ut the
subsequent translation of the shock of the aethereal waves into
consciousness eludes mechanical science. As an oar dipping into the
Cam generates systems of waves, which, speeding from the centre of
disturbance, finally stir the sedges on the river's bank, so do the
vibrating atoms generate in the surrounding aether undulations, which
finally stir the filaments of the retina. The motion thus imparted is
transmitted with measurable, and not very great velocity to the brain,
where, by a process which the science of mechanics does not even tend
to unravel, the tremor of the nervous matter is converted into the
conscious impression of light.

Darkness might then be defined as aether at rest; light as aether in
motion. But in reality the aether is never at rest, for in the
absence of light-waves we have heat-waves always speeding through it.
In the spaces of the universe both classes of undulations incessantly
commingle. Here the waves issuing from uncounted centres cross,
coincide, oppose, and pass through each other, without confusion or
ultimate extinction. Every star is seen across the entanglement of
wave-motions produced by all other stars. It is the ceaseless thrill
caused by those distant orbs collectively in the aether, that
constitutes what we call the 'temperature of space.' As the air of a
room accommodates itself to the requirements of an orchestra,
transmitting each vibration of every pipe and string, so does the
inter-stellar aether accommodate itself to the requirements of light
and heat. Its waves mingle in space without disorder, each being
endowed with an individuality as indestructible as if it alone had
disturbed the universal repose.

All vagueness with regard to the use of the terms 'radiation' and
'absorption' will now disappear. Radiation is the communication of
vibratory motion to the aether; and when a body is said to be chilled
by radiation, as for example the grass of a meadow on a starlight
night, the meaning is, that the molecules of the grass have lost a
portion of their motion, by imparting it to the medium in which they
vibrate. On the other hand, the waves of aether may so strike against
the molecules of a body exposed to their action as to yield up their
motion to the latter; and in this transfer of the motion from the
aether to the molecules consists the absorption of radiant heat. All
the phenomena of heat are in this way reducible to interchanges of
motion; and it is