Chemistry and Physics. 125 
EM has communicated an elaborate memoir upon these important sub- 
jects; we must however refer to the original paper for the details of the 
experiments and content ourselves with stating the results which, in the 
author’s own words, are the following. 
(1.) The double refraction artificially produced, either by traction or 
i If we lay off the weights on the axis of abscissas and the correspond- 
ing lengthenings and shortenings upon the axis of ordinates, the: first 
below and the second above this axis, we obtain two similar, if not 
equal, curves, the first of which is convex and the second concave to- 
d the axis of abscissas. These curves insensibly become straight 
and for linear char es which a rcely measurable by ordinary 
Servers, experiments the results of which were only too uncertain to 
pie. exhibit the truth of the law; this confirmation results especially 
rom the experiments of Mr. Hodgkinson when we calculate them so 
(8) 
'Solropic bodies, whether they have been endowed with negative double 
refraction by pressure or with positive double refraction by traction. 
The double refraction or the difference of path of two rays, ordinary 
aordinary, may be determined very accurately by means of 
> NOWeve ) 
than the first seven half rings: the colors of the transmitted rings are 
paid the ordinary image while the tints of the reflected rings cor- 
Pond to the extraordinary image. 3 é 
(4.) Making no account of the small differences which have just been 
eet Out, the temporary double refraction is independant of the 
ight and length of the piece, proportional to the weight applied and to 
“ doubly refracting power of the substance, and reciprocally propor- 
'0 its breadth and to its coefficient of mechanical elasticity. — 
