SCIENTIFIC USE OF THE IMAGINATION. 109 



and all believe in an aether and its light-producing 

 waves. Let us consider what this belief involves. 

 Bring your imaginations once more into play, and 

 figure a series of sound-waves passing through air. 

 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 aether-waves to their source; 

 remembering at the same time that your aether is 

 matter, dense, elastic, and capable of motions subject 

 to, and determined by, mechanical laws. What then 

 do you expect to find as the source of a series of aether- 

 waves ? Ask your imagination if it will accept a 

 vibrating multiple proportion a numerical ratio in a 

 state of oscillation ? I do not think it will. You 

 cannot crown the edifice with this abstraction. The 

 scientific imagination, which is here authoritative, 

 demands, as the origin and cause of a series of aether- 

 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 intellect, 

 when focussed so as to give definition without penumbral 

 haze, is sure to realise this image at the last. 



With the view of preserving thought continuous 

 throughout this discourse, and of preventing either 

 failure of knowledge or of memory, from causing 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 anxious to make familiar to you 

 all. The waves generated in the aether by the swinging 

 atoms of luminous bodies are of different lengths and 

 amplitudes. The amplitude is the width of swing of 



