REPORT ON PHYSICAL OPTICS. 381 



through the ray, and bisecting the dihedral angle contained by 

 these planes ; while that of the other vras perpendicular to the 

 former, or bisected the supplemental dihedral angle*. 



When a ray of light enters a crystal, the component molecules 

 are supposed, in the theoiy of M. Biot, to receive different mo- 

 tions round their centres of gravity, dependent on the nature of 

 the forces exerted upon them by the particles of the body. 

 Sometimes the molecules of the ray are turned by the operation 

 of these forces, so as to have certain lines in each, denominated 

 axes of polarization, all in the same direction ; and this arrange- 

 ment of the molecules is maintained throughout the whole of 

 their future progress. There are other cases, however, according 

 to this author, in which the molecules oscillate round their cen- 

 tres of gravity in certain periods, during their entire progress 

 through the crystal ; while in others, finally, they receive a 

 motion of continued rotation. To the two latter cases I shall 

 have occasion to advert hereafter. 



The phenomena oi fixed polarization are ascribed by M. Biot 

 to the operation of certain forces, which he denominates polariz- 

 ing forces. In the case of uniaxal crystals these forces are sup- 

 posed to act in the planes containing the two rays and the axis 

 of the crystal, — the ordinary polarizing force tending to arrange 

 the axes of the molecules in the plane containing the ray and 

 the axis, while the extraordinary polarizing force draws them 

 towards the perpendicular plane. If the molecules were simi- 

 larly circumstanced in every respect, they would necessarily obey 

 the stronger of these forces, and there would be but one plane 

 of polarization. This, however, is supposed not to be the case. 

 Owing to the diffei'ent phases of their fits, at their incidence upon 

 the crystal, the molecules are disposed to yield more readily to 

 one or other of these forces ; so that when a polarized ray meets 

 a double refracting medium, some of the molecules fall under 

 the influence of the ordinary polarizing force, and have their 

 axes of polarization turned into the plane containing the ray and 

 the axis of the crystal, while others are actuated by the extra- 

 ordinary force, and have their axes arranged in the perpendicular 

 plane. The number of molecules which yield to one or other 

 of these forces, or the intensity of the two polarized raj'^s, is 

 supposed to depend on the angle which the plane of primitive 

 polarization makes with the two planes just mentioned. When 

 the plane of polarization coincides with the former, the extra- 

 ordinary force has no effect, and the ray receives only the ordi- 

 nary polarization ; the converse takes place when the plane of 

 polarization coincides with the perpendicular plane. Similar 



* Ibid. 



