arise from same cause. H1 = Young. Fresnel. Light is transverse undulations propagated in media by elasticity dependent on axis, when crystalline. I1 = Fresnel. Light is transverse undul^ns. transmitted from one med. to another according to probable hypotheses. J1 = Young. Fresnel. Colours result from interferences, the lengths of undulation being different for different colours. K1 = THE UNDULATORY THEORY OF LIGHT. {{141}} NOVUM ORGANON RENOVATUM. BOOK III. OF METHODS EMPLOYED IN THE FORMATION OF SCIENCE. CHAPTER I. INTRODUCTION. APHORISM XXVII. _The Methods by which the construction of Science is promoted are,_ Methods of Observation, Methods of obtaining clear Ideas, _and_ Methods of Induction. 1. IN the preceding Book, we pointed out certain general Characters of scientific knowledge which may often serve to distinguish it from opinions of a looser or vaguer kind. In the course of the progress of knowledge from the earliest to the present time, men have been led to a perception, more or less clear, of these characteristics. Various philosophers, from Plato and Aristotle in the ancient world, to Richard de Saint Victor and Roger Bacon in the middle ages, Galileo and Gilbert, Francis Bacon and Isaac Newton, in modern times, were led to offer precepts and maxims, as fitted to guide us to a real and fundamental knowledge of nature. It may on another occasion be our business to estimate the value of these precepts and maxims. And other contributions of the same kind to the philosophy of science might be noticed, and some which {142} contain still more valuable suggestions, and indicate a more practical acquaintance with the subject. Among these, I must especially distinguish Sir John Herschel's _Discourse on the Study of Natural Philosophy_. But my object at present is not to relate the history, but to present the really valuable results of preceding labours: and I shall endeavour to collect, both from them and from my own researches and reflections, such views and such rules as seem best adapted to assist us in the discovery and recognition of scientific truth; or, at least, such as may enable us to understand the process by which this truth is obtained. I would present to the reader the Philosophy and, if possible, the Art of Discovery.