of the composition of white light, Newton had
been everywhere triumphant--triumphant in the heavens, triumphant on
the earth, and his subsequent experimental work is, for the most part,
of immortal value. But infallibility is not an attribute of man, and,
soon after his discovery of the nature of white light, Newton proved
himself human. He supposed that refraction and chromatic dispersion
went hand in hand, and that you could not abolish the one without at
the same time abolishing the other. Here Dollond corrected him.

But Newton committed a graver error than this. Science, as I sought to
make clear to you in our second lecture, is only in part a thing of
the senses. The roots of phenomena are embedded in a region beyond the
reach of the senses, and less than the root of the matter will never
satisfy the scientific mind. We find, accordingly, in this career of
optics the greatest minds constantly yearning to break the bounds of
the senses, and to trace phenomena to their subsensible foundation.
Thus impelled, they entered the region of theory, and here Newton,
though drawn from time to time towards truth, was drawn still more
strongly towards error; and he made error his substantial choice. His
experiments are imperishable, but his theory has passed away. For a
century it stood like a dam across the course of discovery; but, as
with all barriers that rest upon authority, and not upon truth, the
pressure from behind increased, and eventually swept the barrier away.

In 1808 Malus, looking through Iceland spar at the sun, reflected from
the window of the Luxembourg Palace in Paris, discovered the
polarization of light by reflection. As stated at the time, this
discovery ushered in the darkest hour in the fortunes of the wave
theory. But the darkness did not continue. In 1811 Arago discovered
the splendid chromatic phenomena which we have had illustrated by the
deportment of plates of gypsum in polarized light; he also discovered
the rotation of the plane of polarization by quartz-crystals. In 1813
Seebeck discovered the polarization of light by tourmaline. That same
year Brewster discovered those magnificent bands of colour that
surround the axes of biaxal crystals. In 1814 Wollaston discovered the
rings of Iceland spar. All these effects, which, without a theoretic
clue, would leave the human mind in a jungle of phenomena without
harmony or relation, were organically connected by the theory of
undulation.

The wave