ON LIGHT. 361 



them within an interval of time inappretiably short, and 

 without prejudice to the continuous perception of the 

 vibratory movement communicated to the ether as light. 

 The act of polarization consists then in the subsequent 

 arrangement, at some definite point in the Hne of progress 

 of the ray, of all these vibratory movements, into parallel- 

 ism with each other, or into a single plane from which 

 they have afterwards no tendency {per sc) to deviate. 

 As the particles of crystallized bodies must be con- 

 ceived to be arranged in definite lines and planes, it 

 is easily conceivable that, whether among them, or in 

 conjunction with them, the ethereal molecules may 

 be confined in their vibrations to two particular 

 planes determined by the internal constitution of 

 the crystal and the incidence of the light ; that a 

 vibratory movement propagated into a body so con- 

 stituted should ipso facto resolve itself into two such 

 movements in these two planes (according to the general 

 mechanical principle of the composition and resolution 

 of motion) ; that it should be so propagated during its 

 progress through the crystal ; and that at its emergence 

 into free space, each vibration should thenceforward sub- 

 sist separately, there being nothing to change it. Again, 

 it is no less conceivable that in these vibrations the 

 molecules of the ether moving in one plane may be 

 differently impeded by, or stand in a different connexion 

 with those of the medium, from those moving in the 

 other, and that, in consequence, their propagation of 

 the movement may be effected with a different velocity, 

 and thus give rise to a difference of refractive power, 



