VERDET'S LAWS. 575 



cases the angle of rotation is proportional, other things being equal, 

 to the thickness of the medium traversed ; but in quartz, in solution 

 of sugar, in essence of turpentine, the rotation is connected with the 

 propagation of light in such a manner that it is always in the same 

 direction for the observer who receives the rays. It follows from 

 this, that if the ray, after having traversed the transparent substance, 

 returns on its own path after having been perpendicularly reflected, 

 it undergoes a rotation which is equal and opposite to the first, 

 and the plane of polarization reverts to its primitive position at 

 its starting-point. 



The magnetic rotation, on the contrary, is independent of the 

 direction of the propagation, and only depends on the direction 

 of the magnetic force. The ray, which returns on its own path 

 after a normal reflection, undergoes a rotation in the same absolute 

 direction, which adds on to the first ; in this way, by causing the 

 ray to be perpendicularly reflected an unequal number of times, 

 272 + 1, we may observe the same rotation as if it had traversed 

 a layer of the substance zn + 1 times the thickness. 



594. VERDET'S LAWS. Verdet proved, experimentally, that for 

 a homogeneously polarized ray, the rotation of the plane of polari- 

 zation is proportional : 



ist To the thickness traversed; 



2nd. To the component of the magnetic force in the direction 

 of the ray ; 



3rd. To a coefficient depending on the nature of the body, 

 and which is positive or negative, according as the body is dia- 

 magnetic or magnetic. 



These laws may be summarised in the following statement : 



The rotation of the plane of polarization between two points is 

 proportional to the difference of magnetic potential between these 

 points. 



Let V and V be the values of the potential at two points A 

 and A' in the path of the ray ; the angle 9 by which the plane of 

 polarization is turned between these two points will be expressed 

 by the ratio 



= <o(V-V), 



w being the rotation which for a given substance corresponds to a 

 difference of potential equal to unity. This quantity is known as 

 Verde 'fs constant ; it defines the magnetic rotatory power of the 

 body. 



