T-i HISTORY OF OPTICS. 



sequently parallel to the sides of the refracted ray. This view of the 

 subject includes some of the leading- features of the case, but still 

 leaves several considerable difficulties. 



No material advance was made in the subject till it was taken up 

 by Mains,' along with the other circumstances of double refraction, 

 about a hundred years afterwards. He verified what had been ob- 

 served by Huyghens and Newton, on the subject of the variations 

 which light thus exhibits ; but he discovered that this modification, in 

 virtue of which light undergoes the ordinary, or the extraordinary, 

 refraction, according to the position of the plane of the crystal, may 

 be impressed upon it many other ways. One part of this discovery 

 was made accidentally. 3 In 1808, Malus happened to be t observing 

 the light of the setting sun, reflected from the windows of the Luxem- 

 bourg, through a rhombohedron of Iceland spar ; and he observed 

 that in turning round the crystal, the two images varied in their 

 intensity. Neither of the images completely vanished, because the 

 light from the windows was not properly modified, or, to use the terra 

 which Malus soon adopted, was not completely polarized. The com- 

 plete polarization of light by reflection from glass, or any other trans 

 parent substance, was found to take place at a certain definite angle, 

 different for each substance. It was found also that in all crvstals in 



v 



which double refraction occurred, the separation of the refracted rays 

 was accompanied by polarization ; the two rays, the ordinary and the 

 extraordinary, being always polarized oppositely, that is, in planes at 

 right angles to each other. The term poles, used by Malus, conveyed 

 nearly the same notion as the term sides which had been employed by 

 Newton, with the additional conception of a property which appeared 

 or disappeared according as the poles of the particles were or were 

 not in a certain direction ; a property thus resembling the polarity of 

 magnetic bodies. When a spot of polarized light is looked at through 

 a transparent crystal of Iceland spar, each of the two images produced 

 by the double refraction varies in brightness as the crystal is turned 

 round. If, for the sake of example, we suppose the crystal to be 

 turned round in the direction of the points of the compass, N, E, S, 

 W, and if one image be brightest when the crystal marks N and S, 

 it will disappear when the crystal marks E^nd "W : and on the con- 

 trary, the second image will vanish when the crystal marks N and S, 



1 Mains, 111. de laDoub. R!-f. p. 296. 



* Arngo, art. Polarization, Sunn. Fnc. Brit 



