LAW OF DOUBLE llEFRACTION. 193 



able to keep a just mean. Huyghens had given his law 

 as the result of an hypothesis ; men rejected it therefore 

 ■without examination. The measures on which it was 

 founded could not redeem it from what was thought 

 vicious in its origin. Newton himself took part among its 

 opponents ; and from this moment the progress of optics 

 ■was arrested for more than a century. Since that period, 

 the numerous experiments and measures of two of the 

 most celebrated members of this Academy, WoUaston 

 and Mains, have replaced the law of Huyghens in the 

 rank to which it is entitled.* 



* Newton had rejected Huyghens's law, and substituted one founded 

 on measures of his own. In 1788 Haiiy repeated the measurements, 

 and showed that Huj'ghens's rule was far more accurate than New- 

 ton's. In 1802 Wollaston repeated similar observations by his new 

 method, in ignorance of Huyghens's law; but found them well repre- 

 sented when that law was pointed oi;t to him — probably by Dr. Young, 

 as the circumstance is stated by him in an article in the Quarterly 

 Revieto, Nov. 1809, p. 338. 



Some idea may be given of the simple geometrical construction de- 

 termining the direction of the extraordinary ray which results from 

 Huyghens's theory, as follows: Supposing portions of the concentric 

 sphere and spheroid within the crj-stal, whose axis a coincides with 

 the axis of revolution of the spheroid; and conceiving a second spher- 

 ical surface concentric, and of greater radius, as that which would 

 have been the wave surface if the velocity had remained undimin- 



