temporarily produced in Isotropic Bodies. 'WSi 



I have attributed to the latter the cause of the observed dif- 

 ferences ; these are my reasons : Mechanical force acts directly 

 only upon ponderable matter ; the differences in the tension of 

 the aether are only a secondary consequence of the differences of 

 the molecular or mechanical tensions. The double refraction is 

 necessarily a function of the temporary linear changes, and, for 

 the same substance, the optic axes depend for magnitude and 

 position on the principal mechanical axes. It is therefore neces- 

 sary that the form of this function should change according to 

 the intensity of the mechanical forces applied externally, and 

 that it should change not only with the amount of the charges, 

 but also with their mode of action ; since the double refractions 

 differ among themselves according as the substance is compressed 

 or elongated by the same weight, provided the latter does not 

 exceed a certain limit. In the first case, we produce artifi- 

 cially a body possessing negative double refraction, in which the 

 extraordinary ray is propagated with greater velocity than the 

 ordinary ; in the second case we obtain a positive body, in which 

 ^ it is the ordinary ray which moves quickest. If, now, with double 

 refractions of different magnitudes in the two cases, the mecha- 

 nical shortenings and lengthenings were still equal to each other, 

 it would follow that the ratio of the indices of refraction of the 

 two rays would vary according as the one or the other moves 

 quickest ; which conclusion is hardly admissible. 



It might be objected, it is true, that the double refraction 

 does not depend upon the linear change which takes place in the 

 direction of the force, but upon the difference between this latter 

 and the changes of length which occur in the two directions 

 perpendicular to that of the force, and that this difference, or what 

 amounts to the same thing, that the law of change of volume 

 might vary with the magnitude and the sign of the force applied ; 

 but this would be a purely gratuitous hypothesis, without any 

 fact to support it. 



On the contrary, we find a decisive proof in favour of the ex- 

 planation which we have given, in the results of the direct mea- 

 • surements of the elongations and compressions. It has often 

 been observed, that the first elongations, which require the most 

 feeble charges, are too small as compared with the subsequent 

 elastic elongations. This remark has been made by M. Poncelet* 

 with reference to the experiments of M. Ardant ; the same thing 

 reproduces itself in my experiments and in those of Mr. Hodg- 

 kinson. On the contrary, the first compressions obtained by 

 the same author are generally too great; only, inasmuch as 

 they refer to differences so small as not to be capable of mea- 



* Mecanique Industrielle, p. 347. 

 PhiL Mac;. S. 4. Vol. 8. No. 52. Oct. 1854. S 



