^Ether-particles in a Rectilinearly Polarised Ray of Light. 191 



Stokes*, Haidingerf, and Lorenz J, have decided for the first 

 view, MM.Babinet§, Holtzmann||, and Jamin ^[ adhere to the 

 latter. Cauchy**, who in his theory assumes independent par- 

 ticles of sether, was originally for Neumann's view, which, how- 

 ever, he afterwards, in a letter to M. Libri, exchanged for Fres- 

 nePs, since he could not assume that the forces which in a 

 normal condition are exerted in vacuo on the sether disappeared* 

 As far as is known to the author, Cauchy has never published 

 the reasons which led him to this assumption. • 



Of the methods of experimentally deciding between the two 

 assumptions, that of MM. Stokes, Holtzmann, and Lorenz, of 

 deducing the direction of the vibrations from the position of 

 the plane of polarization in diffracted rays, had given discordant 

 results ; and according to Fizeau's newest experiments ft no con- 

 clusion appears possible from this method. 



In like manner, Haidinger's experiment for deciding the 

 question by considering the absorption of light in crystals in its 

 dependence on the position of the direction of vibration of the 

 sether-particles, has led to no result ; and objections might be 

 raised against Babinefs proof, who, from the position of the 

 plane of polarization of light which is reflected from paper 

 surfaces at an extremely oblique incidence, concludes that the 

 vibrations are in the plane of polarizition. 



Jamin %%, again, in an elementary proof of Cauchy's for- 

 mula? for the reflexion and refraction of light at the surface of 

 transparent bodies, has defended Neumann's assumption. For 

 if the components of the vibrations normal to the limiting sur- 

 face are constructed for the rays in each of the media vibrating 

 in the plane of incidence, these, according to Jamin, must stand 

 in a ratio independent of the angle of incidence. According 

 to Neumann's assumption, these components are equal ; accord- 



f sm i \ 

 ing to Fresnel's assumption, they are as 1 : ( — — ) , where i 



and r are angles of incidence and refraction. 



In this Jamin sees a refutation of FresnePs assumption ; 



(sm z \ 

 — — ) =the refractive index, and therefore = a con- 

 sin rj 



stant magnitude, by this assumption the required condition 



would be satisfied. 



* Cambridge Phil. Trans. f Wien. Ber. vols. xii. & xv. 



% Pogg. Ann. vol. cxi. p. 315,, and vol. cxiv. p. 250. 



§ Compt. Rend. vol. xxix. p. 514. || Pogg. Ann. vol. xcix. p. 446. 



^ Ann. de Chim. et de Phys. (3) vol. lix. p. 413. 



** Compt. Rend. 1836, vol. ii. p. 342. 



ft ^-nn. de Chim. et de Phys. (3) vol. lxiii. p. 385. 



XX Ibid. vol. lix. p. 413. 



