FRESNEL ON DOUBLE REFRACTION. 241 



SO far ; this curve, and the definitive direction of the refracted 

 ray, must therefore vary according to the azimuth of the 

 plane of polarization of the incident ray. Thus, on following 

 this hypothesis into its consequences, it would be found that the 

 light, instead of dividing itself simply into two rays, ought to 

 separate itself into a multitude of rays, distributed according to 

 all the inclinations comprised between the extreme directions of 

 the ordinary and extraordinary beam. 



The theory here combated, and against which many other 

 objections might be brought, has not led to a single discovery. 

 The skilful calculations of M. de Laplace, however remarkable 

 for an elegant application of mechanical principles, have taught 

 nothing new on the laws of double refraction. 



Now, we do not think that the assistance to be derived from 

 a good theory is to be confined to the calculation of the forces 

 when the laws of the phaenomena are known ; it v.ould contribute 

 too little to the progress of science. There are certain laws so 

 complicated or so singular, that observation alone, aided by 

 analogy, could never lead to their discovery. To divine these 

 enigmas, we must be guided by theoretical ideas founded on a 

 true hypothesis. The theory of luminous vibrations presents this 

 character and these precious advantages ; for to it we ow e the 

 discovery of optical laws the most complicated and most difficult 

 to divine ; whilst all the other discoveries, numerous and im- 

 portant no doubt, which have been made in this science bv 

 experimenters adopting the emission system, are much rather 

 the fruit of their observation and sagacity, commencing with 

 those of Newton, than mathematical consequences deduced from 

 his system*. 



The theory of vibrations, which had suggested to Huygens 

 the idea of ellipsoidal waves, by means of which he has so 



• For the labours of Newton and M. de Laplace I entertain the most lively 

 and sincere admiration ; but I do not admire equally all which they have done ; 

 and I do not consider, for instance, as many persons do, that Newton's ' Optics' 

 is one of his chief titles to fame. It contains many grave errors, and the truths 

 comprised were much less difticuU to discover tiian the mechanical explanation 

 of the celestial motions. What a difference, in fact, between the so-easv ana- 

 lysis of li»ht and that profound glance by which Newton saw that the precession 

 of the equinoxes was occasioned by the oblateness of the earth ! It is his im- 

 mortal ' Principia' and the discovery of the method of fluxions which have 

 placed him in the first rank of geometers and natural philosophers. Ijut how- 

 ever great the intellectual superiority of so prodigious a man, he is not the less 

 subject to error ; it cannot be too often repeated, Errare hiimanum est. Nothing 

 can be more fatal to the progress of science than the doctrine of infallibility. 



