science of optics lends itself particularly well to this mode of
treatment, and on it, therefore, I propose to draw for the materials
of the present course. It will be best to begin with the few simple
facts regarding light which were known to the ancients, and to pass
from them, in historic gradation, to the more abstruse discoveries of
modern times.

All our notions of Nature, however exalted or however grotesque, have
their foundation in experience. The notion of personal volition in
Nature had this basis. In the fury and the serenity of natural
phenomena the savage saw the transcript of his own varying moods, and
he accordingly ascribed these phenomena to beings of like passions
with himself, but vastly transcending him in power. Thus the notion of
_causality_--the assumption that natural things did not come of
themselves, but had unseen antecedents--lay at the root of even the
savage's interpretation of Nature. Out of this bias of the human mind
to seek for the causes of phenomena all science has sprung.

We will not now go back to man's first intellectual gropings; much
less shall we enter upon the thorny discussion as to how the groping
man arose. We will take him at that stage of his development, when he
became possessed of the apparatus of thought and the power of using
it. For a time--and that historically a long one--he was limited to
mere observation, accepting what Nature offered, and confining
intellectual action to it alone. The apparent motions of sun and stars
first drew towards them the questionings of the intellect, and
accordingly astronomy was the first science developed. Slowly, and
with difficulty, the notion of natural forces took root in the human
mind. Slowly, and with difficulty, the science of mechanics had to
grow out of this notion; and slowly at last came the full application