Page images provided by Case Western Reserve University's Digital Preservation Department Experimental Determination of the Velocity of Light Made at the U.S. Naval Academy, Annapolis. By Albert A. Michelson, Master U.S. Navy. Note. The probability that the most accurate method of determining the solar parallax now available is that resting on the measurement of the velocity of light, has led to the acceptance of the following paper as one of the series having in view the increase of our knowledge of the celestial motions. The researches described in it, having been made at the United States Naval Academy, though at private expense, were reported to the Honorable Secretary of the Navy, and referred by him to this Office. At the suggestion of the writer, the paper was reconstructed with a fuller general discussion of the processes, and with the omission of some of the details of individual experiments. To prevent a possible confusion of this determination of the velocity of light with another now in progress under official auspices, it may be stated that the credit and responsibility for the present paper rests with Master Michelson. Simon Newcomb, _Professor, U.S. Navy_, _Superintendent Nautical Almanac_. Nautical Almanac Office, Bureau of Navigation, Navy Department, _Washington, February 20, 1880._ Table Of Contents. Introduction Theory of the New Method Arrangement and Description of Apparatus Determination of the Constants The Formulæ Observations Separate results of Groups of Observations Discussion of Errors Objections Considered Postscript Experimental Determination of the Velocity of Light. By Albert A. Michelson, _Master, U.S.N._ Introduction. In Cornu's elaborate memoir upon the determination of the velocity of light, several objections are made to the plan followed by Foucault, which will be considered in the latter part of this work. It may, however, be stated that the most important among these was that the deflection was too small to be measured with the required degree of accuracy. In order to employ this method, therefore, it was absolutely necessary that the deflection should be increased. In November, 1877, a modification of Foucault's arrangement suggested itself, by which this result could be accomplished. Between this time and March of the following year a number of preliminary experiments were performed in order to familiarize myself with the optical arrangements. The first experiment tried with the revolving mirror produced a deflection considerably greater than that obtained by Foucault. Thus far the only apparatus used was such as could be adapted from the apparatus in the laboratory of the Naval Academy. At the expense of $10 a revolving mirror was made, which could execute 128 turns per second. The apparatus was installed in May, 1878, at the laboratory. The distance used was 500 feet, and the deflection was about twenty times that obtained by Foucault.[1]