EPOCH OF YOUNG AND FRESNEL. 439 



steps of generalization. In both cases, all these 

 steps were made by the same persons; but there 

 is this difference ; all the parts of the law of uni- 

 -^rsal gravitation were worked out in one burst of 

 .spiration by its author, and published at one 

 time ; in the doctrine of light, on the other hand, 

 the different steps of the advance were made and 

 published at separate times, with intervals between. 

 We see the theory in a narrower form, and in de- 

 tached portions, before the widest generalizations 

 and principles of unity are reached ; we see the 

 authors struggling with the difficulties before we 

 see them successful. They appear to us as men 

 like ourselves, liable to perplexity and failure, in- 

 stead of coming before us. as Newton does in the 

 history of Physical Astronomy, as the irresistible 

 and almost supernatural hero of a philosophical 

 romance. 



The main subdivisions of the great advance in 

 physical optics, of which we have now to give an 

 account, are the following : 



1. The explanation of the periodical colours of 

 thin plates, thick plates, fringed shadows, striated 

 surfaces, and other phenomena of the same kind, 

 by means of the doctrine of the interference of 

 undulations. 



2. The explanation of the phenomena of double 

 refraction by the propagation of undulations in a 

 medium of which the optical elasticity is different 

 in different directions. 



