FORMAL AND PHYSICAL OPTICS. 373 



place in the history of science, as in astronomy 

 they do. We may add, that the reason why For- 

 mal Astronomy was almost complete before Phy- 

 sical 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, refrac- 

 tion, chromatic dispersion, achromatization, double 

 refraction, polarization, dipolarization, the colours 

 of thin plates, the colours of thick plates, and the 

 fringes and bands which accompany shadows. All 

 these cases had been studied, and, in most of them, 

 the laws had been in a great measure discovered, 

 before the physical theory of the subject gave to 

 our knowledge a simpler and more solid form. 



