now before us it has manifested itself by transplanting into space,
for the purposes of light, an adequately modified form of the
mechanism of sound. We know intimately whereon the velocity of sound
depends. When we lessen the density of the aerial medium, and
preserve its elasticity constant, we augment the velocity. When we
heighten the elasticity, and keep the density constant, we also
augment the velocity. A small density, therefore, and a great
elasticity, are the two things necessary to rapid propagation. Now
light is known to move with the astounding velocity of 186,000 miles a
second. How is such a velocity to be obtained? By boldly diffusing
in space a medium of the requisite tenuity and elasticity.

Let us make such a medium our starting-point, and, endowing it with
one or two other necessary qualities, let us handle it in accordance
with strict mechanical laws. Let us then carry our results from the
world of theory into the world of sense, and see whether our
deductions do not issue in the very phenomena of light which ordinary
knowledge and skilled experiment reveal. If in all the multiplied
varieties of these phenomena, including those of the most remote and
entangled description, this fundamental conception always brings us
face to face with the truth; if no contradiction to our deductions
from it be found in external nature, but on all sides agreement and
verification; if, moreover, as in the case of Conical Refraction and
in other cases, it actually forces upon our attention phenomena which
no eye had previously seen, and which no mind had previously
imagined--such a conception, must, we think, be something more than a
mere figment of the scientific fancy. In forming it, that composite
and creative power, in which reason and imagination are united, has,
we believe, led us into a world not less real than that of the senses,
and of which the world of sense itself is the suggestion and, to a
great extent, the outcome.

Far be it from me, however, to wish to fix you immovably in this or in
any other theoretic conception. With all our belief of it, it will be
well to keep the theory of a luminiferous aether plastic and capable
of change. You may, moreover, urge that, although the phenomena occur
_as if_ the medium existed, the absolute demonstration of its existence
is still wanting. Far be it from me to deny to this reasoning such
validity as it may fairly claim. Let us endeavour by means of a