68 DOUBLE REFRACTION. 



ordinary ray ; and from the centre C, and with the radius CD, 

 let the sphere DOGr be described. Let the spheroid of revo- 

 lution GrE be described with the same centre, its axis of 

 revolution being in the direction of the optic axis of the 

 crystal, and equal to the diameter of the sphere, while the 

 other axis is greater in the ratio of the ordinary to the 

 extraordinary index. Now, if through F a line be drawn 

 perpendicular to the plane of the diagram, and through that 

 line there be drawn tangent planes, FO and FE, to the 

 sphere and spheroid, the lines CO and CE, drawn from the 

 centre to the points of contact, will be the directions of the 

 ordinary and extraordinary rays. This elegant construction 

 was given by Huygens. 



For this construction Newton substituted another, with- 

 out stating the theoretical grounds on which he formed it, 

 or even advancing a single experiment in its confirmation. 

 In this unsatisfactory position the problem of double refrac- 

 tion was suffered to rest for nearly a century ; and it was 

 not until the period of the revival of physical optics, in the 

 hands of Young, that any new light was thrown upon the 

 question. Young was led by the theory of waves to assume 

 the truth of the law of Huygens ; and, at his instigation, 

 Wollaston undertook the experimental examination, which 

 recalled to it the attention of the scientific world, and ended 

 in its universal admission. The French Institute soon after 

 proposed the question of double refraction as the subject of 

 their prize essay, and the successful memoir of Malus left 

 no doubt remaining as to the accuracy of the Huygenian 

 law. 



(88) It is plain, from the construction of Huygens, that 

 the point of contact of the tangent plane with the sphere, 0, 



/~1T> 



is in the plane BCF, and that the ratio, =r, which is the 



