, showing the uses of
experiment in the cultivation of Natural Knowledge, would materially
promote scientific education in this country. And though such lectures
involved the selection of weighty and delicate instruments, and their
transfer from place to place, I determined to meet the wishes of my
friends, as far as the time and means at my disposal would allow.

Sec. 2. _Subject of the Course. Source of Light employed._

Experiments have two great uses--a use in discovery, and a use in
tuition. They were long ago defined as the investigator's language
addressed to Nature, to which she sends intelligible replies. These
replies, however, usually reach the questioner in whispers too feeble
for the public ear. But after the investigator comes the teacher,
whose function it is so to exalt and modify the experiments of his
predecessor, as to render them fit for public presentation. This
secondary function I shall endeavour, in the present instance, to
fulfil.

Taking a single department of natural philosophy as my subject, I
propose, by means of it, to illustrate the growth of scientific
knowledge under the guidance of experiment. I wish, in the first
place, to make you acquainted with certain elementary phenomena; then
to point out to you how the theoretical principles by which phenomena
are explained take root in the human mind, and finally to apply these
principles to the whole body of knowledge covered by the lectures. The
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 scie