. Carnegie Institution of Washington publication. 8 THE INTERFEROMETRY OF but appearing as a single line, vividly colored above the brightness of the spectrum; or, again, more jet-black than the Fraunhofer lines and located in the position of the coincident wave-lengths of the two superimposed spectra. It is possible, however, as will be shown in § 4, to obtain two spectra in such a way that if their longitudinal axes coincide the Fraunhofer lines intersect at a small angle, and vice versa. In such a case, for coincident Fraunhofer lines, interference occurs in a band around these lines and is

- Image ID: RFRJCW
. Carnegie Institution of Washington publication. 8 THE INTERFEROMETRY OF but appearing as a single line, vividly colored above the brightness of the spectrum; or, again, more jet-black than the Fraunhofer lines and located in the position of the coincident wave-lengths of the two superimposed spectra. It is possible, however, as will be shown in § 4, to obtain two spectra in such a way that if their longitudinal axes coincide the Fraunhofer lines intersect at a small angle, and vice versa. In such a case, for coincident Fraunhofer lines, interference occurs in a band around these lines and is
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Image ID: RFRJCW
. Carnegie Institution of Washington publication. 8 THE INTERFEROMETRY OF but appearing as a single line, vividly colored above the brightness of the spectrum; or, again, more jet-black than the Fraunhofer lines and located in the position of the coincident wave-lengths of the two superimposed spectra. It is possible, however, as will be shown in § 4, to obtain two spectra in such a way that if their longitudinal axes coincide the Fraunhofer lines intersect at a small angle, and vice versa. In such a case, for coincident Fraunhofer lines, interference occurs in a band around these lines and is absent in the rest of the spectrum; whereas, if the longitudinal axes are coincident, the interferences are arranged with reference to these axes. These results seem to bear on the question, but it is difficult to clearly resolve them. The methods used in this paper consist chiefly in bringing the two first- order spectra of a grating, or the second-order spectra or their equivalents, to interfere. In this respect they contain an additional method of inter- ferometry which may be useful, if for any reason it is necessary that the two component beams are not to retrace their paths.. 2. Coincident spectra with one reversed on a given Fraunhofer line.—In figure i, L is a narrow vertical sheet (subsequently broadened by the dif- fraction of the slit) of white sunlight or arc light from a collimator, G the transparent grating ruled on the side g, from which the first or second order of spectra gM and gN originate. M and N are opaque mirrors mounted adjustably on a firm rail, RR, each of them with three adjustment screws relative to horizontal and vertical axes. M is provided with a slide micrometer (not shown). From M and N the beams pass to the smaller paired mirrors, m and n, which should meet in a fine vertical line at a very obtuse angle. A silvered biprism would here be far preferable, but none having the required angle was available. From n, m, the beams pass into the tele

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