410 FOUiira REPORT — 1834. 



glass prisms were placed together with their refracting angles 

 alternately in opposite directions, and the ends of the alternate 

 prisms powerfully pressed by screws. A ray transmitted 

 through the combination was found to be divided into two op- 

 positely polarized *. 



The opposite effects of compression and dilatation may be 

 seen in a thick plate of glass which is bent by an external 

 force. When this body is interposed between the polarizing 

 and analysing plates, so as to form an angle of 45° with the plane 

 of primitive polarization, two sets of coloured bands are seen 

 separated by a neutral line ; and these vanish altogether when 

 the compressing force is withdrawn. By crossing the glass 

 with a plate of mica or sulphate of lime, Sir David Brewster 

 found that the parts towards the convex, or dilated side of the 

 neutral line, had acquired a positive double-refracting structure, 

 and those at the concave, or compressed side, a negative one f- 

 The intimate connexion between the double-refracting property, 

 and the internal state of the body as to condensation or rarefac- 

 tion, is likewise proved by the curious observation of M. Biot, 

 — tliat glass, when in a state of sonorous vibration, possesses 

 the power of depolarizing the light. 



In these cases of induced double refraction, the phenomena 

 are related to the form of the entire mass j and are essentially 

 different from those produced by regidar crystals, in which the 

 law of elasticity and of double refraction depends solely on the 

 direction, and is the same in all parts of the substance. Sir 

 Uavid Brewster has lately succeeded in communicating a regu- 

 lar double-refracting structure to a mixture of resin and white 

 wax, by pressing it into a thin film between two plates of glass. 

 This film had a single axis of double refraction at every point 

 in the direction of the axis of pressure ; and the tint developed 

 depended solely on the inclination of the ray to this line. Sir 

 David Brewster has drawn from this phenomenon some highly 

 interesting conclusions respecting the origin of double refraction 

 in regular crystals. He mentions several facts which seem to 

 prove that this property is not inherent in the molecules them- 

 selves ; and he conceives that it is developed by the unequal 

 pressure caused by the forces of aggregation, which are in gene- 

 ral different in the direction of three rectangular axes. Thus 

 the double-refracting properties and the crystalline form ai'e 

 referred to the same agency J. 



Sir David Brewster and Dr. Seebeck had before observed the 



• Annates de Chimie, torn. xx. t Phil. Trans. 1816. 



J Phil. Trans. 1830. ' 



