240 FRESNEL ON DOUBLE REFRACTION. 



pendicularly to its direction, when the ray is parallel to the axis, 

 than when it is perpendicular to it ; and one does not see why 

 this action should be nothing in the first case, whilst it attains 

 its maximum in the second. 



If, leaving aside all inquiry into the mechanicil cause of this 

 singular law, it be considered as a necessary consequence of 

 facts in the emission system, we ai"e then embarrassed by other 

 diificulties. According to this system, a beam of ordinary light 

 is composed of molecules whose planes of polarization are turned 

 in all azimuths : experiment, moreover, shows that the direction 

 of the plane of polarization of an incident ray does not change 

 abruptly at the moment when it penetrates into the crystal, but 

 gradually and after having traversed a sensible thickness, much 

 greater in general than that to which must be limited the sphere 

 of activity of the ordinary and extraordinary refraction, or the 

 limits of the curved portion of the trajectory. This being 

 est£.blished, in a beam of ordinary light, there can only be a very 

 small portion of rays having their planes of polarization exactly 

 parallel or perpendicular to the principal section : those of 

 nearly the whole of the luminous molecules will be found distri- 

 buted through all the intermediate azimuths. Now, if the repul- 

 sive influence of the axis is nothing on a ray polarized parallel 

 to the principal section, and if it makes itself felt with its full 

 energy when the ray is polarized in a perpendicular direction, 

 this repulsive force must vary gi-adually for the intermediate 

 directions, from the first, where it is nothing, up to the last, 

 where it attains its maximum. Thus, since the molecules which 

 compose the direct light are polarized in an infinite number of 

 different azimuths, they would be found subject to repulsive 

 forces of different intensity ; therefore their trajectories on 

 entei-ing the crystal ought to undergo different inflexions. In 

 order for them not to be sensibly affected by the differences of 

 intensity which the diversity of the planes of polarization of the 

 incident rays must cause in the repulsive energy of the axis, 

 it would be necessary that this action, as well as the refract- 

 ing power of the medium, should be sensible at much greater 

 depths than that to which the luminous molecules preserve 

 nearly the same plane of polarization. Now, it is exactly the 

 contrary which is most probable ; for the thickness of crystal 

 necessary to change the plane of polarization is too sensible, 

 especially in certain cases, to allow of our admitting that the 

 curved portion of the trajectory of the luminous molecule extends 





