THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XV. — The Use of the Grating in Interferometry ;'^ 



by C. Bards. 



1. Introductory. It was pointed out elsewheref that on 

 replacing the symmetrically oblique transparent mirror in 

 Michelson's adjustment by a glass grating,:}; gg, fig. 1 (for 

 instance), it is possible, with ordinary plate glass and a non- 

 silvered grating, to produce interferences between pairs of dif- 

 fracted spectra, D'J)", if returned by nearly equidistant mir- 

 rors M and iV^ to a telescope in the line D. Both of these 

 spectra are very brilliant and not very unequally so, and the 

 coincidence of spectrum lines, both horizontally and vertically, 

 brings ont the phenomenon. This is of the ring type and not 

 of the line type heretofore (1. c.) discussed ; but it occupies the 

 whole field of the spectrum from red to violet. One obtains 

 brilliant large confocal ellipses with horizontal and vertical 

 symmetry, and the spectrum lines, simultaneously in focus, 

 may serve either as major or minor axes. Their interfer- 

 ometer motion is twofold in character, consisting of radial 

 motion combined with a drift of the figure as a whole in a 

 horizontal direction. Naturally a fine slit is of advantage, but 

 the experiment succeeds with a wide slit, especially m the red, 

 even after spectrum lines vanish. 



Such ellipses (as I shall call them, though they are probably 

 ovals) as have their centers in the field are clearly due to 

 reflection from the same surface as shown in figure 1 ; curved 



* Abridged from a Report to the Carnegie Institution of Washington, 

 t Science, xxxii, p. 93, 1910 ; of. Phil. Mag., July, 1910. 

 X My thanks are due to Prof. J. S. Ames, who was good enough to lend 

 me this grating. 



Am. Jour. Sci.— Foueth Series' Vol! XXX, No. 177.— September, 1910. 

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