. Carnegie Institution of Washington publication. ciably changing the direction of therays. In other words, one illuminated spot at d, figure 74, is to move verticallyrelative to the other by a small amount. This may be done by placing athick plate-glass compensator, such as is shown in figure 77, in each of thecomponent beams abd and acd and suitably rotating one plate relative to theother, each on a horizontal axis. Very little rotation is required. In the sameway elliptical fringes may be changed to nearly linear horizontal fringeswhen desirable. If the fringes are to be sharp the slit must
. Carnegie Institution of Washington publication. ciably changing the direction of therays. In other words, one illuminated spot at d, figure 74, is to move verticallyrelative to the other by a small amount. This may be done by placing athick plate-glass compensator, such as is shown in figure 77, in each of thecomponent beams abd and acd and suitably rotating one plate relative to theother, each on a horizontal axis. Very little rotation is required. In the sameway elliptical fringes may be changed to nearly linear horizontal fringeswhen desirable. If the fringes are to be sharp the slit must be very fine. Whensunlight is used with a slit not too fine, each of the coincident sodium lines(D\D^) frequently shows a sharply denned helical or rope-like structure, thedark parts in step with the fringes of the spectrum. It looks like an opticalillusion of slanting lines or a shadow interference of two grids (fringes andsodium lines respectively); but later experiments showed it to be an inde-pendent phenomenon. (Cf. § 63 et seq., 68, 70.).
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