INTRODUCTION. 



FORMAL AND PHYSICAL OPTICS. 



THE history of the 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 conditions 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. As- 

 tronomy, 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 New- 

 ton ; Acoustics had her extreme generalization in 

 view from the first, and her history consists in the 

 correct application of it to successive problems; 

 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 occupied at 

 once ; but in the older and earlier science we still 

 want that palpable and pointed confirmation of the 



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