194 DOUBLE KEFBACTION, ETC. 



(a, 6, etc.) is perpendicular to the plane of symmetry or lies in that 

 plane. In the first case the dispersion of the two optic axes will be 

 unequal. The same crystal, however, with light of different colors, 

 or at different temperatures, may afford an example of each case. 



In crystals of the triclinic system, since the ellipsoids (A, B, etc.) 

 and (A', B', etc.) are determined by considerations of a different 

 nature, and there are no relations of symmetry to cause a coincidence 

 in the directions of their axes, there will not in general be any such 

 coincidence. Therefore the three axes of the ellipsoid (a, b, etc.), that 

 is, the two lines which bisect the angles of the optic axes and their 

 common normal, will vary in position with the color of the light. 



16. It appears from this foregoing discussion that by the electro- 

 magnetic theory of light we may not only account for the dispersion 

 of colors (including the dispersion of the lines which bisect the angles 

 of the optic axes in doubly refracting media), but may also obtain 

 Fresnel's laws of double refraction for every kind of homogeneous 

 light without neglect of the quantities which determine the dispersion 

 of colors. 



But a closer approximation than that of this paper will be neces- 

 sary to explain the phenomena of circularly polarizing media, which 

 depend on very minute differences of wave-velocity, represented 

 perhaps by a few units in the sixth significant figure of the index 

 of refraction. That the degree of approximation which will give the 

 laws of circular and elliptic polarization will not add any terms to 

 the equations of this paper, except such as vanish for media which 

 do not exhibit this phenomenon, will be shown in another number 

 of this Journal. 



