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OPTICS. 

 CHAPTER XXIY. 



LIGHT : REFLECTION AND REFRACTION. 



THE nature of light. The generally accepted 

 explanation of the nature of light is that offered by 

 what is called the UNDULATORY THEORY, a theory 

 proposed by Huyghens, in opposition to the EMISSION 

 OR CORPUSCULAR theory, supported by Newton. 

 The latter theory supposed that luminous bodies gave 

 out in all directions very subtle particles, which, 

 reaching the eye, affected it and gave rise to the 

 sensation that we call light, the intensity of the light 

 being determined by the number of emanations. The 

 former theory, advocated also by Young, views light 

 as a mode of motion, as heat and sound are viewed as 

 modes of motion. A luminous body is thus held to 

 be a body whose particles are in a state of vibration. 

 The vibrations require to be transmitted to our eyes 

 if they are to give rise to a luminous impression. 

 The ordinary atmosphere is the medium by which the 

 vibrations of a sounding body are communicated to 1 

 our ears ; but a luminous body does not become in- 

 visible in a vacuum, as a sounding body becomes 

 inaudible. Hence it became necessary to suppose 

 the existence of a highly elastic medium pervading 

 all space and all bodies, to which luminous bodies 

 communicated their vibrations, and which transmitted 

 them with enormous velocity. The medium is called 



