238 ON LIGHT. 



(21.) It is to this power of "scattering" the incident 

 light in all directions, then, that surfaces owe their visi- 

 bility, and that by its aid we are enabled to trace the 

 course of a ray of light itself as if it were a visible thing. 

 Thus a sunbeam passing through a small hole and re- 

 ceived on smoke is seen, and on a white screen moved 

 rapidly to and fro behind it, appears as a straight lumin- 

 ous line or beam, by the momentary persistence of the 

 sensation caused in the eye at every successive point of 

 its motion ; and so, after reflexion or refraction, may its 

 subsequent course be rendered matter of ocular inspec- 

 tion. A pleasing and elegant experiment is to hold a 

 common reading-glass (or even a spectacle-glass) in the 

 sun, and to move rapidly to and fro behind it a white 

 paper, when the course of the refracted light, convcr-in^ 

 from all parts of the glass to the " focus," will be seen in 

 the air as a solid luminous cone, having the glass for 

 its base and the focus for its apex. 



(22.) The reflexion of light, whether "regular" or 

 " scattered," is, except under very peculiar circumstances 

 to be presently noticed, only partial; so that the re- 

 flected image of an object is seen fainter and less 

 luminous than the object itself directly viewed. This is 

 perceptible in an ordinary looking-glass ; yet more so 

 when the reflecting surface is still water, or unsilvered 

 glass. The most reflective substances are the white 

 metals such as silver, speculum-metal, steel, or quick- 

 silver : transparent or semi-transparent bodies being 

 much inferior in respect of this quality. If the substance 

 on which the light falls be of the kind called opakc, the 



