The probability that the most accurate method of determining the solarparallax now available is that resting on the measurement of the velocityof light, has led to the acceptance of the following paper as one of theseries having in view the increase of our knowledge of the celestialmotions. The researches described in it, having been made at the UnitedStates Naval Academy, though at private expense, were reported to theHonorable Secretary of the Navy, and referred by him to this Office. Atthe suggestion of the writer, the paper was reconstructed with a fullergeneral discussion of the processes, and with the omission of some of thedetails of individual experiments.
To prevent a possible confusion of this determination of the velocity oflight with another now in progress under official auspices, it may bestated that the credit and responsibility for the present paper rests withMaster Michelson.
Simon Newcomb,
Professor, U.S. Navy,
Superintendent Nautical Almanac.
Nautical Almanac Office,
Bureau of Navigation,
Navy Department,
Washington, February 20, 1880.
In Cornu's elaborate memoir upon the determination of the velocity oflight, several objections are made to the plan followed by Foucault, whichwill be considered in the latter part of this work. It may, however, bestated that the most important among these was that the deflection was toosmall to be measured with the required degree of accuracy. In order toemploy this method, therefore, it was absolutely necessary that thedeflection should be increased.
In November, 1877, a modification of Foucault's arrangement suggesteditself, by which this result could be accomplished. Between this time andMarch of the following year a number of preliminary experiments wereperformed in order to familiarize myself with the optical arrangements.The first experiment tried with the revolving mirror produced a deflectionconsiderably greater than that obtained by Foucault. Thus far the onlyapparatus used was such as could be adapted from the apparatus in thelaboratory of the Naval Academy.
At the expense of $10 a revolving mirror was made, which could execute 128turns per second. The apparatus was installed in May, 1878, at thelaboratory. The distance used was 500 feet, and the deflection was abouttwenty times that obtained by Foucault.[1]
[Footnote 1: See Proc. Am. Assoc. Adv. Science, Saint Louis meeting.]
These experiments, made with very crude apparatus and under greatdifficulties, gave the following table of results for the velocity of