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The present research — an extension of a Hne of investiga- 

 tion ptu'sued by the author, Mr. A. A. Michelson, for a year or 

 two past — has in view an increased accnracy in the nieasnre- 

 ment of the wave-lengths of specific spectroscopic lines by 

 means of new apphcations of the method of interference. 



One ultimate object of such greater precision of determina- 

 tion is the employment of particular wave-lengths of Ught as 

 standards of comparison of metrical units. 



As a preliminary investigation of some importance, canied 

 on under the auspices of the Smithsonian Institution, the me- 

 moir is adopted as one deserving a place in the Smithsonian 

 "Contributions to Knowledge." 



S. P. LANGLEY, 

 Secretary Smithsonian Institution. 



Washington, July, 1892. 



