CHAPTER XXI\ 

 MAGNETISM AND LIGHT 



The Faraday rotation Faraday*g successors V.-s-.l.-i's constant and its 

 variation with wave length Effect of rise of temperature Absolute 

 values of Verdet's constant Rotation by jfanes Rotation by films of iron, 

 nickel, and cobalt Representation of the rotation by two equal circularly 

 polarised rays with opposite r..t:it iini; with different velocities 



An electron theory of the rotation -('-. I tlMOfJ 



M;;:riirtic double refraction wh'-n : ray is perj^ndicula- 



liii-n of force V, -i-_'t'> tlj.-oi- - 



Magnetic doul liquids The Kerr magnetic effect 



l!i, Keen -Lorents'a theory Ze*man's Teriflcnt 



The Faraday rotation, l opera hi* paper describ- 



ing his givat di f an action of magnetism on li^lit by 



saying: * I have lon^ held an opinion, ai: 



viction, in common I In-lit \c \\itli many oth r 1 natural 



knowledge, that the vari. I under \\ hidi the forces of ma' 



are made manifest lm\e one common origin : <>r. in other words, are 

 so directly related and mutually dependent that tlit-v are coin 

 tible, as it were, one into another, and possess equivalents of p<> 

 in their action.""* 



He then states that this persuasion had led him formerly to 

 make many unsuccessful attempts to obtain ; D of 



light and electricity. Recently, bo i-dcd in 



" magnetising and eL ^>it. (in</ ng a 



magnetic line of force" a picturex : hat if a 



ray of plane-polarised lio;ht tra\ el> through long 



the lines of force in the field of A permanent magnet or in that of a 

 solenoid through which i* .rrent of electricity tlie j)lane 



of polarisation gradually rotates a> the ra\ on. 



The first substance which exhibit i M TV heavy 



glass, a silicated borate of lead which he had ma ra earlier. 



A plate of this glass two inches .square and half an inch thick was 

 polished on its narrow sides and placed between the poles of an 

 electro-magnet in sucb a position that when the current was on, 

 lines of force went, as nearly as might be, straight through the 

 inches of glass from one side to the other. A ray ot lignl Iron 

 Argand burner was made to fall on a glass plate at the polari'sino; 



/;./;. Re*. III. p. 1 (N 

 320 



