108 REPORT ON PHYSICAL OPTICS. 



Huygens was unable to reconcile the existence of a double 

 wave within the crystal with the supposition of a single vibrating 

 medium ; and ho was accordingly forced to assume the existence 

 of two such media the spherical wave being propagated by the 

 vibrations of the ether alone, while the spheroidal wave arose 

 from the vibrations of the crystal and of the ether jointly. 



For the construction of Huygens Newton substituted another, 

 without stating the theoretical grounds on which he formed it, or 

 even advancing a single experiment in its confirmation.* In this 

 unsatisfactory position the problem of double refraction was suffered 

 to rest for nearly a century ; and it was not until the period of the 

 revival of physical optics in the hands of Young, that any new 

 light was thrown upon the question. This sagacious philosopher 

 was led by the theory of waves to assume the truth of the law of 

 Huygens ; and it was by his advice that Dr. Wollaston undertook 

 the experimental examination! which recalled to it the attention 

 of the scientific world, and ended in its universal admission. The 

 French Institute soon after proposed the question of double refrac- 

 tion as the subject of their prize essay, and the successful memoir 

 of Malus left no doubt remaining as to the accuracy of the Huy- 

 genian law.J 



The examination of Malus was chiefly directed to the case of 

 Iceland spar ; but he made a few similar measurements, also, in 

 quartz, sulphate of barytes, and arragonite. In the first of these 

 crystals he mistook the ordinary for the extraordinary ray ; and 

 the faces which he chose for examination in the two latter not 

 happening to be well adapted to the discovery of their properties, 

 he was satisfied with a hasty generalization of the law observed in 

 Iceland spar, and concluded that it belonged to all double-refract- 

 ing bodies. Malus entered largely, in the same memoir, into 

 several questions connected with the problem of double refraction ; 

 and he showed, in particular, that the laws of extraordinary reflexion 

 at the second surfaces of crystals are deducible from the law of 

 Huygens. In a memoir presented to the Institute, in the follow- 

 ing year, he extended considerably the list of bodies possessing 



* Optics, Look iii., query 25. 



t " On the Oblique Refraction of Iceland Crystal." Phil Trans. 1802. 



' Theorie de la Double Refraction." Mem. List. 



" Sur 1'Axe de Refraction des Cristaux et des Substances organisce,."-^. 

 Jntt. 1811. 



