152 Professor Thomas Preston [May 12, 



and far away from it the force is almost insensible ; and so we say 

 that the field is strong at certain places near the magnet, and that it 

 is weak at places far away from the magnet. The direction of the 

 force at any point is the direction in which the north pole of another 

 magnet would be urged if placed at that point, and the push which 

 this pole experiences may be taken to represent the intensity or 

 strength of the magnetic field at the point in question. This is 

 represented diagrammatically by these drawings [referring to figures 

 suspended before the audience], which show roughly the nature of 

 the field of force surrounding an ordinary bar magnet, a horse-shoe 

 magnet, and the much more powerful form — the electro-magnet. It 

 will be seen that the space outside the iron is filled with a system of 

 curved lines running from the north pole to the south pole of the iron 

 core. Where the lines are closest together there the magnetic force 

 is strongest, and the direction of a line at any point is the direction 

 of the resultant magnetic force at that point — that is, the direction in 

 which a north pole would be urged if placed at that point, 



Faraday always pictured the magnetic field as filled with lines of 

 force in this way, and the importance of the conception can scarcely 

 be over-rated, for it leads us to view the magnetic action as being 

 transmitted continuously through the intervention of some medium 

 filling all space, rather than by the unintelligible process of direct 

 action at a distance. This medium is called the ether ; but as to 

 what it is that is actually going on in the ether around a magnet, we 

 cannot definitely say. It may be that there is a flow of ether along 

 the lines of magnetic force, so that there is an out-flow at one end of 

 the magnet and an in-flow at the other, or it may be that the ether is 

 spinning round the lines of force in the magnetic field. For our 

 present purpose it is not a matter of very much importance what the 

 exact condition of the ether may be in a magnetic field, for if the 

 ether in a magnetic field is either in some peculiar condition of strain 

 or of motion, and if light consists of an undulatory motion propagated 

 through this same ether, then it may be naturally expected that some 

 action should take place when light is propagated through, or radiated 

 in, a magnetic field of force. This is what Faraday suspected ; and 

 in order that we may appreciate the problem with which he had to 

 deal, let us place ourselves in his position and ask ourselves the 

 question : '• In what manner can we test experimentally if there is 

 any magnetic action on light ? " 



In answer to this question, the first thing that occurs to us is to 

 pass a beam of ordinary light through the magnetic field, in some 

 chosen direction, and examine by all the means at our disposal if 

 any action has taken place. When this is done we find that no 

 observable effect is produced. But the scientific investigator does 

 not rest satisfied with one negative result. He varies the conditions 

 of the experiment, and returns to the attack with renewed vigour and 

 1m. pes. In our first trial we passed a beam of light through the air- 

 filled space around the magnet, and we may vary this experiment 



