EPOCH OF YOUNG AND FRESNEL. 4G5 



diffraction further invested the particles with com- 

 plex hypothetical laws of attraction and repulsion ; 

 polarization gave them sides; double refraction 

 subjected them to peculiar forces emanating from 

 the axes of crystals ; finally, dipolarization loaded 

 them with the complex and unconnected contriv- 

 ance of moveable polarization ; and even when all 

 this had been assumed, additional mechanism was 

 wanting. There is here no unexpected success, no 

 happy coincidence, no convergence of principles 

 from remote quarters: the philosopher builds the 

 machine, but its parts do not fit; they hold toge- 

 ther only while he presses them : this is not the 

 character of truth. 



In the undulatory theory, on the other hand, all 

 tends to unity and simplicity. We explain reflec- 

 tion and refraction by undulations ; when we come 

 to thin plates, the requisite "fits," are already 

 involved in our fundamental hypothesis, for they 

 are the length of an undulation: the phenomena 

 of diffraction also require such intervals ; and the 

 intervals thus required agree exactly with the 

 others in magnitude, so that no new property is 

 needed. Polarization for a moment checks us ; but 

 not long; for the direction of our vibrations is 

 hitherto arbitrary; we allow polarization to de- 

 cide it. Having done this for the sake of polariza- 

 tion, we find that it also answers an entirely dif- 

 ferent purpose, that of giving the law of double 

 refraction. Truth may give rise to such a coin- 

 VOL. ii. II H 



