SCIENCE OF COMMON THINGS. 241 



What is light ? Light possesses no weight. 



PAET VII. 



LIGHT, AND HOW WE SEE. 



CIIAPTEE I. 



NATURE AND LAWS OF LIGHT. 



ISIS Through what agency alone are we enabled to enjoy the sense of 

 sight f 



Through the agency of light. 



1613 What is light? 



Light is now believed to be caused by the agitation, 

 vibration, or undulation of an elastic fluid which is sup- 

 posed to occupy and pervade all space. "We call this 

 supposed fluid ether, and its undulations or vibrations, 

 reaching the eye, affect the optic nerve, and produce 

 the sensation which we call light. 



1614 What analogy is there between the eye and the ear f 



The vibrations or undulations of the ether 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 this body and the ear. 



1615 If we collect a large quantity of light in one point by means of a 

 glass, and throw it upon the most sensitive balance, does it indicate any per- 

 ceptible weight f 



It does not, in the slightest degree. 



1616 What are the chief sources of light? 



The sun, the stars, ftre or combustion, electricity, and 

 phosphorescence. 



1617 With what velocity does light move through space 1 



