LAW OF DOUBLE REFRACTION. 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, Wollaston 
and Malus, 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 Huyghens’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 out to him—probably by Dr. Young, 
as the circumstance is stated by him in an article in the Quarterly 
Review, 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 crystal, 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- 
a 
SEC. SER. 
