168 



FROPERTIBS OF LIGHT. 



Double reflec. 

 tioa. 



General re- 

 marks. 



the sine of the angle included between the axis of tlie crys- 

 tal, and the principal axis, a, of the luminous particle. All 

 the particles, of which the axis b is perpendicular to this 

 force, are refracted ordinarily; and all those, of which the 

 axis c is perpendicular to it, are refracted ordinarily. The 

 particles refract^ ordinarily, that escape the repulsive 

 force, are in the same case with those, that escape reflection 

 in the first class of facts 1 have described. 



The phenomena of double reflection at the second sur- 

 face of transparent crystals are analogous to those of the re- 

 fraction in two crystals, the principal sections of which are 

 parallel, and their axes hi o-jposite >:irections; with the ad- 

 dition of this property common to all diaphanous bodies, 

 that, when the reflecting face is parallel to the axis c of the 

 luminous particles, the reflection at a determinate angle is 

 null. 



Thus, without the knowledge of this singular property of 

 transparent substances, the most extraordinary part of the 

 phenomena of double refraction would have remained inex^ 

 plicable. 



I shall not enter ipore largely into the particulars of the 

 application of the tht;ory J have brought forward, but shall 

 content myself with saying, that it refers to one source a 

 number of facts, which seemed to have no analogy to each 

 other, and the want of connexion in which rendered it 

 almost impracticable to measure them. 



I do not pretend to point out the cause of this general 

 property of the repulsive powers that act on light ; 1 merely 

 exhibit the means of connecting the phenomena with each 

 other, of ascertaining them before hand by calculation, and 

 of measuring thenj with accuracy : at the same time in re- 

 ferring the figures of the luminou:, particles to three rectan- 

 gular angles, as those of an octaedron would be, I antici- 

 pate nothing respecting the real figure of these particles ; 

 but I present the result as a consequence of the calculation, 

 %o which I have been led by the analysis of the- phenomena 

 that. J. have observed. 



n. 



