170 



EEPORT — 1892. 



On the Application of Interference Methods to Spectroscopic 

 Measurements. By Albert A. Michelson. 



Plates I.-IV. 

 [Ordered by the General Committee to be printed in extenso among the Reports.] 



The theoretical investigation of the relation between the distribution of 

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

 ' visibility curve ' 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 ' visibility ' there adopted is 



ii + V 



in which I, is the intensity at the centre of a bright interference band, 

 and I2 the intensity at the centre of the adjoining dark band. In order 

 to interpret 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 V,,, 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 their 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 principal section of the polariser and the axis of the first 

 quartz, and to the angle between the axis and the analyser, the intensity 

 of the light transmitted will be 



I = cos^ (co — a) — sin 2a sin 2ijiSm'^Tr 1- — ^ , 



where ty is the thickness through the first quartz, and t^ that through 

 the second. If the analyser and polariser are parallel, cu = a, and 



•whence 



and 



I=l-sin2 2a8in2 7r'^-^^^ 



Ii=l, and l2= 1 — sin^ 2a, 



Y__ I] — I2 __ 1 — cos^ 2a 

 ii + I2 ~ 1 + cos^Ta' 



This curve, together with the mean of a number of eye-estimates, 

 given in fig. 2 on opposite page. From these the following table of cor- 

 rections may be obtained : — 



