PHENOMENA OF DIPOLARIZED LIGHT. 421 



but he had a full conviction of their value, and ven- 

 tures to class them with the great steps in this part 

 of optics." "To Bartholin we owe the knowledge 

 of double refraction; to Huyghens, that of the 

 accompanying polarization; to Malus, polarization 

 by reflection; to Arago, depolarization." Sir D. 

 Brewster was at the same time engaged in a similar 

 train of research; and made discoveries of the same 

 nature, which, though not published till some time 

 after those of Arago, were obtained without a 

 knowledge of what had been done by him. Sir D. 

 Brewster's Treatise on New Philosophical Instru- 

 ments, published in 1813, contains many curious 

 experiments on the "depolarizing" properties of 

 minerals. Both these observers noticed the changes 

 of colour which are produced by changes in the 

 position of the ray, and the alternations of colour 

 in the two oppositely polarized images ; and Sir D. 

 Brewster discovered that, in topaz, the phenomena 

 had a certain reference to lines which he called the 

 neutral and depolarizing axes. M. Biot had en- 

 deavoured to reduce the phenomena to a law ; and 

 had succeeded so far, that he found that in the 

 plates of sulphate of lime, the place of the tint, 

 estimated in Newton's scale, (see p. 414,) was as the 

 square of the sine of the inclination. But the laws 

 of these phenomena became much more obvious 

 when they were observed by Sir D. Brewster with a 

 larger field of view 2 . He found that the colours of 



2 Phil. Trans. 1814. 



