ox THE APPLICATION OF INTERFERENCE METHODS 

 TO SPECTROSCOPIC MEASUREMENTS. 



By Albert A. Michelson.* 



The theoretical investigation of the relation between the distribution of 

 light in a source, as a function of the wave-length, and the resulting " visi- 

 biUty cui-ve " has been given in a paper bearing the same title as the present 

 one in the Philosophical Magazine for April, 1891. 



The physical definition of " visibihty " there adopted is 



in which I^ is the intensity at the center of a bright interference band, and 

 L the intensity at the center of the adjoining dark band. In order to in- 

 terpret the actual curves obtained by observation of interference fringes, it 

 is first necessary to reduce the results of the eye-estimates of visibility, 

 which may be designated by F,, to their absolute values as above defined. 

 For this purpose, two quartz lenses, one concave and the other convex, 

 and of equal curvatures, were mounted with then crystalline axes at right 

 angles to each other between two Nicols. Under these conditions a series of 

 concentric interference rings appeared. If a be the angle between the prin- 



* I take this opportunity of presenting my acknowledgments and thanks to the Smithsonian Insti- 

 tution for the funds necessarj' to caiTy out this research ; to Clark University for the facilities it has placed 

 at my disposal ; and especially to Mr. F. L. 0. Wads\yorth, Assistant in Physics of Clark University, for 

 the valuable services he has rendered and his unflagging zeal in furthering this investigation. 



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