Every period in the history of mankind had its revelation in science. Some periods were most fortunate in this respect. The first half of the nineteenth century saw the great scientific revelation called the Principle of Conservation of Energy, and considered it its greatest glory. Our own American philosopher, Benjamin Thompson, of Woburn, Massachusetts, known in Europe as Count Rumford, was one of several early prophets in science who foresaw the advent of this great dynamical doctrine. Its importance to mankind cannot be overestimated. I am sure that many a scientific man of those days felt grateful to heaven for the blessing of having lived during the age when that great revelation was received by mankind. The scientific men of to-day are grateful for having lived during the second half of the nineteenth century, when the great electromagnetic theory was revealed to man. Its importance, likewise, cannot be overestimated. But there is a radical difference in the historical progress of these two nineteenth-century revelations in science. The existence of the first was intuitively foreseen and may be said to have existed in one form or another in the minds of many scientific men long before it received its final form of statement. Its formulator, Helmholtz, thought that he was not announcing anything new, but was only stating his own view of something that was already well known. After his announcement, in 1847, every scientific man accepted the revelation as an almost self-evident truth. The electromagnetic theory of light and of matter had a different history. It was born as a dim vision in the mind of a single man, Faraday, and nearly fifty years elapsed before it was formulated by Maxwell and experimentally demonstrated by Hertz. It was only then that the world began to understand that a great scientific revelation had appeared to man. To-day we know that new physical concepts requiring a new language for their expression had to be created in the minds of scientific men before the modern electromagnetic doctrine could be revealed to the world. The first glimpses of that revelation I caught on the slope of Goat Fell Mountain, and two years later I saw in Berlin what I believed to be a clear outline of its meaning. When I look back to those days and consider how few were the physicists who had caught this meaning even twenty years after it was stated by Maxwell in 1865, I wonder whether it is possible to-day to convey that meaning to people who are not trained physicists. I think it is, and I believe that the attempt should be made, because the electromagnetic doctrine is to-day recognized to be the very foundation of all our knowledge of physical phenomena. I also think that one of the best methods of conveying that meaning is to describe my early attempts which failed to catch it. [Illustration: _From a photograph by John Watkins, London_ MICHAEL FARADAY