temporarily produced in Isotropic Bodies. 345 



- E 



The quantity I^— !«= + ^r is the true measure of the double 



U J 2 



refraction ; for if we take as such the quantity l^ — Iq^ or ^^y we 



should obtain for the same substance a different birefractive 

 power according as it happened to be temporarily positive or 

 negative, which is inadmissible. 



It might be hoped that these researches would conduct to 

 some simple ratio between the birefractive power which may be 

 called specific, and the other properties of the body. My first 

 experiments, which were made with some specimens of glass only, 

 had furnished a sensibly constant value for the birefractive power ; 

 and if this constancy had been general, important consequences 

 relative to the distribution of force in bodies naturally birefractive 

 would have flowed from it. But since these first experiments, I 

 have observed* that heavy flint has an extremely high coefficient 

 of optical elasticity ; the ratio of the two coefficients could not 

 therefore be the same as for the other glasses, unless the flint 

 possessed a very high coefficient of mechanical elasticity also; 

 this is hardly probable, when we consider the great quantity of 

 lead which enters into its composition. Indeed, experiment 

 ha^ demonstrated the contrary, and the results which I have 

 since obtained on the double refraction produced in isotropic 

 crystalline bodies are in direct contradiction with any theory of 

 this kind. 



The birefractive power does not stand in any simple ratio to 

 the density ; neither is it a function of the velocity Oo of light 

 in isotropic bodies, as might be inferred from the results which 

 M. Broch has obtained by calculation f- 



We are thus obliged to assume that the birefractive power, 

 like the refractive, is inherent in each substance, or what is the 

 same thing, that it depends upon a relation still unknown between 

 the mechanical and optical elasticities of the body. 



Further, to be sure that there is no parallel between the two 

 kinds of double refraction,~the natural and the artificial, it is 

 sufficient to consider the magnitude of the forces which it would 

 be necessary to apply to an isotropic body to cause it to produce, 

 with equal thicknesses, a difference of path equal to that which 

 results from the passage across a plate parallel to the axis of 

 certain birefracting crystals. Let us compare, for example, Ice- 

 land spar with ordinary crown glass ; the difference of the two 



been obliged to suppose its coeflficient of elasticity equal to that of the 

 heavy flint preceding ; but its great density renders it probable that the 

 coefficient is really a little more elevated, 



* Comptes Rendus, vol. xxxii. p. 291. 



t Dove, Repertoire de Physique^ vol. vii. p. 58. 



Phil Mag. S. 4. Vol. 8. No. 53. Nov. 1854. 2 A 



