276 ON LIGHT. 



of contemplation. If the point where the luminous ex- 

 citement originates be near, the perpendiculars from it 

 to the wave-surface diverge conically ; but if so far re- 

 mote that the portion of that surface at the eye may be 

 regarded as sensibly plane, they are to all sense parallel, 

 as in the case of light emanating from the stars or the 

 sun. 



(60.) The reflection of light in this theory is in exact 

 analogy with that of any other undulatory movement. 

 We cannot see the waves of sound, but those on smooth 

 water are easily followed and their reflexion made matter 

 of ocular inspection. Drop a small pebble into still 

 water, and a wave will be seen to spread out in an en- 

 larging ring. Let this be done near the perpendicular 

 and smooth side of any large tank or pond, or near a 

 board held vertically in the water, and the ring will be 

 seen on reaching the board to be reflected, and will 

 thence spread back over the surface, still enlarging, as 

 the segment of another ring whose centre might be sup- 

 posed as far on the land side of the reflecting surface as 

 the place where the pebble was dropped was in reality 

 on the water side. If several pebbles be dropped in 

 succession, or a regular up-and-down movement given 

 to the water at that point, a continued series of circular 

 waves will be generated and reflected, the reflected 

 waves running out and intersecting the direct exactly as 

 if they originated in two distinct centres. What in water 

 is seen to be a reflected wave, in air we recognize as an 

 echo. And in the fact that a sound, though partially re- 

 flected as such from a window, a board partition, or a 



