. Carnegie Institution of Washington publication. t all be adjustable. After preliminary meas-urement for equal distances, the fringes were found without trouble. Theywere strong but fine, beginning with vertical hair-lines and gradually rotatingas they grew coarser, till they rather abruptly vanished. The displacementof the M mirror did not exceed mm., nor the rotation 30°. The spectrabeing non-reversed, the fringes covered the whole field. Nevertheless theselines must probably be regarded as arcs of circles or ellipses with enor-mously distant centers. In fact, the appearance of the whol
. Carnegie Institution of Washington publication. t all be adjustable. After preliminary meas-urement for equal distances, the fringes were found without trouble. Theywere strong but fine, beginning with vertical hair-lines and gradually rotatingas they grew coarser, till they rather abruptly vanished. The displacementof the M mirror did not exceed mm., nor the rotation 30°. The spectrabeing non-reversed, the fringes covered the whole field. Nevertheless theselines must probably be regarded as arcs of circles or ellipses with enor-mously distant centers. In fact, the appearance of the whole of the ellipticsymmetry, in the preceding experiments (§ 21) with gratings, is also tobe associated with a slight difference of length of two overlapping is necessarily the case, since the two gratings G and Gf, figure 25, havenever quite the same constant. The third grating must therefore producetwo spectra, the one slightly incremented and the other decremented in length,respectively, as compared with the case for white One would naturally suppose that the abrupt evanescence of fringes wasdue to the escape of the b beam at the edge of the prism P; but this is notpossible, as the mirror M was traveling toward the rear. Furthermore, thefore-and-aft motion of the prism P over several millimeters had scarcely anyeffect on the fringes. This is unexpected; for the rays, c, c, are compelled toapproach or recede from each other by this motion. Finally, the sodiumdoublets may be moved at some distance (many times their breadth) apartwithout destroying the fringes. They are often most distinct when the Dlines are not superposed. The same is also true for the longitudinal axes,though to a less degree. These features are therefore peculiar. The rays c c were about 5 Unfortunately the faces of the prisms were optically inadequate, sothat the sodium lines were not sharp. For this reason no results were obtainedwith homogeneous light and a wide slit. 50 TH
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