hed 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 the individual particles of the waves. In water-waves it is the
vertical height of the crest above the trough, while the length of the
wave is the horizontal distance between two consecutive crests. The
aggregate of waves emitted by the sun may be broadly divided into two
classes: the one class competent, the other incompetent, to excite
vision. But the light-producing waves differ markedly among
themselves in size, form, and force. The length of the largest of
these waves is about twice that of the smallest, but the amplitude of
the largest is probably a hundred times that of the smallest. Now the
force or energy of the wave, which, expressed with reference to
sensation, means the intensity of the light, is proportional to the
square of the amplitude. Hence the amplitude being one-hundredfold,
the energy of the largest light-giving waves would be ten-thousandfold
that of the smallest. This is not improbable. I use these figures not
with a view to numerical accuracy, but to give you definite ideas of
the differences that proba