126 Netentific Intelligence. 
(5.) The doubly refracting power of an isotropic substance which 
has become temporarily doubly refracting, cannot be expressed but by 
the difference between its ordinary and its extraordinary index ; this 
difference changes its sign only according as we apply pressure or trac 
tion, which would not be the case if we wished to express the doubly 
refracting power by a function of the two indices other than the differ- 
ence of their first powers. 
(6.) The dispersion of double refraction is insensible for substanggs 
which have been submitted to experiment. 
(7.) Glasses which had been submitted to the operation of compres 
sion while in a pasty state, ceased to be optically homogeneous bodies, 
and this alteration, entirely distinct from what is called tempering, did 
not always disappear by annealing. i 
8.) The doubly refracting power is not the same for different iso 
tropic substances ; no connection can be established between this power 
and the ordinary index of refraction or even the density. 
.) By analogy with the ordinary or mechanical coefficient of elas- 
ticity E, we call coefficient of optical elasticity C the ratio between ! 
charge applied to the unit of surface and the double refraction which 
it produces ; we have then the simple equation, 
| Sep a aes. ee 
which serves to determine the doubly refracting power 
p= +t Uo — Te). 
(10.) The value of the doubly refracting power being once known 
for a substance, we may use the phenomena of double refraction 10 d¢ 
termine any one of the quantities which enter into the equation _ 
P(lo—le) = d. E. Sy 
the indices for the ordinary and 
extraordinary ray, d is the difference of path, and La the breadth of 
piece employed. 
_(11.) The most important of these applications consists in the deter 
mination of the force P, whatever be its magnitude and mode of action. 
where P is the charge, lo and I 
the useful and the theoretical effect, to graduate ordinary manometels 
accurately, and even to measure living forces, 
12.) The same formula would serve to determine the coefficient of 
mechanical elasticity if we had a direct method of finding the extract 
dinary index le; but in the mean time it has permitted me to establis! 
the optical coefficient of the diamond, and to fix certain limits betwee® 
which its mechanical coefficient is comprised. oe 
(13.) The difference of path d being independent of the length 0 
the undulation 4, if the ratio z is to remain the same for the differet! 
values of 2, P must vary proportionally to 4; which furnishes an easy 
method of determining the lengths of undulations, and of deciding 
whether a gist ray is homogeneous, or what are the different simp! 
rays of which it is composed. 
