164 PROPAGATION OF LIGHT. SECT. XX. 



are in a state of perpetual agitation, and that they pos- 

 sess the property of exciting regular vibrations in the 

 ethereal medium, corresponding to the vibrations of their 

 own molecules ; and that, on account of its elastic nature, 

 one particle of the ether when set in motion communi- 

 cates its vibrations to those adjacent, which in succession 

 transmit them to those farther off ; so that the primi- 

 tive impulse is transferred from particle to particle y and 

 the undulating motion darts through ether like a wave 

 in water. Although the progressive motion of light is 

 known by experience to be uniform and in a straight 

 line, the vibrations of the particles are always at right 

 angles to the direction of the ray. The propagation of 

 light is like the spreading of waves in water ; but if one 

 ray alone be considered, its motion may be conceived by 

 supposing a rope of indefinite length stretched horizon- 

 tally, one end of which is held in the hand. If it be 

 agitated to and fro at regular intervals, with a motion 

 perpendicular to its length, a series of similar and equal 

 tremors or wavps will be propagated along it ; and if the 

 regular impulses be given in a variety of planes, as up 

 and down, from right to left, and also in oblique direc- 

 tions, the successive undulations will take place in every 

 possible plane. An analogous motion in the ether, 

 when communicated to the optic nerves, would produce 

 the sensation of common light. It is evident that the 

 waves which flow from end to end of the cord in a ser- 

 pentine form, are altogether different from the perpen- 

 dicular vibratory motion of each particle of the rope, 

 which never deviates far from a state of rest. So in 

 ether, each particle vibrates perpendicularly to the di- 

 rection of the ray ; but these vibrations are totally dif- 

 ferent from, and independent of, the undulations which 

 are transmitted through it, in the same manner as the 

 vibrations of each particular ear of corn are independent 

 of the waves that rush from end to end of a harvest field 

 when agitated by the wind. 



The intensity of light depends upon the amplitude or 

 extent of the vibrations of the particles of ether ; while 

 its color depends upon their frequency. The time of 

 the vibration of a particle of ether is by theory, as the 

 length of a wave directly, and inversely as its velocity. 



