Michelson and Morley — Wave-length of Sodium Light. 427 



intermediate neutral parts of iron, the poles becoming as zinc to 

 the other portions, and the currents flowing from the poles through 

 the liquid to the neutral parts of the metal. There is good reason 

 to think that these local currents are the cause of the curious 

 modifications in the chemical behavior of iron in the magnetic 

 field, already described in the pages of this journal, and of the 

 influence of magnetization upon the passivity of that metal. A 

 full account of this research will be published at an early day. 

 June, 1887. 



Aet. XLVII. — On a Method of making the Wave-length of 

 Sodium Light the actual and 'practical standard of length ; by 

 Albert A. Michelsojst and Edward W. Morley. 



The first actual attempt to make the wave-length of sodium 

 light a standard of length was made by Peirce.* 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 tempera- 

 ture 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.f 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 Lepinay4 

 This consists in the calculation of the number of wave-lengths 

 between two surfaces of a cube of quartz. Besides the spec- 

 troscopic 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 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 lumin- 

 iferous ether. 



* Nature, xx, 99, 1879 ; this Journal, III, xviii, 51, 1879. 



f On the absolute wave-lengths of light, this Journal, III, xxxiii, 167, 18S7. 



\ Comptes Rendus, cii, 1153, 1886; Journ. de Phys., II, v, 411, 1886. 



