CHAPTER II. 



LIGHT. 



IN order to understand the relations of light to vegetation, a 

 short description of its properties is necessary. There are two 

 theories respecting the nature of light : one supposes it to be 

 particles of luminous matter emitted, or thrown off by luminous 

 bodies. The other supposes the existence of a substance 

 called ether, which pervades all nature, and is put into a 

 vibrating or wave-like motion by all luminous bodies. 



Rays of light proceed in straight lines from luminous bodies, 

 unless interrupted by some intervening medium. Light moves 

 with the astonishing velocity of 200,000 miles in a second of 

 time. When a ray of light falls on a plane surface, it is 

 disposed of in one of three ways : when the plane is black, the 

 ray is all absorbed: when it is polished, the ray is partly 

 absorbed and partly reflected : when the plane is transparent, 

 as glass or water, it may be partly absorbed, partly transmitted 

 and partly reflected. The law of reflection of light is the same 

 as that of sound: when a ray of light falls obliquely on a 

 reflecting surface, it is reflected in the same angle as the one 

 in which it approached the surface ; thus the angles of reflec- 

 tion and incidence are equal. 



"When a ray of light passes from a rarer to a denser medium, 

 it is refracted, or turned out of its course : when it passes from 

 a rare to a denser medium, as from the air into water, it is 



