Dr Wollaston’s Observations on Double Refractimi. 32^ 
Dr Wollaston considers the result of this comparison as high- 
ly favourable to the Huygenian Theory ; and he adds, that 
though the existence of two refractions at the same time, and 
in the same substance, be not well accounted for, and still less 
their interchange with each other, when a ray of light is made 
to pass through a second piece of spar, situated transversely to 
the first ; yet the oblique refraction, when considered alone, seems 
nearly as well explained as any other optical phenomenon 
Sect. VI. Account of the Investigations of M. La Place, 
The attention of this illustrious mathematician was no doube 
directed to the subject of double refraction, by the labours of 
Dr Young and Dr Wollaston, who had drawn the attention of 
the scientific world to this recondite branch of physical science. 
These investigations are contained in a memoir, entitled^ 
‘‘ Sur les Mouvemens de la Lumiere dans les Milieux Dia* 
phanesf which was read at the Institute on the SOth January 
1808, and published in their Memoirs for ISOO? p. 300, — 342. 
It occurred to M, La Place, that it would be highly interest- 
ing to refer the law of Huygens to attractive and repulsive for- 
ces, as Newton had done the ordinary refraction. In employ- 
ing the principle of least action for this purpose, he remarks^ 
that, in the case of the extraordinary refraction, the velocity of 
the light within the crystal must be independent of the manner 
in which it enters, and must depend only on the position of the 
ray with respect to the axis of the crystal, that is, on the angle 
which the ray forms with a line parallel to the axis. 
In setting out from this datum, M. de La Place arrives at two 
differential equations given by the principle of least action, and 
in which the interior velocity is an indeterminate function of the 
angle, which the refracted ray forms with the axis of the crys- 
tal. In the first case which he examines, the square of the ve- 
locity of the ray is increased in the interior of the medium by 
a constant quantity, (which is the case of ordinary transparent 
mediums), and this constant quantity expresses the action of the 
medium upon light. The two equations, then, shew that the 
^ Phih Transactions 180^. 
