260 On the double Refraction produced in Isotropic Bodies, 



experiments prove that we might safely continue to make use of 

 the s.une coefficient of elasticity in calculating the effects of trac- 

 tions and of compressions. 



But the results acquire a great importance when they are 

 regarded from the point of view of the theory of molecular forces, 

 or those of mechanical oscillations, or of sonorous vibrations ; I 

 believe that they will furnish the solution of a certain number of 

 questions which have hitherto rested in suspense, and to which I 

 propose to return on another occasion. 



5. The difference d 0/ the path passed over is independent of the 

 length of the undulation \; hence the dispersion of the double 

 refraction is insensible. 



The charges P are proportional to the differences of path 

 between the limits which we have defined ; hence, if our propo- 

 sition be true, we ought to have the equation 



in which X and V are the lengths of the undulations of two 

 simple rays, and P, P' the chai'ges which must be applied to the 

 same parallelopiped to obtain extinctions of the same order in 

 the one or the other image. 



This equation is already in part demonstrated by the deter- 

 mination of the fixed points which we have made use of in the 

 construction of our table ; to verify it between the widest limits, 

 I thought it necessary to make some observations with the red 

 and violet rays. 



I have employed successively red glasses of two descriptions ; 

 experiment itself showed me that neither of the two was rigour- 

 ously monochromatic ; but in placing them one above the 

 other, I have obtained a deep red colour without any trace 

 of other rays. The violet light was obtained by the passage 

 of white light through a solution of the ammoniacal sulphate 

 of copper; the light transmitted still contained some yellow 

 rays. 



We shall now make use of our experiments to determine the 

 colours of the spectrum, assuming for point of comparison the 

 mean yellow ray Xy=550. 



