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edge 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 difierent
lengths and amplitudes. The amplitude is the width of
swing of the individual particles of the wave. In water-
waves it is the height of the crest above the trough, while
the length of the wave is the distance between two con-
secutive crests. The aggregate of waves emitted by the sun
may be broadly divided into two classes: the one class com-
petent, 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, ex-
pressed with reference to sensation, means the intensity of
the light, is proportional to the square of the amplitude.
Hence the amplitude being one-hundred-fold, the energy of
the largest light-giving waves would be ten-thousand-fold
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 probably exist
among the light-giving waves. And if we take the whole
range of solar radiation into account—fits non-visual as well
as its visual waves—I think it probable that the force or
energy of the largest wave is a million times that of the

Turned into their equivalents of sensation, the different
light-waves produce different colors. ' Red, for example, is
produced by the largest waves, violet by the smallest, while
green is produced by a wave of intermediate length and
amplitude. On entering from air into more highly refract-
ing substances, such as glass or water, or the sulphide of

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