. Carnegie Institution of Washington publication. 17 Film grating 6,400. .208 Three or more measurements, completed in each case, were in good agree-ment. The attempt was also made to use a 45° prism here, but the spectrawere too small and the fringes could not be found. In each of these casesthe value of for the plate £ = cm., as shown in table 14, series III,IV, V, are virtually the same. The rapid increase of range of displacementwith the dispersion of the system is thus again encountered. 33. Rotation of fringes.—A word must now be given relative to the rotationof fringes, which
. Carnegie Institution of Washington publication. 17 Film grating 6,400. .208 Three or more measurements, completed in each case, were in good agree-ment. The attempt was also made to use a 45° prism here, but the spectrawere too small and the fringes could not be found. In each of these casesthe value of for the plate £ = cm., as shown in table 14, series III,IV, V, are virtually the same. The rapid increase of range of displacementwith the dispersion of the system is thus again encountered. 33. Rotation of fringes.—A word must now be given relative to the rotationof fringes, which is here throughout 180°, whereas in the similar case above(Chapter I, §§ 25, 26) the rotation was but 90°. It will be seen on consultingfigure 43 that if M moves micrometrically, normal to itself, the pencil 6 willslide fore and aft, along the edge of the reflecting prism P. Thus b may beeither in front of or behind b or coplanar with it in a vertical plane. It willnot generally be collinear. This is an essential part of the In figure 49, let a and b be the two patches of light of like color and origin,which produce interferences. The fringes will therefore be arranged in thedirection /, normal to the line ab. Now suppose a is moved toward the rightor b toward the left, or both, parallel to the edge of the prism, as the arrowsin the figure suggest. Then the fringes will successively take the trends ofwhich cases i, 2, 3, 4 are typical examples. In other words, they will bemarkedly accelerated and retarded in passing through the cases 2 and 3 re-spectively. This is precisely what takes place and suggests why the casebetween 2 and 3 may be used as a fiducial mark in interferometry. If a andb also move vertically, in figure 49 there will be no essential difference, unlessthe latter motion is large. In such a case the rotation may become i, 2, 2, i. The displacement of b parallel to itself, for a normal displacement e of themirror M, will be, as above, figure
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