the most eminent
men, and that, as the theory then held had to yield, the undulatory
theory may have to yield also. This seems reasonable; but let us
understand the precise value of the argument. In similar language a
person in the time of Newton, or even in our time, might reason thus:
Hipparchus and Ptolemy, and numbers of great men after them, believed
that the earth was the centre of the solar system. But this deep-set
theoretic notion had to give way, and the helio-centric theory may, in
its turn, have to give way also. This is just as reasonable as the
first argument. Wherein consists the strength of the present theory of
gravitation? Solely in its competence to account for all the phenomena
of the solar system. Wherein consists the strength of the theory of
undulation? Solely in its competence to disentangle and explain
phenomena a hundred-fold more complex than those of the solar system.
Accept if you will the scepticism of Mr. Mill[19] regarding the
undulatory theory; but if your scepticism be philosophical, it will
wrap the theory of gravitation in the same or in greater doubt.[20]

Sec. 11. _The Blue of the Sky_.

I am unwilling to quit these chromatic phenomena without referring to
a source of colour which has often come before me of late in the blue
of your skies at noon, and the deep crimson of your horizon after the
set of sun. I will here summarize and extend what I have elsewhere
said upon this subject. Proofs of the most cogent description could be
adduced to show that the blue light of the firmament is reflected
light. That light comes to us across the direction of the solar rays,
and even against the direction of the solar rays; and this lateral and
opposing rush of wave-motion can only be due to the rebound of the
waves from the air itself, or from something suspended in the air. The
solar light, moreover, is not scattered by the sky in the proportions
which produce white. The sky is blue, which indicates an excess of the
smaller waves. The blueness of the air has been given as a reason for
the blueness of the sky; but then the question arises, How, if the air
be blue, can the light of sunrise and sunset, which travels through
vast distances of air, be yellow, orange, or even red? The passage of
the white solar light through a blue medium could by no possibility
redden the light; the hypothesis of a blue atmosphere is therefore
untenable. In fact, the agent, whatever it be, which sends us the
li