PROPAGATION OF -LIGHT. 409 



OPTICS. 



38. Propagation of Light. Shadow. Photometers. The 

 knowledge which we possess of the various objects in 

 the universe is chiefly obtained in consequence of the 

 property of these objects to emit light, and thus to pro- 

 duce an impression upon our eyes. By the sense of 

 touch we can only discriminate those objects which are 

 very near to us; the ear conveys to our mind only im- 

 pressions of bodies which are sounding, a state which is 

 accidental and comparatively rare ; but the eye informs 

 us of the existence of the most distant objects, provided 

 that they are sufficiently luminous, a condition which 

 is much more common and more lasting than that 

 required for the production of the vibratory movements 

 which render bodies sonorous. A further essential 

 difference between the sensations of light and of sound 

 is that the eye is capable of determining in all cases 

 the direction from which the light proceeds, while 

 the power of the ear to indicate the direction in which 

 the sounding body is situated is very limited and un- 

 certain. On the other hand, the sound produced by a 

 body may reach our ear even if an obstacle be interposed 

 in the path of the sonorous vibrations ; sound may, as 

 it were, go round the obstacle. In this respect the eye 

 is inferior as an instrument for gaining information of 



