Wave-length of Sodium as the Standard of Length. 463 



3. Finally there remains the determination of the velocity 

 of light by observations of the eclipses of Jupiter's satellites. 

 If the improved photometric methods practised at the Harvard 

 observatory make it possible to observe these with sufficient 

 accuracy, the difference in the results found for the velocity of 

 light when Jupiter is nearest to and farthest from the line of 

 motion will give, not merely the motion of the solar system 

 with reference to the stars, but with reference to the lumini- 

 ferous sether itself. 



LIX. 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 Peircet. 

 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 partif. 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§. 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. 



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 



* Communicated bv the Authors. 



t ' Nature,' xx. p. 99 (1879) ; Amer. Journ. Sci. [3-], p. 51 (1879). 



\ " On the Absolute Wave-lengths of Light,'' Amer. Journ. Sci. [3 J, 

 xxxiii. p. 167 (1887) ; Phil. Mag. [5], xxiii. p. 365. 



§ Comptes Rendus, cii. p. 1153 (1886) : Journ. de Phys. [2], v. p. 411 

 (1886). 



