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PHYSICAL OPTICS. 



CHAPTER X. 

 PRELUDE TO THE EPOCH OF YOUNG AND FRESNEL. 



BY Physical Optics we mean, as has already been 

 stated, the theories which explain optical phe- 

 nomena on mechanical principles. No such expla- 

 nation could be given till true mechanical principles 

 had been obtained ; and, accordingly, we must date 

 the commencement of the essays towards physical 

 optics from Descartes, the founder of the modern 

 mechanical philosophy. His hypothesis concerning 

 light is, that it consists of small particles emitted by 

 the luminous body. He compares these particles to 

 balls, and endeavours to explain, by means of this 

 comparison, the laws of reflection and refraction 1 . 

 In order to account for the production of colours by 

 refraction, he ascribes to these balls an alternating 

 rotatory motion c . This first form of the emission 

 theory, was, like most of the physical speculations 

 of its author, hasty and gratuitous ; but was exten- 

 sively accepted, like the rest of the Cartesian doc- 

 trines, in consequence of the love which men have 

 for sweeping and simple dogmas, and deductive 

 1 Diopt. c. ii. 4. * Meteor, c. viii. 0. 



