128 REPORT ON PHYSICAL OPTICS. 



was made by Sir David Brewster. This philosopher observed 

 that in some double-refracting crystals, as carbonate of barytes, 

 the two pencils were differently coloured ;* while in others their 

 intensity was widely different.! The unequal absorption of the 

 two pencils is most remarkable in tourmaline, in which it was 

 observed, nearly at the same time, by M. Biot and Dr. Seebeck ; 

 and the former philosopher inferred from the phenomena that 

 the more refrangible rays of the spectrum are more easily ab- 

 sorbed by the mineral, when polarized parallel to its axis, than 

 when perpendicularly.^ 



Sir David Brewster, to whom we owe the greater part of our 

 knowledge on this subject, has shown that similar properties 

 belong, in a greater or less degree, to most coloured crystals which 

 possess double refraction ; and that the absorption of light by 

 such media varies, in general, both with the colour of the light 

 and with the position of the plane of polarization. When a ray 

 of common light therefore enters a plate of such a crystal, the 

 two pencils into which it is divided will be unequally absorbed, 

 and the emergent light will be partially polarized, the differ- 

 ence of the intensities of the oppositely-polarized portions in- 

 creasing with the thickness of the medium traversed. But the 

 two pencils differ, in general, in colour as well as in intensity ; 

 and this difference, in uniaxal crystals, Sir David Brewster 

 found to depend on the inclination of the ray to the axis, 

 vanishing when the ray coincided with the axis, and becoming 

 a maximum when it was perpendicular to it. A ray of common 

 light, therefore, transmitted perpendicularly through a plate of 

 such a crystal, will emerge coloured; and the resulting colour 

 will, in general, vary with the inclination of the surface to the 

 axis. Thus the phenomena of dichroism, observed by Wollas- 

 ton and others, are reduced to the more general laws of absorp- 

 tion. Analogous properties belong to biaxal crystals, and depend 

 in like manner on the planes of polarization of the two pencils, and 

 on the direction of the ray. These properties Sir David Brewster 

 found could be modified by heat ; and were even communicated by 

 such influences to crystals in which they did not naturally reside. 



* Edin. Trans., vol. vii. ^ p hi i fi. anSf 1814> 



I Traite de Physique, torn, iv., 313. 



" On the Laws which regulate the Absorption of Polarized Light by double- 

 refracting Crystals," Phil. Trant. 1819. 



