358 Molecules, Ultimates, Atoms, and Waves. [July, 



conception of a luminous wave as it exists in the free ether, 

 if attention be confined to a single line of ethereal particles 

 lying in the direction in which the wave motion is travel- 

 ling. Take for example the wave f, one of those produced 

 by hydrogen gas. Its length in hundredths of the millionth 

 of an inch is about 1915, the probable number of ethereal 

 particles embraced in this length is about 40,000 millions, 

 all of which are more or less moving simultaneously in 

 a direction transverse to that in which the wave is advan- 

 cing. The foremost — that at the point of the wave — is just 

 beginning to move. At a quarter of the length of the wave 

 from this point, there is a particle which has performed the 

 first quarter of its motion, and has departed to the farthest 

 limit from its point of rest on one side. It has been in 

 motion one 2448th part of the billionth of a second, and in 

 that interval has travelled one 523,390th part of the mil- 

 lionth of the length of the wave, moving at the average 

 C rate of about one-seventh of an inch in a second. At the 

 middle point of the wave is a particle which is passing 

 through the line of propagation on its way to the other 

 side of that line. It has been double of the above period of 

 time in motion, and has travelled double the space. At 

 three quarters of the wave-length from the front is a particle 

 which has performed three-fourths of its movement. It has 

 gone as far to the other side of the line of propagation 

 as the limit of departure from its point of rest, having been 

 in motion one gi8 % th part of the billionth of a second. 

 At the extreme rear of the wave is a particle which has 

 just returned to its point of rest, having entirely com- 

 pleted its excursion in one 612th part of the billionth of a 

 second. The particles intervening between each pair of the 

 five above specified are, according to their position, in 

 intermediate phases of their movement. 



The perfect elasticity of the ethereal fluid leads to the 

 inference that the rearmost particle, having regained its 

 point of rest, will remain there until disturbed by a fresh 

 impulse travelling along the same line of propagation. It 

 will have delivered over to those in front of it the whole of 

 the motive energy which it had acquired from the primary 

 impulse. Owing, moreover, to the circumstance that all 

 ponderable particles are in a continual state of rapid 

 motion through the ether, it is improbable that any indi- 

 vidual line of ethereal particles will be agitated twice in 

 succession by one and the same ponderable particle, 

 although it may be agitated by one of the same kind, 

 and thus have transmitted along it a succession of 



