C. Barus — Rotation of Interference Fringes. 63 



Art. VIII. — Rotation of Interference Fringes in Case of 

 Non-reversed and of Reversed Spectra ; by C. Barus.* 



1. Non-reversed spectra. — When the slit is oblique, it 

 effectively reproduces the wide slit, locally, and therefore 

 does not destroy the colored fringes. At every elevation 

 in the field the slit is necessarily linear though not verti- 

 cal. In figure 1 let the heavy lines, H, denote the colored 

 fringes for a fine vertical slit and white light, showing 

 nearly the same distance apart, throughout. Let the light 

 lines, Z, denote the fringes for a wide vertical slit and 

 homogeneous light, X. These fringes are due to the suc- 

 cessively increased or decreased obliquity of the rays, in the 

 horizontal plane. Now let acb be the image of the oblique slit 

 in homogeneous light. It is thus merely an oblique strip, cut 

 from the area of light lines or striations, as it were, and con- 

 sists of an alternation of black and bright dot-like vertical 

 elements, in correspondence with the original striated field. 

 We may suppose ab to have rotated around c, so that the ver- 

 tical through c is its position on the colored field (white light 

 and fine vertical slit). 



A color, X' (near the one X), corresponding to the field of the 

 lines, L, in case of a wide slit and homogeneous light, X', 

 will supply nearly the same grid, so far as the distance apart 

 of fringes is concerned. But the grid is displaced laterallv, 

 in consequence of the different angle of diffraction, 6. This 

 is shown by the dotted lines D, in figure 1, the effect being as 

 if the slit had been displaced laterally. If the wide slit for 

 homogeneous light X' is now narrowed and inclined as before, 

 an alternation of bright and dark elements will appear 

 in the image of the slit, ed, corresponding to X'. If we 

 suppose that for white light and the fine vertical slit, the 

 position of the fringe (X') was at c', we may again regard c' as 

 an axis of rotation. To find the fringes such as^, it is then 

 only necessary to connect corresponding black elements on ab 

 and ed. Their inclination is thus opposite to ab and ed, or 

 they have rotated in a direction opposite to that of the slit. 

 If, for, instance, the slit image ab or ed is gradually moved 

 back to the vertical, the points g and h will move with great 

 rapidity and in both directions toward infinity and the 

 fringes ff and ff become vertical lines through c and c', 

 respectively. 



* Work done on a grant from the Carnegie Institution of Washington, 

 D. C. See earlier papers in this Journal, xl, pp. 486-498, 1915 ; xli, pp. 

 414-434, 1916. The phenomena of §1 are most easily produced with two 

 transmitting gratings, parallel and having their ruled faces towards each 

 other. Science, xlii, p. 841, 1915. 



