EPOCH OF YOUXG AND FBESNEL. 93 



The main subdivisions of the great advance in physical optics, of 

 which we have now to give an account, are the following ; 



1. The explanation of the periodical colors of thin plates, thick 

 plates, fringed shadows, striated surfaces, and other phenomena of the 

 same kind, by means of the doctrine of the interference of undulations. 



2. The explanation of the phenomena of double r {fraction by the 

 propagation of undulations in a medium of which the optical elasticity 

 is different in different directions. 



3. The conception of polarization as the result of the vibrations 

 being transverse ; and the consequent explanation of the production 

 of polarization, and the necessary connexion between polarization and 

 double refraction, on mechanical principles. 



4. The explanation of the phenomena of dipolarization, by means 

 of the interference of the resolved parts of the vibrations after double 

 refraction. 



The history of each of these discoveries will be given separately to 

 a certain extent ; by which means the force of proof arising from their 

 combination will be more apparent. 



Sect. 2. Explanation of the Periodical Colors of Thin Plates and 

 Shadows lij the Undulatory Theory. 



THE explanation of periodical colors by the principle of interference of 

 vibrations, was the first step which Young made in his confirmation of 

 the undulatory theory. In a paper on Sound and Light, dated 

 Emmanuel College, Cambridge, 8th July, 1799, and read before the 

 Royal Society in January following, he appears to incline strongly to 

 the Huyghenian theory ; not however offering any new facts or calcu- 

 lations in its favor, but pointing out the great difficulties of the New- 

 tonian hypothesis. But in a paper read before the Royal Society, 

 Xovember 12, 1801, he says, " A further consideration of the colors of 

 thin plaits has converted that prepossession which I before entertained 

 for the undulatory theory of light, into a very strong conviction of its 

 truth and efficiency ; a conviction which has since been most strikingly 

 confirmed by an analysis of the colors of striated surfaces" He here 

 states the general principle of interferences in the form of a proposition. 

 (Prop, viii.) " When two undulations from different origins coincide 

 cither perfectly or very nearly in direction, their joint effect is a combi- 

 nation of the motions belonging to them." He explains, by the help 

 of this proposition, the colors which were observed in Coventry's 



