orld. Laying the
theoretic conception at the root of matters, we determine by deduction
what are the phenomena which must of necessity grow out of this root.
If the phenomena thus deduced agree with those of the actual world, it
is a presumption in favour of the theory. If, as new classes of
phenomena arise, they also are found to harmonise with theoretic
deduction, the presumption becomes still stronger. If, finally, the
theory confers prophetic vision upon the investigator, enabling him to
predict the occurrence of phenomena which have never yet been seen,
and if those predictions be found on trial to be rigidly correct, the
persuasion of the truth of the theory becomes overpowering.

Thus working backwards from a limited number of phenomena, the human
mind, by its own expansive force, reaches a conception which covers
them all. There is no more wonderful performance of the intellect than
this; but we can render no account of it. Like the scriptural gift of
the Spirit, no man can tell whence it cometh. The passage from fact to
principle is sometimes slow, sometimes rapid, and at all times a
source of intellectual joy. When rapid, the pleasure is concentrated,
and becomes a kind of ecstasy or intoxication. To any one who has
experienced this pleasure, even in a moderate degree, the action of
Archimedes when he quitted the bath, and ran naked, crying 'Eureka!'
through the streets of Syracuse, becomes intelligible.

How, then, did it fare with the Emission Theory when the deductions
from it were brought face to face with natural phenomena? Tested by
experiment, it was found competent to explain many facts, and with
transcendent ingenuity its author sought to make it account for all.
He so far succeeded, that men so celebrated as Laplace and Malus, who
lived till 1812, and Biot and Brewster, who lived till our own time,
were found among his disciples.

Sec. 3. _The Undulatory Theory of Light_.

Still, even at an early period of the existence of the Emission
Theory, one or two great men were found espousing a different one.
They furnish another illustration of the law that, in forming
theories, the scientific imagination must draw its materials from the
world of fact and experience. It was known long ago that sound is
conveyed in waves or pulses through the air; and no sooner was this
truth well housed in the mind than it became the basis of a theoretic
conception. It was supposed that light, like sound, might also b