574 OPTICAL PHENOMENA. 



CHAPTER VIII. 

 OPTICAL PHENOMENA. 



591. FARADAY'S DISCOVERY. After prolonged researches, which 

 for a long time were unfruitful, Faraday discovered in 1845 that 

 a transparent body, though itself destitute of rotatory power, becomes 

 capable, under the influence of magnetism, of rotating the plane of 

 polarization of a luminous ray. The effect is at its maximum when 

 the polarized ray traverses the body parallel to the lines of force ; 

 it is zero when the two directions are at right angles. 



This phenomenon, which was first observed in the case of heavy 

 flint glass, is produced in all single refracting liquids and solids ; 

 the action of magnetism is less perceptible in double refracting 

 bodies ; it is extremely feeble in gases and vapours, and it is only by 

 quite recent experiments that it has been ascertained to exist. 

 Bodies naturally endowed with rotatory power give rise to the 

 same phenomenon ; the two rotations become added or substracted 

 according to their respective directions. 



592. POSITIVE AND NEGATIVE BODIES. All the bodies examined 

 by Faraday rotate the plane of polarization in the same direction 

 under the influence of magnetism ; it is that direction of the current 

 which, revolving around the ray, would give to the field its actual 

 direction. All these substances are diamagnetic. Verdet found that 

 most magnetic substances (for instance, solutions of ferric chloride 

 in alcohol or ether) cause the plane of polarization to rotate in the 

 opposite direction. If we consider the former rotation as positive, 

 we may in general, though not with absolute strictness, assert that 

 diamagnetic substances turn the plane of polarization in the positive 

 direction, and magnetic substances in the negative direction. 



593. There is an important difference in the way in which the 

 rotation of the plane of polarization takes place, according as we 

 consider the natural rotation or the magnetic rotation. In both 



