POLARIZATION TRANSVERSAL VIBRATIONS. 8.'] 



The erroneous views of mathematicians on this subject, accord- 

 ing to Fresnel, have arisen from the imperfect physical conceptions 

 which they have made the basis of their reasoning. Elastic fluids 

 have been represented as composed of particles in contact, capable 

 only of condensation and dilatation ; and accordingly the accelerat- 

 ing forces have been conceived to arise solely from the difference 

 of density of the consecutive shells of the fluid. In this case, it is 

 evident that if any row of particles is displaced in the direction of 

 the connecting line, this row will slide upon the succeeding one, 

 and the motion will be resisted by no elastic force. But when we 

 regard these bodies as they really are composed of molecules 

 separated by intervals which are probably considerable as compared 

 with their magnitude, and acting on one another according to 

 some law varying with the distance, the whole question is altered. 

 When any row or line of such molecules is similarly displaced, and 

 through a space which is small compared with the separating 

 intervals, the molecules of the succeeding row will be moved in the 

 same direction by the forces which are thus developed with the 

 change of distance ; so that the vibrations of the particles compos- 

 ing the first row will be communicated to those of the second, and 

 thus the vibratory motion will be propagated in a direction per- 

 pendicular to that in which it takes place.* The rapidity of the 

 propagation will depend on the magnitude of the force developed by 

 the displacement. To account for the fact that there are no sen- 

 sible vibrations in a direction normal to the wave, we have only to 

 suppose the repulsive force between the molecules to be very great, 

 or the resistance to compression very considerable ; for in this case, 

 it will be seen, the force which resists the approach of two strata 

 of the fluid is much greater than that which opposes their sliding 

 on one another. Fresnel's views on this subject are contained in 

 a short paper, entitled "Considerations mecaniques sur la po- 

 larisation de la lumiere,t" and in his celebrated memoir on 

 double refraction.* 



The principle of transversal vibrations, however, has not been 



* The existence of transversal vibrations has been fully established in other cases 

 of vibratory motion. M. Savart and Mr. Wheatstone have shown that in many 

 instances the elementary motions of the molecules of bodies which transmit sound 

 are transverse to the direction of the propagation. 



t Bulletin de la Soc. Philom., 1824. 



t Mt'moires dc I'lnstitut, torn. vii. 



r, 2 



