356 0/1 the double Refraction produced in Isotropic Bodies, 



this difference only changes the sign, according as we apply 

 pressure or traction ; this could not take place if the birefracting 

 power were expressed as any function of the two indices other 

 than the difference of the first powers. 



6. The dispersion of the double refraction is insensible for the 

 substances which we have submitted to experiment. 



7. The glasses which have been subjected to the process of 

 refoulement have ceased to be optically homogeneous bodies; 

 and this alteration, entirely distinct from that which we call the 

 temper of the glass, does not always disappear by annealing. 



8. The birefracting power is not the same for different isotropic 

 substances ; no relation can be established between this power 

 and the index of ordinary refraction, or even the density. 



9. By analogy with the coefficient of ordinary or mechanical 

 elasticity E, we apply the name of coefficient of optic elasticity 

 to the ratio of the charge applied to the unit of surface and 

 the double refraction which it produces; we have then the 

 simple equation 



E 



Io-Ie = 



which serves to determine the birefractive power, 



P=±(Io-le)- 



10. The value of the birefracting power of any substance 

 being once known, the phsenomena of double refraction may be 

 made use of to determine any one of the quantities which enter 

 into the equation 



±P(I,-I,)=(/.E.La. 



11. The most important of these applications consists in de- 

 termining the force P, whatever be its magnitude or mode of 

 action. The chromatic dynamometer gives immediately, and 

 without the employment of any coefficient of correction, the 

 effective pressure of a vice, press, hydrauhc press, lever, &c. ; 

 it can be made use of to ascertain in all machines the ratio be- 

 tween the useful and the theoretic effect, to graduate in an exact 

 manner ordinary manometers, and even to measure vis viva. 



12. The same formula would serve to determine the coefficient 

 of mechanical elasticity, if we possessed the means of finding 

 directly the index of extraordinary refraction !<. ; but, in the 



. mean time, it has enabled me to establish the optical coefficient 

 of the diamond, and to fix certain limits between which its 

 mechanical coefficient is comprised. 



13. The difference of path d being independent of the wave- 

 length \, if the ratio - remains the same for different values of 



