218 HISTORICAL REMARK [APR H. 



incited Maxwell, many years later, to begin developing his 

 electro-magnetic theory of light. 



But how do the titles strike us now ? Do they not 

 indicate some extraordinary unconscious insight, such as 

 is frequently experienced by a great discoverer in the 

 enthusiasm of discovery ? Remember that the Hall effect, 

 the Zeeman effect, the Aurora Borealis, and Faraday's 

 rotation are all closely connected with each other by 

 means of the electron theory. 



In the cathode ray tube the flying electrons are deflected 

 by a cross magnetic field; or if they fly along the lines 

 they are twisted into a spiral path round them. In the 

 Aurora Borealis this effect is carried out in the upper 

 region of the air on a gigantic scale, and the earth's 

 magnetic "lines of force are illuminated" by flying elec- 

 trons from the sun entangled and guided by them. In the 

 Hall effect this same influence is felt by the slowly moving 

 crowd of electrons as they are handed on from one atom to 

 the next, causing a curvature of the current path in which 

 either positive or negative may predominate. In the Zeeman 

 effect the same cause operates on the revolving and vibrating 

 electrons, associated with a radiating atom and constituting 

 a source of light ; wherefore we may truly say that the 

 " light is magnetised," for the source of light is magnetised 

 directly, and the effect is impressed on and retained by the 

 light emitted, and is made visible by spectrum analysis. 



The first intimation of that magnetic influence on light 

 which lies at the base of all these at first sight apparently 

 diverse phenomena was detected by Faraday in his slight 

 differential rotation of the plane of polarisation in one 

 direction or the other by a magnet, according as the posi- 

 tive or the negative element in the dense substance was 

 most affected. 



Hence the title which he affixed to his discovery " The 



