40 



nesses : but the reverse will happen in each case if one glass be 

 compressed and the other dilated. 



If a compressing force be applied to the centre of a plate of gla^~ 

 it will exhibit the black cross and other phenomena to be seen by 

 means of doubly-refracting crystals. 



If a plate so compressed be inclined to the polarized ray, the tints 

 of colour will ascend or descend, according to the direction in which 

 it is inclined. 



If a plate to which the power of depolarization has been given by 

 heat be compressed, the tints of the interior fringes rise in the scale, 

 and those of the exterior descend, when the axis of pressure is per- 

 pendicular to their direction. 



The same effects which are thus produced upon glass by com- 

 pression, are produced in a similar manner upon such crystallized 

 bodies as do not possess these properties in their natural state. But 

 those bodies which already possess the doubly-refracting structure 

 in a high degree, as calcareous spar, rock crystal, beryl, &c., suffer 

 no change by any degree of mechanical compression to which the 

 author has subjected them. 



Since the tints of colour communicated to polarized light depends 

 on the degree of force applied to glass, through which it is trans- 

 mitted, Dr. Brewster conceives that a convenient instrument might 

 be constructed for measuring the intensity of forces, which he would 

 call a Chromatic Dynamometer ; and in the same manner might 

 variations of temperature, or humidity of bodies be measured, with 

 the assistance of a little ingenuity, by chromatic thermometers and 

 hygrometers. 



In the prosecution of these experiments, the author examined the 

 properties communicated to jelly by variations of its density from 

 drying, and contrived means of giving it permanent power of depo- 

 larization, by the constrained position in which it was allowed to 

 harden. And he found that the polarizing force of distended isinglass 

 far exceeds any which can be given to glass, either by heat or pres- 

 sure, and is even greater than that of beryl, which owes its power 

 to crystalline texture. 



In conclusion, the author expresses his hope that the principles 

 here investigated afford a solution of the most important part of the 

 problem of double refraction, by ascertaining the mechanical con- 

 dition of both classes of doubly-refracting crystals, although the di- 

 vision of incident light into two portions oppositely polarized yet 

 remains to be accounted for : and he thinks we must remain satis- 

 fied with referring this to the operation of some peculiar fluid, which 

 he conceives to be the principal agent in producing all the pheno- 

 mena of crystallization and double refraction. 



Dr. Brewster adds, that a recent experiment (which he does not 

 describe) upon the polarizing qualities of a body of which the den- 

 sities vary in regular minute strata, induces him to think more fa- 

 vourably than heretofore of the uiidulatory system of light. 



