ON LIGHT. 223 



ous : " a state either natural to it, as in the flame of 

 a candle or the sun ; or induced, by being placed in pre- 

 sence of another luminous object, as when a sheet of 

 white paper is laid in the sun or before a candle. Nor 

 is it then seen if a screen of metal or any of the class of 

 substances called "opake" be interposed anywhere in 

 the direct straight line of communication ; while on the 

 other hand, when so hidden from direct vision, it may 

 be rendered visible "by reflexion" from a polished sur- 

 face held at a fitting angle, anywhere out of that direct 

 line, provided only such surface be not similarly screened 

 either from the object or from the eye. Thus we learn 

 two things : First, that the line of uninterrupted lumin- 

 ous communication is a straight one; and, secondly, that 

 any point whatever in a sphere of indefinite radius sur- 

 rounding a luminous object (in other words, in infinite 

 space) may become included in the line of indirect or 

 deflected luminous communication between any two 

 places. The agency, whatever its nature, is there and 

 ready, requiring only a fitting arrangement of material 

 and tangible substances to make it available. 



(6.) Light, though the cause of vision, is itself invis- 

 ible. A sunbeam, indeed, is said to be seen when it 

 traverses a dark room through a hole in the shutter or 

 when in a partially clouded sky luminous bands or rays 

 are observed as if darted through openings in the clouds, 

 diverging from the place (unseen) of the sun as the 

 vanishing point of their parallel lines seen in perspective. 

 But the thing seen in such cases is not the light, but the 

 innumerable particles of floating dust or smoky vapour 



