22 REPORT ON PHYSICAL OPTICS. 



laws of reflexion and refraction. The phenomena of vision have 

 been in like manner omitted, as involving also the science of phy- 

 siology ; and the relations of light to other agents, as heat, elec- 

 tricity, and magnetism, because these relations are as yet little 

 understood, and in the present state of the kindred sciences, the 

 science of light can hope to derive little aid from their examina- 

 tion. These interesting subjects would, each of them, well merit a 

 separate consideration. 



PART I. UNPOLARIZED LIGHT. 

 I. Propagation of Light. Principle of Interference. 



The first property of light which claims our notice is its pro- 

 gressive movement. Light, we know, travels from one point of 

 space to another in time, with a velocity of about 195,000 miles 

 in a second. The inquiry concerning the mode of this propaga- 

 tion involves that respecting the nature of light itself. 



There are two distinct and intelligible ways of conceiving such 

 a motion. Either it is the self -same body which is found at dif- 

 ferent times in distant points of space ; or there are a multitude 

 of moving bodies, occupying the entire interval, each of which 

 vibrates continually within certain limits, while the vibratory mo- 

 tion is communicated from one to another, and so advances uni- 

 formly. Nature affords numerous examples of each of these modes 

 of propagated movement ; and in adopting one or other to account 

 for the phenomena of light, we fall upon one or other of the two 

 rival systems the theories of Newton and of Huygens. 



The Newtonian theory, in the shape in which it is usually pre- 

 sented, is undoubtedly simpler in conception than its rival ; but 

 this simplicity is only apparent. Newton himself was far too 

 clear-sighted to suppose that the forces of attraction and repulsion, 

 by which the molecules of light were supposed to be refracted and 

 reflected, were adequate to account for all the phenomena ; and it 

 is remarkable that, when he proceeds to speculate on the physical 

 theory of light, he has found it necessary to admit all the appa- 

 ratus required in the theory of waves. In fact, Newton felt, and 

 distinctly stated, that the vibrations of an ethereal medium were 

 necessary in his hypothesis,* although he denied that those vibra- 



* Phi!. Tram. 1672. 



