This invitation, conveyed to me by my accomplished friend Professor Lesley, of Philadelphia, and preceded by a letter of the same purport from your scientific Nestor, the celebrated Joseph Henry, of Washington, desired that I should lecture in some of the principal cities of the Union.  This I agreed to do, though much in the dark as to a suitable subject.  In answer to my inquiries, however, I was given to understand that a course of lectures, 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 science has sprung.