MAGNETISM AND LIGHT 



angle and then passed plane-polarised in thd horixontal direction 

 through the glass close to the poles and in the direction of the 

 lines of force. It was then received through a Nicol prism into 

 the observer's eye. The prism could be rotated about the ray as 



. and the rotation could be measured. 



Before the current was put on, the Nicol was turned into the 

 position of extinction. On making the current the field of view 

 lighted up. If the Nicol was then rotated, a new position of 

 extinction was found showing that the light was still plane- 

 polarised, but that the plane was turned round into a new position. 

 He found that there was no visible effect if the magnet were so 

 turned that the lines of force were perpendicular to the ray. The 

 (ields produced by a permanent magnet, and by a helical coil or 

 solenoid in which an electric current flowed, were effective, and as 

 far as he could estimate he found that the rotation with a given 

 length of glass was proportional to the strength of field, and with 

 different lengths of glass proportional to the length. He examined 

 a large number of substances besides the glass, and found that 

 m. my exhibited the effect, such as flint and crown glass, water and 

 alcohol. The solenoid arrangement was useful for the liquids, 

 which could be enclosed in tubes with glass ends, placed along the 

 axis of the solenoid. It was this solenoid method, in which an electric 

 current was effective, which led him to say that he had electrified a 

 ray of light. With optically active bodies such as turpentine or 

 sugar he found that the magnetic rotation was merely added to or 

 subtracted from the natural rotation according to its sign. He 

 was unable to detect any rotation in air. 



In every case which Faraday observed, the rotation was in the 

 direction of the current which would produce the lines of force, if 

 we imagine them due to a solenoid surrounding them. Thus if the 

 lines of force are supposed to come out from the paper to the 

 reader's eye, the creating current would circulate counter-clock wise, 

 and that would be the direction of rotation of the plane of polari- 

 sation. The rotation would be reversed if the lines of force went 

 from the eye to the paper. 



Subsequent researches have shown that all substances which 

 light traverses exhibit the Faraday effect, and for the most part with 

 the rotation in the direction which he observed and which is described 

 as positive. But salts of iron and some few others, though not salts 

 of nickel and cobalt, exhibit a negative rotation. The distinction 

 in no way corresponds to the distinction of paramagnetic and dia- 

 magnetic, for iron nickel and cobalt, though enormously para- 

 magnetic, coincide with the great majority of diamagnetics in giving 

 positive rotation, while the diamagnetic chloride of titanium gives 

 a negative rotation. 



The magnetic rotation differs in one very important respect 

 from the rotation by a naturally active substance such as sugar 

 solution. In the sugar solution the rotation is always "right- 



