My eminent friend, Prof. Joseph Henry, of Washington, did me the honor of taking these lectures under his personal direction, and of arranging the times and places at which they were to be delivered. Deeming that my home-dutics could not, with propriety, be suspended for a longer period, I did not, at the outset, expect to be able to prolong my visit to the United States beyond the end of 1872. Thus limited as to time, Prof. Henry began in the North, and, proceeding southwards, arranged for the successive delivery of the lectures in Boston, New York, Philadelphia, Baltimore, and Washington. By this arrangement, which circumstances at the time rendered unavoidable, the lectures in New York were rendered coincident with the period of the presidential election. This was deemed unsatisfactory, and when the fact was represented to me I at once offered to extend the time of my visit so as to 'make the lectures in New York succeed those in Washington. The proposition was 'cordially accepted by my friends. To me personally this modified arrangement has proved in the highest degree satisfactory. It gave me a much-needed holiday at Niagara Falls; it, moreover, rendered the successive stages of my work a kind of growth, which reached its most impressive develop ment in New York and Brooklyn. In every city that I have visited, my reception has been that of a friend; and, now that my visit has become virtually a thing of the past, I can look back upon it with unqualified pleasure. It is a memory without a stain-an experience of deep and genuine kindness on the part of the American people never, on my part, to be forgotten. This relates to what may be called the positive side of my visit-to the circumstances. attending the work actually done. My only drawback relates to work undone; for I carry home with me the consciousness of having been unable to respond to the invitations of the great cities of the West; thus, I fear, causing, in many cases, disappointment. Would that this could have been averted! But the character of the lectures, and the weight of instrumental appliances which they involved, entailed loss of time and heavy labor. The need of rest alone would be a sufficient admonition to me to pause here; but, besides this, each successive mail from London brings me intelligence of work suspended and duties postponed through my absence. These are the considerations which prevent me from responding, with a warmth commensurate with their own, to the wishes of my friends in the West. On quitting England, I had no intention of publishing these lectures, and, except a fragment or two, not a line of them was written when I reached this city. They have been begun, continued, and ended in New York, and bear only too evident marks of the rapidity of their production. I thought it, however, due, both to those who heard them with such marked attention, and to those who wished to hear them, but were unable to do so, to leave them behind me in an authentic form. The execution of this work has cut me off from many social pleasures; it has also prevented me from making myself acquainted with institutions in the working of which I feel a deep interest. But human power is finite, and mine has been expended in the way which I deemed most agreeable, not to my more intimate friends, but to the people of the United States. In the opening lecture are mentioned the names of gentlemen to whom I am under lasting obligations for their friendly and often laborious aid. The list might readily be extended, for in every city I have visited willing helpers were at hand. I must not, however, omit the name of Mr. Rhets, Professor Henry's private secretary, who, not only in Washington, but in Boston, gave me most important assistance. To the trustees of the Cooper Institute my acknowledgments are due; also to the directors of the Mercantile Library at Brooklyn. I would add to these a brief but grateful reference to my high-minded friend and kinsman, General Hector Tyndale, for his long-continued care of me, and for the thoughtful tenderness by which he and his family softened, both to me and to the parents of the youth, the pain occasioned by the death of my junior assistant in Philadelphia. Finally, I have to mention with warm commendation the integrity, ability, and devotion, with which, from first to last, I have been aided by my principal assistant, Mr. John Cottrell." NEW YORK, February, 1873. LECTURE I. the auspices of a man who is untiring in his INTRODUCTORY: Uses of Experiment: Early Scien- efforts to diffuse sound scientific knowledge tific Notions: Sciences of Observation: Knowl- among the people of this country; whose edge of the Ancients Regarding Light: Nature judged from Theory defective: Defects of the energy, ability, and single-mindedness, in the Eye: Our Instruments: Rectilineal Propagation prosecution of an arduous task, have won for of Light: Law of Incidence and Reflection: him the sympathy and support of many of us Sterility of the Middle Ages: Retraction: Dis-in "the old country." I allude to Professor covery of Snell: Descartes and the Rainbow: Newton's Experiments on the Composition of Solar Light: His Mistake as rega:ds Achromatism: Synthesis of White Light: Yellow and Blue Lights proved to produce White by their Mixture: Colors of Natural Bodies: Absorption: Mixture of Pigments contrasted with Mixture of Lights. .a Mode of Motion." SOME twelve years ago I published, in England, a little book entitled the "Glaciers of the Alps." and, a couple of years subsequently, a second volume, entitled "Heat as These volumes were followed by others, written with equal plainness, and with a similar aim, that aim being to develop and deepen sympathy between science and the world outside of science. I agreed with thoughtful men* who deemed it good for neither world to be isolated from the other, or unsympathetic towards the other, and, to lessen this isolation, at least in one department of science, I swerved