37B FOURTH REPORT 1834. 



vibrations of the ether alone, while the spheroidal wave arose 

 from the vibrations of the crystal and the ether jointly. 



For the construction of Huygens, Newton substituted another, 

 Avithout stating the theoi*etical 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 I'evival of physical optics in the hands of Young, 

 that any new light was thrown upon the question. This saga- 

 cious 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 t which re- 

 called to it the attention of the scientific world, and ended in its 

 universal admission. The French Institute soon after proposed 

 the question of double refraction as the subject of their prize 

 essay, and the successful memoir of Malus left no doubt remain- 

 ing as to the accui-acy of the Huygenian lawj. 



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 harytes and arragomte. 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 pro- 

 perties, he was satisfied with a hasty generalization of the law 

 observed in Iceland spar, and concluded that it belonged to 

 all double refracting 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 following year §, he extended consider- 

 ably the list of bodies possessing the property of double refrac- 

 tion ; and arrived at the conclusion that this property belonged 

 to all crystals, excepting those whose primitive form was 

 the cube or regular uctohedron. Most organized substances, 

 whether vegetable or animal, were found to possess the same 

 properties. 



In Iceland spar the extraordinary refractive index is less than 

 the ordinary. The extraordinarj- ray consequently is always 

 refracted from the axis of the crystal ; and the same law had 

 been supposed to belong to all double-refracting substances. 



* Optics, book iii., query 25. 



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

 X " Th^orie de la Double Refraction," j1/«w. Itist. 



§ " Sur I'Axe de Refraction des Cristaux et des Substances organisecs," Mem. 

 Inst. 1811. 



