/ OF THE \ 



f UNIVERSITY ) 



LIGHT. 17 



off from it, or whether we suppose it to consist of vi- 

 brations, which are made to glance off as the vibrations 

 of the air in the case of echoes. 



19. The first, or Newtonian theory, ena- 

 advantagl of ^ es us to explain the leading facts more 

 the Newtonian simply and clearly, and is therefore em- 



theory ? . . 



ployed in this work for this purpose. The 

 definitions and laws of light are stated in the language 

 of that theory. 



What is a ray 20. RAY AND MEDIUM DEFINED. A ray 



lustrate ? the ^ ^ nt * S a ^ me ^ P art i c ^S of light. In 



subject. such rays or lines of particles, light is con- 



stantly passing off from all visible objects. From every 

 part of the book before the student, for example, it 

 passes into the eye, enabling him to know the nature 

 of the object. If the book be taken into a dark room 

 it is no longer visible, because it obtains no light which 

 it may afterward reflect to the eye. 

 W1 , . A medium is any space or substance 



w liat is a me- 

 dium? through which light passes. 



n . .-, , 21. LAWS OF LIGHT. The more im- 



Give the laws 



of light ? portant laws of the radiation of light are 

 the following : 



1. Rays of light proceed from ev- 

 ery point of luminous objects in every 

 direction. They proceed, for exam- 

 ple, from every point of the sun's sur- 

 face. 



2. They proceed in straight lines. Light, for example, 

 comes to us in straight lines from the sun. 



3. They diverge as they proceed. This is illustrated 



