[25] “Gases of the Atmosphere: History of Their Discovery.” Third edition, with portraits. London and New York, Macmillan, 1906. Precise Measurement as a Means of Discovery. As measurements become more and more precise they afford an important means of discovery. Sir William Crookes tells us:--“It is well known that of late years new elementary bodies, new interesting compounds have often been discovered in residual products, in slags, flue-dusts, and waste of various kinds. In like manner, if we carefully scrutinize the processes either of the laboratory or of nature, we may occasionally detect some slight anomaly, some unanticipated phenomenon which we cannot account for, and which, were received theories correct and sufficient, ought not to occur. Such residual phenomena are hints which may lead the man of disciplined mind and of finished manipulative skill to the discovery of new elements, of new laws, possibly even of new forces; upon undrilled men these possibilities are simply thrown away. The untrained physicist or chemist fails to catch these suggestive glimpses. If they appear under his hands, he ignores them as the miners of old did the ores of cobalt and nickel.”[26] [26] Nineteenth Century Magazine, London, July 1877. It was a residual effect which led to the discovery of the planet Neptune. The orbit of Uranus being exactly defined, it was noticed by Adams and Leverrier that after making due allowance for perturbations by all known bodies, there remained a small disturbance which they believed could be accounted for only by the existence of a planet as yet unobserved. That planet was forthwith sought, and soon afterward discovered, proving in mass and path to be capable of just the effect which had required explanation. [Illustration: Photograph by Cox, Chicago. PROFESSOR A. A. MICHELSON, UNIVERSITY OF CHICAGO.] [Illustration: Michelson interferometer.] Measurements Refined: the Interferometer.