temporarily prod/aced m Isotropic Bodies. 355 



are not rigorously equal to each other, nor exactly proportional 

 to the weights, as long as the latter are small ; but these dif- 

 ferences disappear as the charges become more considerable, and 

 long before any sensible permanent changes have been produced. 

 If we take the weights as abscissse, and the lengthenings or 

 shortenings corresponding as ordinates, the first below, and the 

 second above, two similar, if not equal curves, will be obtained, 

 the first of which is convex, and the second concave towards 

 the axis of the abscissa? ; the curves straighten insensibly, and, 

 for linear changes hardly measurable by ordinary means, become 

 confounded with a straight line, which pro\^es the proportionality 

 between the charges and their temporary effects. These facts 

 are confirmed by direct experiments made by different observers, 

 experiments the results of which were however too uncertain to 

 be regarded as a demonstration of the law. This confirmation 

 is exhibited in a notable degree by the experiments of Mr. 

 Hodgkinson, when they are calculated with reference to the 

 temporary effects alone, and when we take into account the 

 sources of error which influence all direct experiments by means 

 of compression. 



3. The optic axes coincide with the mechanical ones for all 

 really isotropic bodies, whether these bodies have been endowed 

 with a negative double refraction by means of pressure, or with 

 a positive double refraction by means of traction. The double 

 refraction, or the difference of path between the two rays, ordi- 

 nary and extraordinary, may be determined very exactly by 

 means of the complementary colours assumed by the two images 

 of a white beam, when the principal section of the polarizing 

 Nichol and the analysing birefracting prism form an angle of 45 

 degrees with the direction of the force applied to the body placed 

 between these two prisms. In both cases the colours ascend 

 with the charges, following exactly the series of the coloured rings 

 of Newton ; but, for actual measurements, we can hardly make 

 use of more than the colours of the first seven half rings : the 

 colours of the transmitted rings are those of the ordinary image, 

 while the tints of the reflected rings correspond to the extraor- 

 dinary image. 



4. Neglecting the small differences which we have just referred 

 to, the temporary double refraction is independent of the height 

 and length of the piece, proportional to the weight applied and 

 to the birefractive power of the substance, and reciprocally pro- 

 portional to its width and to its coefficient of mechanical elas- 

 ticity. 



5. The birefracting power of an isotropic substance, which has 

 become temporarily birefractive, can only be expressed by the 

 difference between its ordinary index and its extraordinary one ; 



