DOUBLE KEFRACTIOK OF CRYSTALS, 105 



die finest discoveries of that eminent genius. He was led to it 

 by the idea he had formed of the propagation of light, which His hypothesis 

 . 1 , , , , . , . r u 1 of the propa-- 



he supposed to be produced by the undulations ot ^^ ethereal ^j^j^QfUg^jj^ 



fluid. According to him, the velocity of these undulations was 

 less in common transparent mediums, than iti a vacuum, and 

 the same in all directions. But he supposed there were two 

 kinds of undulations in Iceland crystal : and that the velocity of 

 one was the same in all directions, as in ordinary mediums j 

 but that the velocity of the other was variable, and repre- 

 sented by the radii of an oblate ellipsoid of revolution, the 

 centre of which was at the point of incidence of the lumi- 

 nous ray on the face of the crystal, and the axis parallel to the 

 axis of the crystal. Huygens had also found, that, to render 

 the hypothesis answerable to experiment, the velocity of the 

 undulations respecting the 'ordinary refraction must be repre- 

 sented by half the conjugate axis of the ellipsoid ; which con- 

 nects in a very remarkable manner the two refractions, the or- 

 dinary and extraordinary. This great geometrician did not 

 assign the cause of this variety of the undulations j and the 

 singular phaenomenon exhibited by the light in passing from 

 one crystal to another, which will be noticed at the end of this 

 paper, is inexplicable on his hypothesis. This, added to the His law re- 

 great ditficuities offered by the theory of waves of light, occa- J"*^^ 

 sioned Newton, and most of the philosophers who have fol- 

 lowed him, to reject the law of refraction, that Huygeris had 

 attached to it. But Mr, Malus having proved the precision of but Malus hat 



this law by a number of very accurate experiments, we should Proved its ac- 



■' '^ curacy, 



separate it altogether from the hypothesis that led to its disco- 

 very*. It would be very interesting to connect it, as Newton 

 has done ordinary refraction, with the attractive or repulsive 

 forces, the action of which is sensible only at imperceptible 

 distances. It is, in fact, very probable, that it depends on theno ; The result of 

 and I have satisfied myself of it by the following considerations. ^^"^'^ ■°"* 



The principle of a minimum of action generally takes place principle of 

 in the motion of a point subjected to forces of this kind. On least action ap 

 applying this principle to light, we may set aside the imper- ^ ' 



* Dr. WoUastOQ had before shown the hypothesis of Huygens to be 

 agreeable to experience: See Phil. Trans, for 1802, or Journal, vol. 

 IV, p. 148. C. 



ceptibk 



