410 VELOCITY OF LIGHT. 



surrounding bodies ; for light travels solely in a straight 

 line, and if a body which is not transparent is placed 

 in its path, all light is at once stopped. 



From a careful examination of certain phenomena of 

 light, it has been ascertained that light, like sound, is a 

 vibratory motion. The vibrations of light differ, however, 

 in many respects from the vibrations of sound. It is 

 not intended in this work to enter into an investigation 

 of these differences; it will be sufficient to state that 

 light does not consist in the vibrations of air or other 

 bodies, but that the vibrations of light are supposed 

 to take place in a medium which pervades space and 

 all bodies in space, and to which the name c ether ' has 

 been given. 



All bodies from which light proceeds that is, all 

 bodies which we can see are called l luminous.' Most 

 bodies are luminous in consequence of their reflecting 

 the light which they receive from other bodies, 'such as 

 the sun, a burning lamp, &c. ; but other bodies are 

 4 self-luminous ' for example, the sun, the fixed stars, 

 and any burning or glowing substance. 



Light is propagated in every direction from a luminous 

 body with a velocity which is incomparably greater 

 than that of sound. The velocity of light is indeed 

 exceedingly great, for light travels in one second 

 through a distance of nearly 190,000 miles. The 

 measurement of this enormous velocity was first at- 

 tained by careful observations of the eclipses of Jupiter's 

 satellites. 



The planet Jupiter moves round the sun in an orbit 

 the diameter of which is five times as great as the 

 diameter of the Earth's orbit. Jupiter has four satel- 



