PHYSICAL OPTICS. 



CHAPTER X. 



PRELUDE TO THE EPOCH OF YOUNG AND FRESXEL. 



BY Physical Optics we mean, as has already been stated, the theo- 

 ries which explain optical phenomena on mechanical principles. 

 jSTo such explanation 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 endeavors to explain, by means 

 of this comparison, the laws of reflection and refraction. 1 In order to 

 account for the production of colors by refraction, he ascribes to these 

 balls an alternating rotatory motion. 3 This form of the emission 

 theory, was, like most of the physical speculations of its author, hasty 

 and gratuitous ; but was extensively accepted, like the rest of the Car- 

 tesian doctrines, in consequence of the love which men have for sweep- 

 ing and simple dogmas, and deductive reasonings from them. In a 

 short time, however, the rival optical theory of undulations made its 

 appearance. Hooke in his Micrographia (1664) propounds it, upon 

 occasion of his observations, already noticed, (chap, viii.,) on the colors 

 of thin plates. He there asserts 3 light to consist in a " quick, short, 

 vibrating motion," and that it is propagated in a homogeneous medium, 

 in such a way that " every pulse or vibration of the luminous body will 

 generate a sphere, which will continually increase and grow bigger, 

 just after the same manner (though indefinitely swifter) as the waves 

 or rings on the surface of water do swell into bigger and bigger circles 

 about a point in it."* He applies this to the explanation of refraction, 



1 Diopt c. ii. 4. a Meteor, c. viii. 6. 3 Micrographia, p. 56 



4 Micrographia, p. 57. 



