chap, xxiv.] THE NATURE OF LIGHT. 297 



the LUMINIFEROUS ETHER. The undulations of light 

 are in a particular direction, namely, transverse to 

 the direction of propagation of the wave. If one 

 watches the movement of two or three pieces of cork 

 on the surface of water thrown into Avaves, the trans- 

 verse vibration will be understood. As the wave 

 reaches one piece of cork, the cork rises, occupying 

 different levels with the progress of the wave, till it 

 has reached its highest level, corresponding to the 

 crest of the wave. As the wave progresses still 

 farther the cork begins to descend on its backward 

 side, and is at its lowest level in the trough of the 

 wave. If several pieces of cork happen to have been 

 properly disposed, one piece may be just beginning 

 the forward ascent of the slope when another is half 

 way up, another at the crest, another descending the 

 backward slope, and another in the trough of the 

 wave. If one wave succeeds another, then each piece 

 of cork will be seen bobbing up and down as the 

 wave advances and passes, each piece being at a 

 different level according to the part of the wave that 

 has reached it. When the wave has passed, however, 

 all the pieces of cork will be found to occupy the 

 position they occupied before ; they have only bobbed 

 up and then down in the same place, while the wave 

 has passed onwards. Now if one could conceive of 

 the material of the wave being formed of a large 

 number of particles, then one could see how the wave 

 form is produced by the transverse movements of the 

 particles, in a way similar to that of the piece of cork. 

 Thus the wave form progresses, but the vibrating par- 

 ticles simply move to and fro across the direction of 

 the propagation. 



What is called the period of vibration is 

 the time occupied by one of the particles from the 

 moment it leaves one position to the moment when it 

 returns to the same position in the same direction. 



