views of light. But the most surprising
differences of statement regarding it exist among the very first
astronomers. They do not see it alike. Herschel says a Herculis is red;
Struve says it is yellow. They dispute about its nature, motion, and
quantity. Some astronomers believe the sun to be the great source of
light, at least to our system. But Nasmyth informs the Royal
Astronomical Society that "the true source of latent light is not in the
solar orb, but in space itself, and that the grand function of the sun
is to act as an agent for the bringing forth into existence the
luciferous element, which element I suppose to be diffused throughout
the boundless regions of space."[349] The nature of light is however
still as great a mystery as when Job demanded, "Where is the way where
light dwelleth?" The undulatory theory of light, now generally accepted,
assumes that light is caused by the vibrations of the ether in a plane
transverse to the direction of propagation. In order to transmit motions
of this kind, the parts of the luminiferous medium must resist
compression and distortion, like those of an elastic solid body; its
transverse elasticity being great enough to transmit one of the most
powerful kinds of physical energy, with a speed in comparison with
which that of the swiftest planets of our system is inappreciable, and
its longitudinal elasticity immensely greater--both of these
elasticities being at the same time so weak as to offer no perceptible
resistance to the motion of the planets, and other visible bodies.[350]
Is the velocity of light uniform? Or, if variable, is the variation
caused by the original difference of the projectile force of the
different suns, stars, comets, etc.? or by the different media through
which it passes? Arago alleges that light moves more rapidly through
water than through air; but Brequet asserts that the fact is just the
reverse.[351] Both admit that its velocity varies with the medium.
Jacobs alleges that during the trigonometrical survey of India he
observed the _extinction_ of light reflected through sixty miles of
horizontal atmosphere.[352] How, then, can astronomers make any reliable
calculations of the velocity of light reaching us through regions of
space filled with unknown media? Newton calculated the velocity of light
at one hundred and fifty-five thousand five hundred and fifty-five and
five-ninth miles a second; but Encke shows he erred thirty per cent.
Other emin