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poses of light, an adequately modified form of the mechan-
ism of sound. We know intimately whereon the velocity
of sound depends. When we lessen the density of a medium
and preserve its elasticity constant we augment the velocity.
When we heighten the elasticity and keep the density con-
stant we also augment the velocity. A small density,
therefore, and a great elasticity, are the two things neces-
sary to rapid propagation. N ow light is known to move
with the astounding velocity of 185,000 miles a second.
How is such a velocity to be obtained ? By boldly dif-
fusing in space a medium of the requisite tenuity and

Let us make such a medium our starting-point, endow-
ing it with one or two other necessary qualities; let us
handle it in accordance with strict mechanical laws ; let us
give to every step of our deduction the surety of the syl-
logism ; let us carry it thus forth from the world of imagi-
nation ,into the world of sense, and see whether the final
outcrop of the deduction be not the very phenomena of
light which ordinary knowledge and skilled experiment re~
veal. If in all the multiplied varieties of these phenomena,
including those of the most remote and entangled descrip-
tion, 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 agree-
ment and verification ; if, moreover, as in the case of Coni-
cal Refraction and in other cases, it has actually forced
upon our attention phenomena which no eye had previously
seen, and which no mind had previously imagined, such a
conception, which never disappoints us, but always lands
us on the solid shores of fact, must, we think, be something
more than a mere figment of the scientific fancy. In form-
ing it that composite and creative unity in which reason
and imagination are together blent, has, we believe, led us
into a world not less real than that of the senses, and of

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