30 REPOKT ON PHYSICAL OPTICS. 



tion ; that around that direction its properties were on all sicks 

 alike; and that if the ray were supposed to revolve round that line 

 as an axis, the resulting phenomena would be unaltered. 



Huygens was the first to observe that this was not always the 

 case. In the course of his researches on the law of double refrac- 

 tion, he found that when a ray of solar light is received upon a 

 rhomb of Iceland crystal,* in any but one direction, it is always 

 subdivided into two of equal intensity. But on transmitting 

 these rays through a second rhomb, he was surprised to observe 

 that the two portions into which each of them was subdivided 

 were no longer equally intense ; that their relative brightness 

 depended on the position of the second rhomb with regard to the 

 first ; and that there were two such positions in which one of the 

 rays vanished altogether. 



From this " wonderful phenomenon," as Huygens justly called 

 it, it appeared that each of the rays refracted by the first rhomb 

 had acquired properties which distinguished it altogether from 

 solar light. It had, in fact, acquired sides ; and it was evident 

 that the phenomena of refraction depended, in some unknown 

 manner, on the relation of these sides to certain planes within the 

 crystal. Such was the conclusion of Newton : " This argues," 

 says he, "a virtue or disposition in those sides of the rays, which 

 answers to, and sympathizes with, that virtue or disposition of the 

 crystal, as the poles of two magnets answer to one another." 



This conception was followed out by Malus, whose varied and 

 important discoveries respecting the nature and laws of polarized 

 light have justly placed him in the rank of founder in this most 

 interesting branch of science. The molecules of a polarized ray 

 were supposed by him to have all their homologous sides turned 

 in the same directions. He adopted the term "polarization" to 

 express the phenomenon, and compared the effect to that of a 

 magnet which turns the poles of a series of needles all to the same 

 side. M. Biot has modified the hypothesis of Malus in order to 

 embrace the other phenomena of light, and assumes that there is 

 one line, or axis, similarly placed in each molecule, and that these 

 axes in a polarized ray are all turned in the same direction. 

 The molecules, however, are at liberty to revolve round these 

 axes, and thus to assume different dispositions with respect to the 

 attracting or repelling forces to which they are exposed when they 

 encounter the surface of a new medium. 



