CHAPTEE XII. 



(208) IT has been stated (81, 87), that soon after the dis- 

 covery of double refraction in Iceland crystal, Huygens 

 succeeded in embracing its laws in the theory of waves, by a 

 bold and happy assumption. He had already shown that 

 the form of the wave which gives rise to the ordinary re- 

 fracted ray, in glass and other uncrystallized substances, was 

 the sphere ; or, in other words, that the velocity of undula- 

 tory propagation was the same in all directions. One of the 

 rays in Iceland crystal, too, was found to obey the same law ; 

 and judging that the law which governed the other, though 

 not so simple, was yet next in simplicity, he assumed the 

 form of its wave to be the spheroid; that is, he supposed the 

 velocity of propagation to be different in different directions, 

 in accordance with the following construction : " Let an 

 ellipsoid of revolution be described round the optic axis, 

 having its centre at the point of incidence; and let the 

 greater axis of the generating ellipse be to the less in the 

 ratio of the greatest to the least index of refraction : then the 

 velocity of any ray will be represented by the radius vector 

 of the ellipsoid which coincides with it in direction." 



(209) The law of Huygens was found to hold in many 

 crystals besides that to which it was originally applied ; and 

 in all of' these there was one optic axis, or one line along 

 which a ray of light passed without division. But when the 

 researches of Brewster made known a class of crystalline 



