864 M. 6. Wertheim on the doubk Refraction 



substances. I have subjected to compression a parallelopiped, 

 very little different from a cube, cut from a perfectly pure dia- 

 mond, and of which two pairs of faces were exactly parallel to 

 each other ; the three dimensions of this parallelopiped are 2*847, 

 2-874, and 2*760 millimetres*. After having eliminated, as 

 much as possible, the effect of the very energetic laminar polar- 

 ization in this diamond, we find as means of six experiments 

 which were made by taking successively each of the dimensions 

 for height, for width and for length, the following numbers : — 



Px P^x Pa 



j^=ll-20kilogs.; -^ = ll*09kilogs.; -j-^-rr: 12*50 kilogs.; 



and finally, the coefficient of optic elasticity C = 43985 ; we have 

 besides Io = 2*470 (Brewster), but the value of Ig is unknown. 

 Let it be remarked, however, that for all the bodies which we 



have examined, the value of the ratio -^ — ^ varies between 0*04 



and 0*14; hence, assuming for the diamond a value equal to 

 that of fluor-spar, or in round numbers, equal to 0*1, we shall 

 have Ie — Io = 0-247 and E = 10865. The coefficient of elasti- 

 city of diamond would thus be nearly equal to that of copper, 

 and not at all in proportion to its great hardness. 



C. Determination of the length of an undulation of a given light. 



We have already remarked that these experiments furnish us 

 with a ready and precise means of recognizing whether the light 

 in question is homogeneous, and of determining the heteroge- 

 neous rays which are mixed with the principal colour. We have 

 also shown how the length of an undulation of any ray whatever 

 may be found, when once we have determined for a homogeneous 

 ray of known length, the charge which, applied to any isotropic 

 substance, produces an extinction of any order whatever of one 

 or the other image. 



Conclusions. 



1. The double refraction produced artificially, whether by 

 traction or compression, is, for the same substance, proportional 

 to the linear changes which this force produces in the directions 

 of the principal axes, and consequently proportional also to the 

 change of volume of the body. 



2. The temporary lengthenings and shortenings produced by 

 a given weight, according as it acts by traction or by pressui*e, 



* M. Halphen has had the. extreme kindness to have this diamond cut 

 and poUshed in his manufactory; everybody will comprehend the diffi- 

 culties which he bad to surmount to obtain the paralleUsm of the faces. 



