52 PHYSICS 



close relation to non-symmetrical chemical structure by LeBel (1874) 

 and van 't Hoff (1875), and a tentative molecular theory was ad- 

 vanced by Sohncke (1876). 



Boussinesq (1868) adapted Cauchy's theory (1842) to these phe- 

 nomena. Independent elastic theories were propounded by Mac- 

 Cullagh (1837), Briot, Sarrau (1868); but there is naturally no diffi- 

 culty in accounting for rotary polarization by the electromagnetic 

 theory of light, as was shown by Drude (1892). 



Among investigational apparatus of great importance the Soleil 

 (1846, 1847) saccharimeter may be mentioned. 



Theories 



In conclusion, a brief summary may be given of the chief mechan- 

 isms proposed to account for the undulations of light. Fresnel sug- 

 gested the first adequate optical theory in 1821, which, though 

 singularly correct in its bearing on reflection and refraction in the 

 widest sense, was merely tentative in construction. Cauchy (1829) 

 proposed a specifically elastic theory for the motion of relatively 

 long waves of light in continuous media, based on a reasonable 

 hypothesis of molecular force, and deduced therefrom Fresnel's 

 reflection and refraction equations. Green (1838), ignoring molecular 

 forces and proceeding in accordance with his own method in elastics, 

 published a different theory, which did not, however, lead to Fresnel's 

 equations. Kelvin (1888) found the conditions implied in Cauchy's 

 theory compatible with stability if the ether were considered as 

 bound by a rigid medium. The ether implied throughout is to have 

 the same elasticity everywhere, but to vary in density from medium 

 to medium, and vibration to be normal to the plane of polariza- 

 tion. 



Neumann (1835), whose work has been reconstructed by Kirchhoff 

 (1876), and MacCullagh (1837), with the counter-hypothesis of an 

 ether of fixed density but varying in elasticity from medium to 

 medium, also deduced Fresnel's equations, obtaining at the same 

 time better surface conditions in the case of seolotropic media. The 

 vibrations are in the plane of polarization. 



All the elastic theories essentially predict a longitudinal light-wave. 

 It was not until Kelvin in 1889-90 proposed his remarkable gyro- 

 static theory of light, in which force and displacement become torque 

 and twist, that these objections to the elastic theory were wholly 

 removed. MacCullagh, without recognizing their bearing, seems 

 actually to have anticipated Kelvin's equation. 



With the purpose of accounting for dispersion, Cauchy in 1835 gave 

 greater breadth to his theory by postulating a sphere of action of 

 ether particles commensurate with wave-length, and in this direction 



