440 HISTORY OF OPTICS. 



3. The conception of polarization as the result 

 of the vibrations being transverse ; and the conse- 

 quent 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 di- 

 polarization, by means of the interference of the 

 resolved parts of the vibrations after double refrac- 

 tion. 



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 combi- 

 nation will be more apparent. 



Sect. 2. Explanation of the Periodical Colours 

 of Thin Plates and Shadows by the Undulatory 

 Theory. 



THE explanation of periodical colours by the prin- 

 ciple of interference of vibrations, was the first step 

 which Young made in his confirmation of the undu- 

 latory theory. In a paper on Sound and Light, 

 dated Emmanuel College, Cambridge, 8th July, 

 1799, and read at the Royal Society in January 

 following, he appears to incline strongly to the Huy- 

 ghenian theory ; not however offering any new facts 

 or calculations in its favour, but pointing out the 

 great difficulties of the Newtonian hypothesis. But 

 in a paper read before the Royal Society, Novem- 

 ber 12, 1801, he says, " A further consideration of 



