330 FRESXEL ON DOUBLE REFRACTION. 



The theory which Me have adopted, and the very simple con- 

 structions we have deduced from it, present this remarkable 

 character, that all the unknown quantities are determined at the 

 same time by the solution of the problem. We find at the same 

 time the velocity of the ordinary ray, that of the extraordinary 

 ray, and their planes of polarization. Philosophers who have 

 studied with attention the laws of nature will feel that this sim- 

 plicity, and these intimate relations between the various parts of 

 the phaenomenon, offer the greatest probabilities in favour of the 

 theory by which they are established. 



A long time before having conceived it, and by the sole con- 

 sideration of facts, I had perceived that the true explanation of 

 double refraction could not be discovered without explaining at 

 the same time the phcenomenon of polarization which constantly 

 accompanies it ; thus it was after having found what mode of 

 vibration constituted the polarization of light, that I first caught 

 sight of the mechanical causes of double refraction. It appeared 

 to me still more evident that the velocities of the ordinary and 

 extraordinary beams ought to be in some sort the two roots of 

 one and the same equation ; I have never been able to admit for 

 a single instant the hypothesis, according to which there would 

 be two different media, the refracting body and the aether which 

 it contains, by one of which the extraordinary rays are trans- 

 mitted, by the other the ordinary ones ; in fact, if these two 

 media could transmit separately the luminous waves, one does 

 not see why the two velocities of propagation should be rigo- 

 rously equal in the greater number of refracting bodies, and why 

 prisms of glass, water, alcohol, &c. should not thus divide the 

 light into two distinct beams. 



We have supposed it to be the same vibrating medium which, 

 in bodies endowed with double refraction, propagates the ordi- 

 nary and extraordinary waves, but without specifying whether 

 the molecules of the body participate in the luminous vibrations, 

 or whether these latter were alone propagated by the aether con- 

 tained in the body ; our theory is equally well conciliated with 

 either hypothesis. It is indeed more easy to comprehend, in the 

 first case, how the elasticity of one and the same refracting me- 

 dium may vary with the direction along which the molecular 

 displacements take place ; but it is conceivable also, in the 

 second, that the molecules of the body must influence the mutual 



