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ADDRESS BY ALBERT A. MICHELSON. 73 very interesting and important experiments of Quincke on the compressibility of liquids; of Jamin on the variation of index of refraction of water and of Ketteler and of Mascart on the index of refraction of gases. It seems somewhat curious that, while the immense advantage of the refractrometer as an accurate means of measuring indices of refraction has been so fully appreciated, its use should be limited to differential measurements. Thus, while it is easy to measure indices of gases, since the difference in optical path for gas and vacuum is so small, the indices of solids and liquids can only be determined in thin plates, and the accuracy of smell measurements must be limited to that of the estimation of the thickness. Such experiments may furnish the data for very interesting and important conclusions concerning the index of refraction and specially the anomalous dispersion of intensely opaque substances, such as metals and quasi metallic bodies. In such work the advantage of the interference method over the prismatic must be quite apparent; but I hope to show that for all measurements of refraction and dispersion—for solids and liquids as well as for gases—this method promises results which may far surpass those given by the prism. Suppose a piece of the substance cut in the form of a plane parallel plate. The accuracy, parallelism and distance of the surfaces in wave lengths may be determined exactly as in the case of the proposed standard cubic decimeter. Next the nearest whole number of waves in the solid can be determined either by actual count or perhaps more conveniently by a method described in a previous paper. The residual fractional parts of a wave may also be found as there described, or by direct observation of the interference rings between the two surfaces. The measurement of the index of refraction of a liquid is even more simple. A vessel is made with plane parallel sides, and the number of waves between the inner surfaces determined, first, when empty and then when filled with liquid. The ratio of these two numbers will be the index of refraction. It will be noted that the only observations required in this process are the counting of two numbers; and as Professor Mendenhall has aptly remarked, a mistake in counting of a whole number is not an error but a blunder. A blunder very easy to make, be it noted, in dealing with such large numbers as two or three hundred thousand, but whose chance | 74 SECTION B. of occurring may be indefinitely diminished by making several independent observations with different kinds of light. Perhaps one of the mbstr'important applications of the method is the determination of the^wave length of standard lines, both relative and absolute. In the paper above referred to, it was stated that the maximum difference of path at which interference fringes are visible, had been increased to over two hundred thousand waves (Fizeau’s number is 50,000) by using light from highly rarefied sodium vapor in^an exhausted tube. Since then I have observed interference under similar conditions with thallium with a difference of over three hundred and seventy thousand waves, and with mercury, with a difference of more than five hundred and forty thousand waves. By repeated measurements of the diameters of the interference rings, the fractions of a wave can be found to within a fiftieth—which means that the number of waves in this fixed distance can be found to within less than one part in twenty-five million. Any two kinds of light of this degree of purity can be compared with this same precision. The accuracy of the measurement of absolute wavelengths will of course depend on the accuracy with which the fixed distance can be compared with the standard meter; and this may be estimated as one part in two million. The results of the remarkable work of Rowland do not claim a much greater degree of ao^uracy than one part in half a million for relative determinations; while the elaborate research of Bell on absolute wave-lengths claims but one in two hundred thousand. We have thus at any rate a very promising method of excelling by far the best results that can possibly be obtained by the most perfect gratings. It may possibly help to realize the very considerable superiority of this instrument over the grating—at any rate for the class of work in question — if I recall to your attention the fact that by its means it has been possible to show that the red line of hydrogen is a very close double. A short time ago the same was found true of the green thallium line. Both these lines are something like a fiftieth of the distance of the sodium lines, and like these are of unequal intensity. It is even possible to measure this very small interval easily to within a fourth of one per cent. Following are the numbers obtained for the distance from one maximum or minimum of distinctness to the next:— |