theory was applied and verified in all directions, Airy being
especially conspicuous for the severity and conclusiveness of his
proofs. A most remarkable verification fell to the lot of the late Sir
William Hamilton, of Dublin, who, taking up the theory where Fresnel
had left it, arrived at the conclusion that at four special points of
the 'wave-surface' in double-refracting crystals, the ray was divided,
not into two parts but into an infinite number of parts; forming at
these points a continuous conical envelope instead of two images. No
human eye had ever seen this envelope when Sir William Hamilton
inferred its existence. He asked Dr. Lloyd to test experimentally the
truth of his theoretic conclusion. Lloyd, taking a crystal of
arragonite, and following with the most scrupulous exactness the
indications of theory, cutting the crystal where theory said it ought
to be cut, observing it where theory said it ought to be observed,
discovered the luminous envelope which had previously been a mere idea
in the mind of the mathematician.

Nevertheless this great theory of undulation, like many another truth,
which in the long run has proved a blessing to humanity, had to
establish, by hot conflict, its right to existence. Illustrious names
were arrayed against it. It had been enunciated by Hooke, it had been
expounded and applied by Huyghens, it had been defended by Euler. But
they made no impression. And, indeed, the theory in their hands lacked
the strength of a demonstration. It first took the form of a
demonstrated verity in the hands of Thomas Young. He brought the waves
of light to bear upon each other, causing them to support each other,
and to extinguish each other at will. From their mutual actions he
determined their lengths, and applied his knowledge in all directions.
He finally showed that the difficulty of polarization yielded to the
grasp of theory.

After him came Fresnel, whose transcendent mathematical abilities
enabled him to give the theory a generality unattained by Young. He
seized it in its entirety; followed the ether into the hearts of
crystals of the most complicated structure, and into bodies subjected
to strain and pressure. He showed that the facts discovered by Malus,
Arago, Brewster, and Biot were so many ganglia, so to speak, of his
theoretic organism, deriving from it sustenance and explanation. With
a mind too strong for the body with which it was associated, that body
became a wreck long