

EXPLANATION OF ROTATORY POLARIZATION. 577 



the rotation is towards an observer's right that is to say, inversely 

 as the current which would produce the magnetisation. It is 

 difficult to assert whether this is a simple magnetic rotation due 

 to the gas, or, as Kerr believes, a new phenomenon. 



597. EXPLANATION OF ROTATORY POLARIZATION. The 

 theoretical principles put forth by Fresnel to explain the rotatory 

 polarization of quartz and of active liquids, may be applied to 

 magnetic rotatory polarization. 



It is known that a ray of light polarized in a plane is equivalent 

 to two rays polarized circularly in opposite directions of the same 

 period, moving with the same velocity, and the amplitude of whose 

 vibration is half that of the resultant rectilinear vibration. 



In order to get a conception of each of these circular rays, 

 we shall assume that, all the molecules in the same straight line 

 being disturbed from their position of equilibrium, and arranged 

 along a helix having this right line as axis, a uniform rotation 

 about the axis is imparted to the system. Each point of the 

 system, which represents a vibrating molecule, describes a circum- 

 ference about the axis, and at the same time the helix acquires 

 an apparent longitudinal motion which represents the propagation 

 of the undulation. The wave length, which is the distance traversed 

 during a period, is represented by the thread of the screw. If the 

 screw is right-handed, like an ordinary one, the vibration is in the 

 direction of the hands of a watch for an observer towards whom 

 the propagation takes place. The ray is said to be circularly 

 polarized to the right, or more simply, that it is a right circular 

 ray. The circular ray is left when the vibration is in the contrary 

 direction to the hands of a watch for an observer towards whom 

 it is moving. 



If we superpose in this way two helices in opposite directions, 

 starting from the same point A, and if each molecule of the 

 medium shares this double motion, the successive positions which 

 it will occupy in consequence of the two circular vibrations, will 

 always be symmetrical in reference to the plane passing through 

 the ray, and the point A ; the resultant vibratory motion is always 

 in this plane, and therefore the ray remains rectilinearly polarized 

 in its original plane. 



It may be assumed that matters take place in this way when 

 a ray polarized rectilinearly traverses a transparent isotropic sub- 

 stance in the natural state, like Faraday's flint glass. But if this 

 glass is placed in the magnetic field for instance, inside a cylindrical 

 coil and if the light is propagated parallel to the lines of force, 



p P 



