170 PROPAGATION OF LIGHT. SECT. XX. 



in water ; so that light is motion, and therefore subject to the 

 laws of dynamics and mathematical analysis. 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 ra}^ 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 horizontally, 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 waves 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 directions, 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 serpentine form are altogether different from the perpendi- 

 cular 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 direction of the ray ; but these 

 vibrations are totally different from and independent of the un- 

 dulations 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 colour 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 in- 

 versely as its velocity. Now, as the velocity of light is known 

 to be 190,000 miles in a second, if the lengths of the waves of 

 the different coloured rays could be measured, the number of 

 vibrations in a second corresponding to each could be computed. 

 That has been accomplished as follows : All transparent sub- 

 stances of a certain thickness, with parallel surfaces, reflect and 

 transmit white light ; but, if they be extremely thin, both the 

 reflected and transmitted light is coloured. The vivid hues on 

 soap-bubbles, the iridescent colours produced by heat on polished 

 steel and copper, the fringes of colour between the laminse of 



