322 MAGNETISM 



handed 11 or clockwise in a rav coining to the olei -ver. It is 

 therefore counter-clockwise to him in a ray going from him. So 

 that if a ray is sent along a solution from the observing end and 

 then reflected back to the observer the rotations in the two journeys 

 are opposite and cancel each other, and the ray emerges polarised 

 parallel to its original plane. But with a " magnetised " ray the 

 rotation is always in the direction of the current which would 

 produce the lines of force, so that for each journey along a line of 

 force the rotation is increased by the same amount. Thus Faraday * 

 found that the rotation for one passage through a piece of glass 

 was 12, but on reflecting it to and fro so that it made five passages 

 the rotation was increased to 60. 



Faraday's successors. Faraday does not appear to have 

 made further experiments after the publication of his paper, his 

 attention being called off' by his investigations into paramagnetism 

 and diamagnetism ; but the subject was pursued by many work 

 E. Becquerel was the first to observe that different colours were 

 rotated by different amounts, red least, violet most a phenomenon 

 known as rotary dispersion. By making the magnetic rotation 

 "left-handed" and then passing the rotated light through a sugar 

 tube of suitable length to neutralise the rotation for one colour. 

 it was neutralised for all as nearly as he could judge, or the 

 magnetic rotation was proportional to the natural rotation for all 

 colours. G. Wiedemann used different parts of a solar spectrum as 

 the source of light, a solenoid to produce the field, and a tangent 

 galvanometer to measure the current. He was thus able to show that 

 the rotation was proportional to the current, and therefore to the 

 field intensity within the coil. He found that the magnetic rotary 

 dispersion of carbon bisulphide, for instance, was not very far from 

 being proportional to the natural rotary dispersion in a sir 

 solution but yet sensibly different, and it was not far from being 

 inversely as the square of the wave length, though again quite 

 sensibly different. 



Verdet's constant, and its variation with wave 

 length. Verdetf made a long series of experiments on various 

 substances with an accuracy probably superior to that attained 

 up to his time, and he proved conclusively that the rotation 

 per unit length of a substance traversed is proportional to the 

 component of the intensity of field in the direction of the ray. If, 

 then, ds is an element of the path of the ray in a substance, 

 and the intensity H makes an angle with ds, the rotation R of 

 the plane of polarisation between 8 = a and s = b, usually expressed 

 in minutes of arc, is 



* Exp, Res. III. p. 453. 



f CEurres, tome i. ; or Gordon, Electricity and Magnetism, vol. ii. 



