PHYSICAL FORCES 103 



a " pencil" and is said to consist of " rays " of light, and 

 these can cross each other in the same point of space 

 without either hindrance to their action or even diminu- 

 tion of their intensity. This may be shown on a vertical 

 white surface opposite a minute aperture admitting the 

 rays of the sun into a room otherwise quite dark. Then 

 any external object e.g., a tree will be represented 

 upside down on the white surface. The rays of light 

 emanate from every point of that tree in all directions, 

 but none but those from its upper part can reach the 

 bottom of the surface opposite the small hole, while none 

 but those from the bottom of the tree can ascend towards 

 the top of that surface, and the same consideration 

 applies to all the rays emitted from each intermediate 

 point of the tree's surface. Therefore any object, thus 

 viewed, must appear inverted. 



Light radiates as heat does, and its intensity also 

 varies inversely as the square of the distance, a rate 

 practically diminished by the absorption it may undergo 

 in traversing bodies as, e.g., the air. Bodies which 

 light can traverse are called transparent. Those which 

 entirely absorb it are opaque. Bodies which are opaque 

 may sometimes be rendered transparent very easily. Such 

 is the case, e.g., with a kind of agate known as Hydro- 

 pkane. In its ordinary condition it is only half -trans- 

 parent, but can be made perfectly so by immersion in 

 water. Similarly, paper by being oiled, can, as the reader 

 knows, be made much more transparent than before. 



Light travels with amazing velocity at not less a 

 rate than 186,330 miles a second, yet this does not 

 prevent what is called the " aberration " of light, 

 which will be explained * when we come to the 



* See post, p. 176. 



