USB OP THE IMAGINATION. 423 
necessary. Many chemists of the present day refuse to 
speak of atoms and molecules as real things. Their caution 
leads them to stop short of the clear, sharp, mechanically 
intelligible atomic theory enunciated by Dalton, or any 
form of that theory, and to make the doctrine of " mul- 
tiple proportions" their intellectual bourne. I respect the 
caution, though I think it is here misplaced. The chemists 
who recoil from these notions of atoms and molecules 
accept, without hesitation, the Uudulatory Theory of 
Light. Like you and me they one and ail believe in an 
ether 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 ether-waves to their source; 
remembering 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 multiple 
proportion a numerical ratio in a state of oscillation? 
I do not think it will. You cannot crown the edifice 
with this abstraction 1 . 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 intellect, when focused so as to give definition 
without penumbral haze, is sure to realize 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 ether by the swinging atoms of luminous bodies are 
of different lengths and amplitudes. The amplitude is the 
