two notes of given pitch, that is of 

 given frequency of vibration, are sounded together, their superposition 

 produces two other notes of frequencies which are respectively the 

 sum and the difference of the frequencies of the two given notes. 

 These are known as the summation and the difference tones of the 

 two given notes. Corresponding to these are the two side lines of the 



