nt, the
shorter. On wave-length colour depends. In all cases the vibrations are
transverse. The undulatory movement passes onward at the rate of 192,000
miles in a second. The mean length of a wave of light is 0.0000219 of an
inch; an extreme red wave is about twice as long as an extreme violet
one. The yellow is intermediate. The vibrations which thus occasion
light are, at a mean, 555 in the billionth of a second. As with the air,
which is motionless when a sound passes through it, the ether is
motionless, though traversed by waves of light. That which moves forward
is no material substance, but only a form, as the waves seen running
along a shaken cord, or the circles that rise and fall, and spread
outwardly when a stone is thrown into water. The wave-like form passes
onward to the outlying spaces, but the water does not rush forward. And
as we may have on the surface of that liquid waves the height of which
is insignificant, or those which, as sailors say, are mountains high in
storms at sea, their amplitude thus differing, so in the midst of the
ether difference of amplitude is manifested to us by difference in the
intensity or brilliancy of light.

[Sidenote: The human eye; its capabilities.] The human eye, exquisitely
constructed as it is, is nevertheless an imperfect mechanism, being
limited in its action. It can only perceive waves of a definite length,
as its fellow organ, the ear, can only distinguish a limited range of
sounds. It can only take note of vibrations that are transverse, as the
ear can only take note of those that are normal. In optics there are two
distinct orders of facts; the actual relations of light itself, and the
physiological relations of our organ of vision, with all its limitations
and imperfections. Light is altogether the creation of the mind. The
ether is one thing, light is another, just as the air is one thing and
sound another. The ether is not composed of the colours of light any
more than the atmospheric air consists of musical notes.

[Sidenote: Chemical influences of light.] To the chemical agency of
light much attention has in recent times been devoted. Already in
photography, it has furnished us an art which, though yet in its
infancy, presents exquisite representations of scenery, past events, the
countenances of our friends. In an almost magical way it evokes
invisible impressions, and gives duration to fleeting shadows. Moreover,
these chemical influences of light give b