Barus — Use of the Grating in Interferometry. 171 



or small, and in the last table it was about •00025''"' per fringe. 

 The displacement at the mirror would be more than twice this. 

 The radial motion per fringe is of the order of wave-leno-th. 

 Naturally the position p of the grating changes parallel to itself 

 linearly with the thickness e' of the compensator, supposing 

 other conditions the same, so that ^, 2, and dz/dn all vary lin- 

 early with e', the thickness of tlie compensator. Finally, the 

 virtual thickness of the air film can only be brought to vanish 

 by compensation, otherwise spectra if equidistant are unequal 

 in length from red to violet, and vice versa. 



The full equations for the amounts of displacement, etc., 

 requires an evaluation of dd/dn, which in turn must take into 

 consideration that reflection from the mirrors cannot in gen- 

 eral be normal, except for the one color instanced above. 

 This investigation will have to be reserved for another com- 

 munication. 



Brown University, Providence, R. I. 



