DOUBLE REFRACTION. 1Q7 



III. Double Refract ion. 



The phenomenon of double refraction was first discovered by 

 Erasmus Bartholinus, in Iceland spar. After a long series of ob- 

 servations, he found that one of the rays within the crystal observed 

 the known law of refraction discovered by Snellius, while the other 

 was bent according to a new and extraordinary law. An account 

 of these experiments was published at Copenhagen in the year 

 1669, under the title " Evperimenta Crystalli Islandici Disdiactastici, 

 quibus mira et insolita refract io deteyitur." 



The success of Huygens in deriving the laws of ordinary re- 

 fraction from the hypothesis of waves naturally led him to examine 

 whether these new phenomena could be reconciled to the same 

 theory ; and in his desire to assimilate the two classes of pheno- 

 mena, he was happily led to assign the true law of extraordinary 

 refraction. Huygens had already shown that the direction of the 

 refracted ray, in glass and other uncrystallized substances, could 

 be deduced from the supposition that the ethereal wave within the- 

 substance was a sphere, or, in other words, that the velocity of 

 undulatory 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 of revolution, the greater and the lesser axis 

 of the generating ellipse being in the ratio of the greatest and 

 least index of refraction. The form of the wave being known, the 

 law of refraction is derived from the principle of the superposition 

 of small motions. Conceive three surfaces having their common 

 centre at the point of incidence, and representing respectively the 

 simultaneous positions of three waves diverging from that point, 

 the first in air, the other two within the crystal. Let the incident 

 ray be produced to meet the air icave, and at the point of intersec- 

 tion let a tangent plane be drawn. Through the line of intersection 

 of this plane with the refracting surface let planes be drawn 

 touching the two refracted waves ; the lines connecting the centre 

 with the points of contact are the directions of the two refracted 

 rays. This beautiful construction, and the other speculations of 

 Huygens on the subject of extraordinary refraction, are contained 

 in the fifth chapter of his Traite de la Lin// fire. 



