LIGHT. 293 



yn. jv • 641. In order to explain how such a result is possible, or 



of light have in Other words, to account for the origin of light, two theories 

 been proposed? Yiare been proposed, which are called the Corpusculak and 

 the Undulatobt Theories, 



What is the The Corpuscular Theory supposes that a 

 Theor^"^of distant object becomes visible to us by emit- 

 ^'^'*'' ting particles of matter from its surface, which 



particles of matter, passing through the intervening space 

 between the visible object and the eye, enter the eye, and 

 striking upon the nervous membrane, so affect it as to 

 produce the sensation of light, or vision. 



According to this theory, there is a striking analogy or resemblance be- 

 tween the eye and the organs of smelling. Thus, we recognize the odor of 

 an object in consequence of the material particles which pass from the object 

 to the organs of smelling, and there produce a sensation. In the same 

 manner, a visible object at any distance may be supposed to send forth parti- 

 cles of light, which move to the eye and produce vision, by acting mechan- 

 ically on its nervous structure, as the odoriferous particles of a rose produce a 

 sensible effect upon the organs of smelling. 



What is the I'li^ Undulatory Theory supposcs that 

 Theo"^*?"^^ there exists throughout all space an ethereal, 

 elastic fluid, which, like the air, is capable of 

 receiving and transmitting undulations, or vibrations. 

 These, reaching the eye, affect the optic nerve, and pro- 

 duce the sensation which we call li2:ht. 



According to this theory, there is a striking analogy between the eye and 

 the ear ; the vibrations, or undulations of the ethereal medium being supposed 

 to pass along the space intervening between the visible object and the eye in 

 the same manner that the undulations of the air, produced by a sounding body, 

 pass through the air between it and the ear. 



Whi h f th '^^° Corpuscular Theory was sustained by Newton, and waa 



two theories of for a long time generally believed. At the present day it is 

 ^y'recefved ?' almost entirely discarded, and the Undulatory Theory is now 

 received by scientific men as substantially correct ; since it 

 explains in a satisfactory manner nearly all the phenomena of light, which 

 the Corpuscular Theory does not. 



If the Corpuscular Theory be correct, a common candle is able to fill for 

 - hours, with particles of luminous matter, a circle four miles in diameter, since 

 it would be visible, under favorable circumstances, in every portion of this 

 space. Light, moreover, has no weight ; the largest possible quantity col- 

 lected in one point and thrown upon the most sensitive balance, does not 

 affect it in the sUghtest degree. 



