ELEMENTS 



OF 



THE WAVE-THEORY OF LIGHT. 



CHAPTER I. 



PROPAGATION OF LIGHT. 



(1) NATURAL bodies may be divided into two classes in 

 relation to Light. Some possess, in themselves, the powe'r of 

 exciting the sense of vision, and of producing the sensation of 

 light ; while others are devoid of that property. Bodies of 

 the former class are said to be toamows ; those of the latter, 

 non-luminous. The Sun and the fixed stars are all luminous 

 bodies ; terrestrial bodies are luminous, in the states of incan- 

 descence, combustion, or phosphorescence. 



Non-luminous bodies acquire the power of exciting the 

 sensation of light in the presence of a luminous body. Thus, 

 a lamp or candle illuminates all the objects in a room, and 

 renders them visible ; and the light of the Sun illuminates the 

 Earth and the planets. This property of bodies is due to 

 their capacity of reflecting light, and belongs ^to them in dif- 

 ferent degrees. 



(2) The foregoing distinction of bodies, obvious as it seems, 

 was not fully comprehended by the ancients. According to 



