. Carnegie Institution of Washington publication. REVERSED AND NON-REVERSED SPECTRA. 21 of the grating is so cut up by the strong, stationary interferences that it is unavailable. The grating plate must, of course, be slightly wedge-shaped, otherwise all the spectra would be superposed. In case the ruled face is half-silvered, however, the stationary interferences are practically absent, while two strong spectra are reflected from the silvered side. The phenome- non may then be produced at all distances of G from M and N (2 meters and less), but best at distances within i meter. It is, however
. Carnegie Institution of Washington publication. REVERSED AND NON-REVERSED SPECTRA. 21 of the grating is so cut up by the strong, stationary interferences that it is unavailable. The grating plate must, of course, be slightly wedge-shaped, otherwise all the spectra would be superposed. In case the ruled face is half-silvered, however, the stationary interferences are practically absent, while two strong spectra are reflected from the silvered side. The phenome- non may then be produced at all distances of G from M and N (2 meters and less), but best at distances within i meter. It is, however, frequently hard to find unless different distances apart of the mean D lines are tested. This may be due to the fact that the silver film is not quite equally thick. Besides the symmetrical position, gT, figure 12, the two corresponding unsymmetrical positions g'T' were tested with success; and it appeared that while in the case gT the phenomenon is virtually linear, dark or bright, like a Fraunhofer line, a succession of dark lines inclined to the vertical may appear for the unsymmetrical position g'T. Dark lines are apt to be broadened. Questions relative to the effect of oblique incidence were also tested by aid of the concave-mirror method shown in figure 11, the white light from C to G being conducted in an inch tube of pasteboard, immediately under the concave mirror, m. Figure 13,0, shows the general disposition of The angle of incidence i is gradually increased, until the return rays from N meet the grating at nearly grazing incidence. No essential difference in the phenomenon was observed, however, except that it was apt to be broader in the non-symmetrical positions and to suggest fine new lines in parallel with the old. In a return to the symmetrical position, sharp lines were especially distinct, usually showing one dark and two bright lines, while two dark and one bright occurred less frequently. It could be seen quite vividly with the naked eye. When
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