ON LIGHT. 397 



circles, and we fall back upon the circular rings and 

 cross proper to that class of bodies. 



(172.) Neglecting the bending which the rays undergo 

 at their emergence from the posterior surface of the 

 crystal, or conceiving the eye as immersed within its 

 substance, it is evident that when looking in the direc- 

 tion of either of the foci of the ovals, the visual ray will 

 be directed along one of two axes, or lines of no double 

 refraction; while if looking towards any point in the 

 circumference of any one of the ovals, the visual ray will 

 traverse the crystal in such a direction that an ordinary 

 and extraordinary ray following that path shall gain or 

 lose on each other so many semi-undulations, or parts 

 of one, as shall correspond to the tint developed in that 

 direction ; and that, therefore, in all the directions 

 marked out by the circumference of each individual 

 oval, the tints being the same, the phase-difference, and 

 therefore the difference of velocities of the interfering 

 rays, and therefore again, the amount of double refraction 

 in that direction is the same. The forms of these ovals, 

 therefore, stand in immediate and intimate connexion 

 with the law of double refraction in such crystals, and 

 with the forms of the two wave surfaces belonging to the 

 ordinary and extraordinary rays. The theory of these 

 wave-surfaces belongs, however, to a higher department 

 of geometry than we could hope to make intelligible in 

 these pages. Suffice it to say that as delivered by M. 

 Fresnel and his followers it explains all the facts in the 

 most complete and satisfactory manner, and has even 

 led to the prediction, antecedent to observation, of some 



