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XLTX. Interferometry with the Aid of a Grating. By Carl 

 Bar us, Ph.D., LL.D., Professor of Physics at Brown 



University, Providence, U.S.A.* 



Part I. — Introduction. 



1. R<mi%rks on the Phenomena. — In the earlier papers f I 

 described certain of the interferences obtained when the 

 oblique plate gg, fig. 1, of Michelson's adjustment, is replaced 



by a plane diffraction-grating on ordinary plate glass. Some 

 explanation of these is necessary here. In the figure, L is 

 the source of white light from a collimator. Such light is 

 therefore parallel relative to a horizontal plane, bnt con- 

 vergent relatively to a vertical plane ; M and N are the usual 

 silver mirrors. A telescope adjusted for parallel rajs in the 

 line GrE must, therefore, show sharp white images of the slit. 

 As the grating is usually slightly wedge-shaped, there will be 

 (normally) four such images, two returned by M after re- 

 flexion from the front (white) and rear face (yellowish) of 

 the plate gg, and two due to N. There will also be two 

 other, not quite achromatic slit images from N or M, re- 

 spectively, due to double diffraction before and after reflexion. 

 These will be treated below. In the direction GD there 

 will thus be a corresponding number of diffraction spectra, 

 more or less coincident in all their parts, and therefore 

 adapted to interfere in pairs throughout their extent. If 



* Communicated by the Author. 



f Abridged from a Report to the Carnegie Institution of Washington. 

 U.S.A. See also Am. Journ. Sci. xxx. 1910, pp. 161-171 ; Science, 

 July 15. 1910, p. 92 ; and C. & M. Barns, Phil. Mas-. July, 1910. pp. 45 59. 



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