02 INTRODUCTION. 



Optics, like Astronomy, has for its object of inquiry, 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 ad- 

 vance, many of the laws of phenomena were studied and discovered in 

 immediate reference to the theoretical cause, and do not occupy a 

 separate place in the history of science, as in Astronomy they do. 

 We may add, that the reason why Formal Astronomy was almost 

 complete before Physical Astronomy began to exist, was, that it was 

 necessary to construct the science of Mechanics in the mean time, in 

 order to be able to go on ; whereas, in Optics, mathematicians were 

 able to calculate the results of the undulatory theory as soon as it had 

 suggested itself from the earlier facts, and while the great mass of 

 facts were only becoming known. 



We shall, then, in the first nine chapters of the History of Optics, 

 treat of the Formal Science, that is, the discovery of the laws of 

 phenomena. The classes of phenomena which will thus pass under 

 our notice are numerous ; namely, reflection, refraction, chromatic 

 dispersion, achromatization, double refraction, polarization, dipolariza- 

 tion, the colors of thin plates, the colors of thick plates, and the 

 fringes and bands Avhich accompany shadows. All these cases had 

 been studied, and, in most of them, the laws had been in a great mea- 

 sure discovered, before the physical theory of the subject gave to our 

 knowledge a simpler and more solid form. 