aside from those original researches which had previously been the pursuit and pleasure of my life. These books were, for the most part, republished by the Messrs. Appleton, under *Among whom may be mentioned, specially, the late Sir Edmund Head, Bart. Youmans, of this city. Quite as rapidly as in Engiand, the aim of these works was understood and appreciated in the United States, and they brought me from this side of the Atlantic innumerable evidences of good-will. Year after year, invitations reached me to visit America, and last year I was honored with a request so cordial, and signed by five-and-twenty names so distinguished in science, in literature, and in administrative position, that I at once resolved to respond to it by braving, not only the disquieting oscillations of the Atlantic, but the far more disquieting ordeal of appearing in person before the people of the United States. This request, conveyed to me by my accomplished friend, Professor Lesley, of Philadelphia, and preceded by a letter of the same purport from your scientific Nestor, Professor Joseph Henry, of Washington, desired that I would lecture in some of the principal cities of the Union. This I agreed to do, though much in the dark as to what form such lectures ought to to take. In * One of the earliest came from Mr. John Amory Lowell, of Boston. answer to my inquiries, however, I was given | single increment is made good by the indefi nite number of such increments, summed up in what may be regarded as practically infinite time. 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 a certain stage of his development, when, by evolution or sudden endowment, he became possessed of the apparatus of thought and the power of using it. For a time-and that to understand (by Professor Youmans principally) that a course of experimental lectures would materially promote scientific education in this country, and I at once resolved to meet this desire, as far as my time allowed. Experiments have two uses-a use in discovery, and a use in tuition. They are the investigator's language addressed to Nature, to which she sends intelligible replies. These replies, however, are, for the most part, at first too feeble for the public ear; for the investigator cares little for the loudness of Na-historically a long one-he was limited to ture's voice if he can only unravel its meaning. But after the discoverer comes the teacher, whose function it is so to exalt and modify the resu ts of the discoverer as to render them fit for public presentation. This secondary function I shall endeavor, in the present instance, to fulfil. mere observation, accepting what Nature offered, and confining intellectual action to it. 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 dif culty, the notion of natural forces took root I propose to take a single department of in the mind, the seedling of this notion being natural philosophy, and illustrate, by means the actual observation of electric and magof it, the growth of scientific knowledge under netic attractions. Slowly, and with difficulty. the guidance of experiment. I wish, in this the science of mechanics had to grow out of fist lecture, to make you acquainted with cer- this notion; and slowly at last came the full tain elementary phenomena; then to point application of mechanical principles to the out to you how those theoretic principles hymotions of the heavenly bodies. We trace which phenomena are explained, take root, and flourish in the human mind, and afterwards to apply these principles to the whole body of knowledge covered by the lectures. The science of optics lends itself to this mode of :reatment, 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 at struse discoveries of modern times. All our notions of Nature, however exalted or however grotesque, have some 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 antecedents of phenomena all science has sprung. The development of man, indeed, is ultimately due to his interaction with Nature. Natural phenomena arrest his attention and excite his questionings, the intellectual activity thus provoked reacting on the intellect itself, and adding to its strength. The quantity of power added by any single effort of the intellect may be indefinitely small; but the integration of innumerable increments of this kind has raised intellectual power from its rudiments to the magnitude it possesses today. In fact, the indefinite smallness of the the progress of astronomy through Hipparchus and Ptolemy; and, after a long halt, through Copernicus, Galileo, Tycho Brahe, and Kepler; wile, from the high table-land, of thought raised by these men, Newto shoots upward like a peak, overlooking ali others from his dominant elevation. But other objects than the motions of the stars attracted the attention of the ancient world. Light was a familiar phenomenon, and from the earliest times we find men's minds busy with the attempt to render some account of it. But, without experiment, which belongs to a later stage of scientific development, little progress could be madein this subject. The ancients, accordingly, were far less successful in dealing with light than in dealing with solar and stellar motions. Still, they did make some progress. They satisfied themselves that light moved in straight lines; they knew, also, that these lines or rays of light were reflected from polished surfaces, and that the angle of incidence was equal to the angle of reflection. These two results of ancient scientific curiosity constitute the starting-point of our present course of lectures. But, in the first place, it may be useful to say a few words regarding the source of light to be employed in our experiments. The rusting of iron is, to all intents and purposes, the slow burning of iron. It develops heat, and, if the heat be preserved, a high temperature may be thus attained. The destruction of the first Atlantic cable was probably due to heat developed in this