154 Professor Thomas Preston [May 12, 



Yon will Lave noticed that in the foregoing experiment the source 

 of light was placed quite outside the field of magnetic force, while 

 the beam of light was transmitted through the field for examination. 

 Now we might place the source of light itself in the magnetic field, 

 and then examine if the light emitted by it is in any way affected by 

 the magnetic force. This variation of the experiment suggests itself 

 at once, and was indeed also tried by Faraday — in fact it formed his 

 last experimental research of 1862, but without success. The same 

 experiment has been tried, no doubt, by many other physicists, with 

 the same negative result. 



The first recorded success, or at least partial success, was by 

 M. Fievez in 1885. He placed the source of light — a gas flame 

 impregnated with sodium vapour — between the pole-pieces of a 

 powerful electro-magnet. This being done, the light radiated by the 

 flame was passed through the slit of a highly dispersive spectroscope 

 and examined. What M. Fievez observed was that the bright spectral 

 lines became broadened by the action of the magnetic field on the 

 radiating source. His account is, perhaps, somewhat confused, owing 

 to his imperfect apprehension of the true nature of the phenomenon 

 which he observed ; but, without doubt, he observed a true magnetic 

 effect on the radiated light — namely, this broadening of the spectral 

 lines. But he did not convince the scientific world that he had made 

 any new discovery, and so the matter fell into neglect until it was 

 revived again in 1897 by the now celebrated work of Dr. P. Zeemau. 



The credit which attaches to Dr. Zeeman's work is that he not 

 only, after prolonged effort, succeeded in obtaining this new magnetic 

 effect, but he also convinced the world that the effect was a true one, 

 arising from the action of the magnetic field on the source of light. 

 That Dr. Zeeman was able to do this was due, perhaps, as much to 

 the present advanced state of our theoretical knowledge of this sub- 

 ject as to his own skill and perseverance as an observer ; and this is 

 a striking example of the great assistance which well-founded theory 

 affords to experimental investigation. The theory connects the facts 

 already known in reasonable and harmonious sequence, predicts new 

 results, and points out the channels through which they must be 

 sought. Without such scientific theory this general systematic ad- 

 vance would be impossible, and new results would be stumbled on 

 only by accident. 



To see how this applies to our case, we revert to the fact deter- 

 mined by Dr. Zeeman — namely, that when the source of light is 

 placed in a strong magnetic field the spectral lines become broadened. 

 [Slide shown here.] As soon as this was announced Professor 

 Lorentz, and subsequently Dr. Larmor, examined the question from 

 the theoretical point of view. They analysed the subject mathe- 

 matically, and came to the conclusion that each spectral line should 

 be not merely broadened, but should be actually split up into three 

 ■^-that is, each line should become three lines, or, as we shall say in 

 future, a triplet. They also arrived at the further most important 



