This book is chie?y intended for those who feel interested in the methodical procedure of scientific investigation, and although some parts of it may appeal most strongly to science teachers, yet the fact that scientific method is now destined to play so great a part not only in the whole of the educational field but also in every other field of thought and activity, may suffice to make the book welcome to a much wider circle than those whose interests are con fined to the laboratory and the lecture table. The method adopted by men of science in their work is far different now from what it was in the time of Aristotle or even in the time of Bacon. Fundamentally, the main processes of the method of science are, in character, not direct but inverse, and inverse processes almost always present great difficulties. The method of science has thus been a thing of exceedingly slow growth, and even now is by no means fully developed. We smile at the methods of Descartes, who himself found serious fault with the methods of the ancients, and there is no doubt at all that our descendants will greatly improve on the methods of the present day.