462 HISTORY OF SCIENCE. 



what point or in what direction soever it enters the crystal. The 

 other or " extraordinary " portion of the ray, on the contrary, propa- 

 gates undulations which move with less velocity in a certain direc- 

 tion than in any other. This direction is parallel to the line joining 

 those two corners of an equilateral rhomb of Iceland spar which are 

 formed by the union of three obtuse angles of the surface, and which, 

 from the properties we are considering, is called its optic axis. The 

 velocity of propagation in all directions perpendicular to the axis is 

 uniform and at its maximum. It will, however, be quite unnecessary to 

 here pursue further the geometry of double refraction in Iceland spar, 

 which is not without a certain difficulty. Enough has been said to 

 enable the reader to understand the importance of a confirmation of 

 all its conclusions, which was effected by Wollaston when he deter- 

 mined the values of the indices of reflection in Iceland spar by an inge- 

 nious method of his own invention. What renders this confirmation 

 the more striking is the fact that at the time Wollaston made his dis- 

 covery he was unacquainted with Huyghens' hypothesis. We may 

 infer from this how little ground the undulatory theory had gained 

 among men of science in 1802. 



The discovery by Malus of polarization by reflection and by trans- 

 mission was a surprise to the scientific world, for polarization had 

 until then been considered as merely the special property of two or 

 three minerals. The generality of this property which the discovery 

 of Malus revealed, and the certain allied phenomena first observed 

 in the early years of the present century by Fresnel and Arago, de- 

 manded a theoretical explanation of a more comprehensive kind than 

 the advocates of the Newtonian hypothesis were able to bring for- 

 ward. Young, who had been attracted to the general undulatory 

 theory of light by analogies he had observed between light and sound, 

 had entertained the idea of transverse vibrations in rays of light, but 

 hesitated to advance that hypothesis on account of some dynamical 

 difficulty. This had reference to the mode in which a force could 

 be conceived to produce, in such a medium as the supposed ether, 

 vibrations transverse to the direction in which the ray is propagated. 

 Fresnel, however, came to the conclusion that only by the supposi- 

 tion of transverse vibrations could certain phenomena, which he ex- 

 amined in conjunction with Arago, be explained in terms of the un- 

 dulatory theory. Although Arago had been associated with Fresnel 

 in the experimental investigations, the account of which was published 

 under their joint names, the second part of the paper, where the 

 hypothesis of transverse vibrations is first definitely proposed and 

 justified, stands in the name of Fresnel alone, Arago shrinking from 

 joining in the publication of so bold a conception. The difficulty of 

 reconciling lateral vibrations with the force determining the direction 

 of the ray will doubtless occur to most readers. It would hardly be 

 possible to explain here the mechanical and mathematical principles 



