But, though the region of physical theory lies thus behind the world of senses, the verifications of theory occur in that world.  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 be the product of wave-motion.  But what, in this case, could be the material forming the waves?  For the waves of sound we have the air of our atmosphere; but the stretch of imagination which filled all space with a luminiferous ether trembling with the waves of light was so bold as to shock cautious minds.  In one of my latest conversations with Sir David Brewster, he said to me that his chief objection to the undulatory theory of light was, that he could not think the Creator capable of so clumsy a contrivance as the filling of space with ether to produce light.  This, I may say, is very dangerous ground, and the quarrel of science with Sir David, on this point as with many estimable persons on other points, is, that they profess to know too much about the mind of the Creator.