190 PKOPAGATION OF LIGHT. [SECT. xx. 



oppose one another, the resulting motion will be nearly 

 equal to their difference ; and, if the undulations be equal 

 and opposite, the resultant will be zero, and the particle 

 will remain at rest. 



The preceding experiments, and the inferences deduced 

 from them, which have led to the establishment of the doc- 

 trine of the undulations of light, are the most splendid me- 

 morials of our illustrious countryman Dr. Thomas Young, 

 though Huygens was the first to originate the idea. 



It is supposed that the particles of luminous bodies are in 

 a state of perpetual agitation, and that they possess the pro- 

 perty 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 communicates its vibrations to those adja- 

 cent, which in succession transmit them to those farther off; 

 so that the primitive impulse is transferred from particle to 

 particle, 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 con- 

 sidered, 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 propa- 

 gated 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 perpendicular vibratory 



