MAGNETISM AND LIGHT 



333 



a salt with a strong absorption band in the yellow, Wood found 

 that though the rotation had the same sign on the two sides of the 



red side 



blue 



^ Ijound- 

 decre as mtj ^ 



FIG. 246. 



bjuui it did not appear to rise towards the band on both sides, but 

 rather to be as indicated in Fig. 246.* 



Magnetic double refraction when the light ray is 

 perpendicular to the lines of force. Voigt's theory. 

 \Ve have seen that if we resolve a vibration which is perpendicular 

 to the field into two circular vibrations with opposite rotations 

 in a plane also perpendicular to the field, the natural periods of 

 the two are one a little greater and the other a little less than 

 the natural period before the field existed. If we assume that 

 the>e two periods t-xist for a ray travelling perpendicularly to the 

 lines of force in which the motion of the electrons is also perpen- 

 dicular to the lines of force, the equation for the velocity v before 

 the field exists, which is 



1 _ R+ ft 

 must be replaced by the less simple form 



-i,=K + 



ft 



W-v-n- 



, neglecting higher powers. 



The velocity before the field is established is also the velocity of 

 a rav which has its vibrations along the lines of force, and which 

 i> therefore unaffected by the field. Let us put it v p = v. If we 

 take off' the field, v becomes = and v n v p . This requires that 

 2ft 1 = ft. We have then 



_ _ 

 v* v p * (N 2 -/* 2 ) 3 * 



It will be noted that this is a small quantity of a higher order 

 than the corresponding difference which accounted for the rotation 



* P!,U. Miiy. ix. p. 7'2~> (11)0.")). 



