2 5 6 ( 42 ) 



Follow them up to their origin, and what do you there 

 find ? A definite, tangible, vibrating body. It may be 

 the vocal chords of a human being, it may be an organ 

 pipe, or it may be a stretched string. Follow in the 

 same manner a train of ether waves to their source, re- 

 membering at the same time that your ether is matter, 

 dense, elastic, and capable of motions subject to and 

 determined by mechanical laws. What then do you ex- 

 pect to find as the source of a series of ether waves ? 

 Ask your imagination if it will accept a vibrating multi- 

 ple proportion a numerical ratio in a state of oscilla- 

 tion ? I do not think it will. You cannot crown the 

 edifice by this abstraction. The scientific imagination, 

 which is here authoritative, demands as the origin and 

 cause of a series of ether waves a particle of vibrating 

 matter quite as definite, though it may be excessively 

 minute, as that which gives origin to a musical sound. 

 Such a particle we name an atom or a molecule. I 

 think the imagination when focused so as to give defini- 

 tion without penumbral haze is sure to realize this im- 

 age at last. 



To preserve thought continuous throughout this dis- 

 course, to prevent either lack of knowledge or failure of 

 memory from producing any rent in our picture, I here 

 propose to run rapidly over a bit of ground which is 

 probably familiar to most of you, but which I am anx- 

 ious to make familiar to you all. 



The waves generated in the ether by the swinging 

 atoms of luminous bodies are of different lengths and 

 amplitudes. The amplitude is the width of swing of 

 the individual particles of the wave. In water waves 

 it is the hight of the crest above the trough, while the 

 length of the wave is the distance between two con- 