way. Other metals are still more combustible than iron. You may light strips of zinc in a candleflame, and cause them to burn almost like strips of paper. But, besides combustion in the air, we may also have combustion ir a liquid. Water, for example, contains a store of oxygen which may unite with and consume a metal immersed in it. It is from this kind of combustion that we are to derive the heat and light employed in the present course. Their generation merits a moment's attention. Before you is an instruinent-a small voltaic battery-in which zinc is immersed in a suitable liquid. Matters are so arranged that an attraction is set up between the metal and the oxygen, actual union, however, being in the first instance avoided. Uniting the two ends of the battery by a thick wire, the attraction is satisfied, the oxygen unites with the metal, the zinc is consumed, and heat, as usual, is the result of the combustion. A power, which, for want of a better name, we call an electric current, passes at the same time through the wire. Cutting the thick wire in two, I unite the severed ends by a thin one. It glows with a white heat. Whence comes that heat? The question is well worthy of an answer. Suppose in the first instance, when the thick wire was employed, that we had permitted the action to continue until 100 grains of zinc were consumed, the amount of heat generated in the battery would be capable of accur te numerical expression. Let the action now continue, with this thin wire glowing, until 100 grains of zinc are consumed. Will the amount of heat generated in the battery be the same as before? No, it will be less by the precise amount generated in the thin wire outside the battery. In fact, by adding the internal heat to the external, we obtain for the combustion of 100 grains of zinc a total which never varies. By this arrangement, then, we are able to burn our zinc at one place, and to exhibit the heat and light of its combustion at a distant place. In New York, for example, we have our grate and fuel; but the heat and light of our fire may be made to appear at San Francisco. I now remove the thin wire and attach to the severed ends of the thick one two thin rods of coke. On bringing the rods together we obtain a small star of light. Now, the light to be employed in our lectures is a simple exaggeration of this star. Instead of being produced by ten cells, it is produced by fifty. Placed in a suitable camera, provided with a suitable lens, this light will give us all the beams necessary for our experiments. And here, in passing, let me refer to the common delusion that the works of Nature, the human eye included, are theoretically perfect. The degree of perfection of any organ is determined by what it has to do. Looking at the dazzling light from our large battery, you see a globe of light, but entirely fail to see the shape of the coke-points whence the light issues. The cause may be thus made clear: On the screen before you is projected an image of the carbon-points, the whole of the lens in front of the camera being employed to form the image. It is not sharp, but surrounded by a halo which nearly obliterates it. This arises from an imperfection of the lens, called its spherical aberration, due to the fact that the circumferential and central rays have not the same focus. The human eye labors under a similar defect, and, when you looked at the naked light from fifty cells, the blur of light upon the retina was sufficient to destrov the definition of the retinal image of the carbons. A long list of indictments might indeed be brought against the eye-its opacity, its want of symmetry, its lack of achromatism, its absolute blindness, in part. All these taken together caused an eminent German philosopher to say that, if any optician sent him an instrument so full of defects, he would send it back to him with the severes censure. But the eye is not to be judged from the standpoint of theory. As a practi cal instrument, and taking the adjustments by which its defects are neutralized into account, it must ever remain a marvel to the reflecting mind The ancients, as I have said, were aware of the rectilineal propagation of light. They knew that an opaque body, placed between the eye and a point of light, intecepted the light of the point. Possibly the terms "ray" and "beam" may have been suggested by those straight spokes of light which, in cer tain states of the atmosphere, dart from the sun at his rising and his setting. The rectilineal propagation of light may be illustrated at home in this way: Make a small hole in a closed window-shutter, before which stands a house or a tree, and place within the darkened room a white screen at some distance from the orifice. Every straight ray proceeding from the house or tree stamps its color upon the screen, and the sum of all the rays forms an image of the object. But, as the rays cross each other at the orifice, the image is inverted. Here we may illustrate the subject thus: In front of our camera is a large opening, closed at present by a sheet of tinfoil. Pricking by means of a common sewing-needle a small aperture in the tin-foil, an inverted image of the carbon-points starts forth upon the screen. will give a dozen images, a hundred a hundred, a thousand a thousand. But, as the apertures come closer to each other, that is to say, as the tin-foil between the apertures vanishes, the images overlap more and more. Removing the tin-foil altogether, the screen becomes uniformly illuminated Hence the light upon the screen may be regarded as the overlapping of innumerable images of the carbon-points. In like manner the light upon every white wall on a cloudless day may be regarded as produced by the superposition of innumerable images of the sun. A dozen apertures The law that the angle of incidence is equal to the angle of reflection is illustrated in this simple way: A straight lath is placed as an index perpendicular to a small looking |