372 FORMAL AND PHYSICAL OPTICS. 



general principle, which the undulatory theory re- 

 ceives from optical phenomena. Astronomy has 

 amassed her vast fortune by long-continued in- 

 dustry and labour; Optics has obtained hers in a 

 few years by sagacious and happy speculations; 

 Acoustics, having early acquired a competence, has 

 since been employed rather in improving and adorn- 

 ing than in extending her estate. 



The successive inductions by which Optics made 

 her advances, might, of course, be treated in the 

 same manner as those of Astronomy, each having 

 its prelude and its sequel. But most of the dis- 

 coveries in Optics are of a smaller character, and 

 have less employed the minds of men, than those 

 of Astronomy ; and it will not be necessary to ex- 

 hibit them in this detailed manner, till we come 

 to the great generalization by which the theory was 

 established. I shall, therefore, now pass rapidly in 

 review the earlier optical discoveries, without any 

 such division of the series. 



Optics, like Astronomy, has for its object of in- 

 quiry, first, the laws of phenomena, and next, their 

 causes; and we may hence divide this science, 

 like the other, into Formal Optics and Physical 

 Optics. The distinction is clear and substantive, 

 but it is not easy to adhere to it in our narrative ; 

 for, after the theory had begun to make its rapid 

 advance, many of the laws of phenomena were 

 studied and discovered in immediate reference to 

 the theoretical cause, and do not occupy a separate 



