. Carnegie Institution of Washington publication. , mirrors;G, grating; T, telescope). A small prism angle, <p, is essential (<p= 18°, about),as a large divergence of rays wouldnot be accommodated on the interfer-ometer, figure 3 . The fringes were foundwithout difficulty, in the second order,the arc lamp being used. They are alsoeasily distorted, if the edge of the prismis not parallel to the rulings of the grat-ing. In such a case the symmetricalarrow-shaped forms become one-sidedand, as it were, curved or faintlyfringed beyond the limits of the strip .To get the best adjustment, the l
. Carnegie Institution of Washington publication. , mirrors;G, grating; T, telescope). A small prism angle, <p, is essential (<p= 18°, about),as a large divergence of rays wouldnot be accommodated on the interfer-ometer, figure 3 . The fringes were foundwithout difficulty, in the second order,the arc lamp being used. They are alsoeasily distorted, if the edge of the prismis not parallel to the rulings of the grat-ing. In such a case the symmetricalarrow-shaped forms become one-sidedand, as it were, curved or faintlyfringed beyond the limits of the strip .To get the best adjustment, the lampshould shed about the same amount of undeviated light from both faces ofthe prism, on a screen temporarily placed behind it. The illuminated stripson the grating must coincide to the eye while making the fore-and-aft adjust-ment. Finally, the grating is to be slowly rotated on the axis normal toitself, until fringes of satisfactory shape and size appear. Naturally thisis done through the telescope, and a readjustment of the longitudinal axes. 22 THE INTERFEROMETRY OF of the spectra is necessary after each step of rotation. Fringes so obtainedare as good as those obtained by any other method. The range within which the fringes are sharp is small, not exceeding 2 displacement of the micrometer mirror, M. A partial reason for this willappear from figure 9 and results from the fact that the illumination on thegrating due to M moves laterally across the stationary strip due to N. Clearlyif the latter were also on a micrometer it might, in turn, be displaced relativelyto the direction of M and restore the fringes to full brilliancy. The rangein this case may be increased till either illuminated strip gets beyond theedges of the grating. This test will presently be made. If the prism angle is <p and the angle of diffraction for normal incidence is 6,the angle d, between the incident and reflected ray at M, is d = 6 — p Thus e tan 8/2 is the displacement of the strip of
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