A man who could thus arouse popular enthusiasm for science a century and a half ago must have been a remarkable genius.  Trusted students of Linnaeus were sent on botanical exploring expeditions throughout the world.  The high renown in which Linnaeus was held was shown in the significant title, almost universally bestowed upon him, of “The Flower King.”—­Western Druggist.

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ON A METHOD OF MAKING THE WAVE LENGTH OF SODIUM LIGHT THE ACTUAL AND PRACTICAL STANDARD OF LENGTH.

BY ALBERT A. MICHELSON AND EDWARD W. MORLEY.

The first actual attempt to make the wave length of sodium light a standard of length was made by Peirce.[1] This method involves two distinct measurements:  first, that of the angular displacement of the image of a slit by a diffraction grating, and, second, that of the distance between the lines of the grating.  Both of these are subject to errors due to changes of temperature and to instrumental errors.  The results of this work have not as yet been published; but it is not probable that the degree of accuracy attained is much greater than one part in fifty or a hundred thousand.  More recently, Mr. Bell, of the Johns Hopkins University, using Rowland’s gratings, has made a determination of the length of the wave of sodium light which is claimed to be accurate to one two hundred thousandth part[2].  If this claim is justified, it is probably very near the limit of accuracy of which the method admits.  A short time before this, another method was proposed by Mace de Lepinay.[3] This consists in the calculation of the number of wave lengths between two surfaces of a cube of quartz.  Besides the spectroscopic observations of Talbot’s fringes, the method involves the measurement of the index of refraction and of the density of quartz, and it is not surprising that the degree of accuracy attained was only one in fifty thousand.

   [Footnote 1:  Nature, xx, 99, 1879; this Journal, III, xviii, 51, 1879.]

   [Footnote 2:  On the absolute wave lengths of light, this Journal,
   III, xxxiii, 167, 1887.]

   [Footnote 3:  Comptes Rendus, cii, 1153, 1886; Journal, de Phys.,
   II, v, 411, 1886.]

Several years ago, a method suggested itself which seemed likely to furnish results much more accurate than either of the foregoing, and some preliminary experiments made in June have confirmed the anticipation.  The apparatus for observing the interference phenomena is the same as that used in the experiments on the relative motion of the earth and the luminiferous ether.

Light from the source at s (Fig. 1), a sodium flame, falls on the plane parallel glass, a, and is divided, part going to the plane mirror, c, and part to the plane mirror, b.  These two pencils are returned along cae and bae, and the interference of the two is observed in the telescope at e.  If the distances, ac and ab, are made equal, the plane, c, made parallel with that of the image of b, and the compensating glass, d, interposed, the interference is at once seen.  If the adjustment be exact, the whole field will be dark, since one pencil experiences external reflection and the other internal.