1899.] Magnetic Perturbations of the Spectral Lines. 151 



WEEKLY EVENING MEETING, 



Friday, May 12, 1899. 



His Grace The Duke of Northumberland, E.G. F.S.A., President, 



in the Chair. 



Professor Thomas Preston, M.A. D.Sc. F.E.S. 



Magnetic Perturbations of the Spectral Lines. 



The subject which we are about to consider this evening forms a 

 connecting link between two of the most interesting branches of 

 human knowledge — namely, that which treats of magnetism and that 

 which treats of light. Almost as soon as the properties of magnets 

 became known, mere curiosity alone must have prompted philosophers 

 to ascertain if any relation existed between magnetism and " the other 

 forces of nature," as they were generally termed. We are consequently 

 led to expect, amongst the records of early experimental investiga- 

 tions, some accounts which treat of the action of magnetism on light. 



When we seek for such accounts, however, we find that they are 

 almost wholly absent from the literature of science ; and this arises, I 

 believe, from the great difficulty of the investigation and from the 

 circumstance that only negative results were obtained, rather than 

 that no such inquiry suggested itself or was undertaken. Even in 

 quite recent times this inquiry has been prosecuted, but wit u out 

 success, by physicists who have published no account of their experi- 

 ments. We may take it, therefore, that the inquiry is in itself an 

 old one, although it is only now that it has been carried to a successful 

 issue. 



The earliest recorded attempt to solve this problem with which 

 we are acquainted, is that of a celebrated British physicist whose 

 name must for ever shed lustre on the annals of the Royal Institution 

 — I speak of Michael Faraday. In order to understand the nature of 

 the investigation which Faraday took in hand, and which has led up 

 to the discourse of this evening, it is best to consider briefly some 

 elementary facts concerning magnetism and light. 



In the first place I shall assume that we know in a general way 

 what the peculiarities of a body are which lead us to say that it is 

 magnetised, or a magnet. These are that, when freely suspended, it 

 sets itself in a definite direction over the earth's surface, as illustrated 

 by the compass needle, and that in the space around it there is 

 " magnetic " force exerted on pieces of iron, and in a smaller degree 

 on other substances. For this reason we say that a magnet is 

 surrounded by a magnetic field of force. The field of force is simply 

 the space surrounding the magnet, and it extends to infinity in all 

 directions from the magnet. 2s ear the magnet the force if. strong, 



