PIIEXOMEXA OF DIPOLAKIZED LIGHT. 81 



this part of optics. " To Bartholin we owe the knowledge of double 

 refraction ; to Iluyghens, that of the accompanying- polarization ; to 

 Mai us, polarization by reflection ; to Arago, depolarization." Sir D. 

 Brewster was at the same time engaged in a similar train of research ; 



o o 



and made discoveries of the same nature, which, though not pub- 

 lished till some time after those of Arago, were obtained without a 

 knowledge of what had been done by him. Sir D. Brewster's Trea- 

 tise on New Philosophical Instruments, published in 1813, contains 

 many curious experiments on the " depolarizing " properties of mine- 

 rals. Both these observers noticed the changes of color which are 

 produced by changes in the position of the ray, and the alternations 

 of color 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 hail 

 endeavored to reduce the phenomena to a law ; and had succeeded so 

 for, that he found that in the plates of sulphate of lime, the place of 

 the tint, estimated in Newton's scale (see ante, chap, vii.), was as the 

 square of the sine of the inclination. But the laws of these pheno- 

 mena became much more obvious when they were observed by Sir D. 

 Brewster with a larger field of view.* He found that the colors of 

 topaz, under the circumstances now described, exhibited themselves in 

 the form of elliptical rings, crossed by a black bar, " the most brilliant 

 class of phenomena," as he justly says, " in the whole range of optics." 

 In 1814, also, "Wollaston observed the circular rings with a black 

 cross, produced by similar means in calc-spar; and M. Biot, in 1815, 

 made the same observation. The rings in several of these cases were 

 carefully measured by M. Biot and Sir D. Brewster, and a o;reat mass 



/ / O 



of similar phenomena was discovered. These were added to by various 

 persons, as M. Seebeck, and Sir John Herschel. 



SirD. Brewster, in 1818, discovered a general relation between the 

 crystalline form and the optical properties, which gave an incalculable 

 impulse and a new clearness to these researches. He found that there 

 was a correspondence between the degree of symmetry of the optical 

 phenomena and the crystalline form ; those crystals which are uniaxal 

 in the crystallographical sense, are also uniaxal in their optical proper- 

 ties, and give circular rings ; those which are of other forms are, gene- 

 rally speaking, biaxal ; they give oval and knotted isochromatic lines, 

 ivith two poles. He also discovered a rule for the tint at each point 



2 Phil Trans. 1SH. 

 VOL. II. 6. 



