262 FRESNEL ON DOUBLE REFRACTION. 



of fluid than those of the other wave, the oscillations parallel to 

 the rays would have much less amplitude than those perpen- 

 dicular to them, and consequently could only impress on the 

 optic nerve much smaller vibrations ; for the amplitude of its 

 vibrations cannot exceed that of the vibrations of the aether in 

 which it is plunged [qui le baigne). Now it is natural to r-up- 

 pose that the intensity of the sensation depends on the amplitude 

 of the vibrations of the optic nerve, and that thus the sensation 

 of hght resulting from vibrations normal to the waves will be 

 sensibly nothing compared to that produced by the vibrations 

 parallel to their surface. 



Moreover, it will be conceived that during the oscillation of 

 the illuminating molecule, the equilibrium of tension is restored 

 so rapidly between that portion of the aether which it approaches 

 and that which it recedes from, that there is no sensible con- 

 densation or dilatation ; and that the displacement of the aetherial 

 molecules which surround it is reduced to an oscillatory circular 

 movement, which bears them on the spherical surface of the wave 

 from the point which the illuminating molecule approaches 

 towards that from which it recedes, 



I think 1 have sufficiently proved that there is no mechanical 

 absurdity in the definition of luminous vibrations which the pro- 

 perties of polarized rays have compelled me to adopt, and which 

 has led me to the discovery of the true laws of double refraction. 

 If the equations of motion of fluids imagined by geometers are 

 not reconcilable with this hypothesis, it is because they are 

 founded on a mathematical abstraction, the contiguity of the 

 elements, which, without being true, may nevertheless represent 

 a part of the mechanical properties of elastic fluids, -when it is 

 admitted besides that these contiguous elements are compressible. 

 But from this very circumstance, that such is not the i-eaHty and 

 merely a pure abstraction, we ought not to expect to find in it 

 all the kinds of vibration of which elastic fluids are susceptible, 

 and all their mechanical properties ; thus, for example, according 

 to the equations of which we speak, there would be no friction 

 between two indefinite layers of fluid which slide one on the 

 other. It would then be but little philosophical to reject an 

 hypothesis to which the phaenomena of optics so naturally lead, 

 for no other reason than because it does not agree with these 

 equations. 



