OPTICAL PRINCIPLES. 167 



CHAPTER XIV. 



OPTICAL PRINCIPLES. 



WE shall now devote a few pages to the consideration 

 of the nature of light, and the optical principles in- 

 volved in the construction and use of the microscope. 

 Two theories of light have been propounded. Accord- 

 ing to one, light consists of minute particles emana- 

 ting from self-luminous bodies, as the sun, a candle, 

 or a red-hot piece of iron ; this is called the corpuscular 

 theory. According to the other, light consists of 

 waves or undulations like those of water or the 

 ears of corn set in motion by the wind, of the mole- 

 cules of an extremely subtle and rarified elastic mat- 

 ter, called ether, existing everywhere, and set in mo- 

 tion by the causes which produce light ; this is called 

 the undulatory theory. The consideration of the 

 merits of these two theories would be foreign to our 

 purpose : suffice it to say that the evidence in favour 

 of the undulatory theory preponderates, so that the 

 corpuscular theory is now laid aside. 



It will often be requisite to make use of the term 

 ray of light, by which must be understood the small- 

 est bundle of luminous undulations which can be 

 separated from a mass of light as by passing light 

 through a small hole in an opake body, or by any 

 equivalent method. 



The most casual observer must have noticed that 

 the rays of light move in straight lines; as when the 

 sun's rays are seen entering a dark room through a 

 small window or other aperture, their direction being 

 then distinctly visible ; the manner in which ordinary 

 shadows are formed also illustrates the same fact. 



