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Vol. VII

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

[Read December 27, 1887.]

Abridged.

The first actual attempt to make the wave length of sodium light the standard of length was made by Pierce. This method involves two distinct measurements. First, of the angular displacement of the image of a slit by a, diffraction grating; second, of the distance between the lines of the grating. Both of these are subject to errors due to temperature changes and instrumental errors. The results of this work have not as yet been published, but it is probable that the degree of accuracy attained is not much greater than one part in 50 or 100 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. If this claim is justified, it is probably very near to the limit of accuracy of which the method admits.

A short time before this another method was proposed by Macé de Lepinay. 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 the quartz, and it is not surprising that the degree of accuracy attained was only one in fifty thousand.

Several years ago a method suggested itself which seemed likely to furnish results much more accurate than either of the foregoing, and