THE NATURE OF LIGHT. 369 



infinite number of points in the surface of a visible object. If the 

 infinite number of lines were drawn from each of the infinite number 

 of points, there would be no vacant spaces in 

 the figure ; the rays really intersect at every 

 point from which the object is visible. 



581. Transparent, Translu- 

 cent and Opaque Bodies. Bodies 

 are transparent, translucent or opaque 

 according to the degree of freedom which 



they afford to the passage of the luminiferous waves. 

 Transparent bodies allow objects to be seen distinctly 

 through them, e. g., air, glass and water. Translucent 

 bodies transmit light, but do not allow bodies to be seen 

 distinctly through them, e.g., ground glass and oiled paper. 

 Opaque bodies cut off the light entirely and prevent 

 objects from being seen through them at all. The light 

 is either reflected or absorbed. So much of the radiant 

 energy as is neither reflected nor transmitted is changed 

 to absorbed heat. 



582. Luminous Rays. A single line of light is 

 called a ray. The ray of light is perpendicular to the 

 wave of ether. The ray may, without considerable error, 

 be deemed the path of the wave. 



583. Luminous Beams and Pencils. A col- 

 lection of parallel rays constitutes a beam ; a cone of rays 

 constitutes a pencil. The pencil may be converging or 

 diverging. If a beam or pencil should dwindle in thick- 

 ness to a line, it would become a ray. 



584. Rectilinear Motion of Light. A medium 

 is homogeneous when it has an uniform composition and 

 density. In a homogeneous medium, light travels 



