INTRODUCTION. 

 Formal and Physical Optics. 



THE history of tlie science of Optics, written at length, would be 

 very voluminous ; but we shall not need to make our history so ; 

 since our main object is to illustrate the nature of science and the con- 

 ditions of its progress. In this way Optics is peculiarly instructive ; 

 the more so, as its history has followed a course in some respects 

 different from both the sciences previously reviewed. Astronomy, as 

 we have seen, advanced with a steady and continuous movement from 

 one generation to another, from the earliest time, till her career was 

 crowned by the great unforeseen discovery of Newton ; Acoustics had 

 her extreme generalization in view from the first, and her history 

 consists in the correct application of it to successive pro*blems ; Optics 

 advanced through a scale of generalizations as remarkable as those 

 of Astronomy ; but for a long period she was almost stationary ; and, 

 at last, was rapidly impelled through all those stages by the energy of 

 two or three discoverers. The highest point of generality which 

 Optics has reached is little different from that which Acoustics occu- 

 pied at once ; but in the older and earlier science we still want that 

 palpable and pointed confirmation of the general principle, which the 

 undulatory theory receives from optical phenomena. Astronomy has 

 amassed her vast fortune by long-continued industry and labor ; Optics 

 has obtained hers in a few years by sagacious and happy speculations,' 

 Acoustics, having early acquired a competence, has since been em- 

 ployed rather in improving and adorning 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 Astro- 

 nomy, each having its prelude and its sequel. But most of the 

 discoveries in Optics are of a smaller character, and have less employ- 

 ed the minds of men, than those of Astronomy ; and it will not be 

 necessary to exhibit 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. 



