266 M. G. Wertheim on the double Refraction 



direction of refoulement, that is to say, in the direction in which 

 the molecules had been previously approximated. 



It might be supposed that this difference ought not to be ob- 

 served except at the commencement of the experiment, and that a 

 slight traction applied in the direction of refoulementj or a slight 

 pressure in the directions perpendicular to the latter, would be 

 sufficient to restore the equilibrium, and to bring the glass to the 

 state of homogeneity which constitutes the true zero of the expe- 

 riment. But, far from this, the difference exists under the 

 greatest pressures, which proves that there is an alteration in 

 the homogeneity of the matter, which does not manifest itself by 

 any optic phsenomenon, as long as the body is not subjected 

 anew to the action of mechanical forces. 



This fact appears to me to be very important, not only because 

 it is in opposition to the opinion generally admitted, according 

 to which all defects of homogeneity are indicated by an action on 

 polarized light, but also because the same anomaly is often met 

 with in crystals, and can furnish us with indications regarding 

 the molecular pressures which such a crystal must have experi- 

 enced at the moment of its formation. 



4. The double refraction, or the difference of path between the 

 two rays, is proportional to the mechanical lengthening or shorten^ 

 ing, but these are not rigoroushj proportional to the charges. 

 Taking for abscisses the weights, and for ordinates the shortenings 

 and lengthenings which these weights produce, we obtain for the 

 pressures a curve concave to the abscisses, and for the tractions a 

 curve convex to the abscissa; these curves become less as the 

 charges increase, and finally confound themselves ivith the same 

 straight line, ivhich corresponds to the coefficient of elasticity gene- 

 Tally adopted. 



Our last table shows the comparative route of the double 

 refractions and the charges ; the value of P;^ is always the most 



T 

 feeble, the charges augment afterwards up to Pa or P^x., and 



2 2 



from this point forward seem to rest stationary. The inverse 

 takes place with the tractions ; the greatest values are those of 

 Tx ; they diminish afterwards ; and although the experiments 



have not been pursued as far as those of compression, they suf- 

 fice to show that the two curves approximate to one and the 

 same straight line. 



Before going further, it will be necessary for us to resolve the 

 following question : — Is there an absence of proportionality be- 

 tween the double refraction and the purely mechanical effect, or 

 between the mechanical effect and the charge ? 



