CHAPTER XIX. 



SPECIAL SENSES (Concluded.) 



VISION. 



THOSE bodies are said to be luminous which especially affect the 

 >rgan of vision. Some are luminous in themselves,, others become 

 by reflection. Since there is no direct contact between the- visual 

 ipparatus and the object which .makes the impression, and since the 

 istance which separates them is often infinite, it is impossible not 

 admit the existence of a' particular intervening agent between the 

 iter of .radiation and the eye. This agent is ether. 



How Does Light Transmit Itself? The accepted theory to-day 

 with regard to its propagation is the undulatory , or wave, theory. 

 Its doctrines make light, like heat and sound, a mode of motion. 



A luminous body is one whose particles are in a state of vibration. 

 That they may give rise to a luminous impression it is necessary that 

 ley be transmitted to the eye. Ordinarily the atmospheric air is the 

 isual medium for the transmission of the vibrations of a soun.ding 

 jody to our ears. However, a luminous body does not become invisi- 

 >le in a vacuum, as does a sounding body become inaudible. Hence, 

 lere must be supposed the existence of a highly elastic medium that 

 pervades all spaces and all bodies. To this especial medium luminous 

 bodies communicate their vibrations to be transmitted with enormous 

 velocity. This medium is known to physicists as ether. 



Suppose a luminous body isolated in a gas or suspended in a 

 vacuum ; it will be visible in all directions. Imagine, also, a point of 

 ice lighted up by its radiations. The line which joins this point 

 one of the elements of the luminous body represents the direction of 

 ray of light. So long as no obstruction intervenes the ray of light 

 mrsues an even, straight course. Should, however, a mirror inter- 

 cept its path, the greater portion of it will be beint out of its regular 

 )urse. That is, it is reflected. In all cases of reflection it is well to 

 ^member that "the angle of reflection always equals the angle of in- 

 idence." 



Again, the passage of light through transparent media of various 

 lensities presents peculiarities: its straight course is modified 

 >roken. To convey a conception of this phenomenon the term re- 

 fraction is used. 



(771) 



