REVERSED AND NON-REVERSED SPECTRA. 69 



must be ascribed to the assumed value of B and to the difficulty of placing 

 the plate normal to the rays. The method of reversion of fringes, used else- 

 where, is not sufficiently sensitive here. The data for x are again in excess 

 of y by about 13 per cent. 



Finally, a series of consecutive measurements were made of the ranges of 

 displacement, 2y, for different dispersive powers at G, figure 43, all under 

 (otherwise) like circumstances. The mean results were for 



60 prism dd/d\= 760. 2jy =0.060 cm. 



Ruled grating 2,880. .117 



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 spectra 

 were too small and the fringes could not be found. In each of these cases 

 the value of o.y for the plate = 0.434 cm., as shown in table 14, series III, 

 IV, V, are virtually the same. The rapid increase of range of displacement 

 with the dispersion of the system is thus again encountered. 



33. Rotation of fringes. A word must now be given relative to the rotation 

 of 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 consulting 

 figure 43 that if M moves micrometrically, normal to itself, the pencil 6 will 

 slide fore and aft, along the edge of the reflecting prism P'. Thus b may be 

 either in front of or behind b' or coplanar with it in a vertical plane. It will 

 not generally be collinear. This is an essential part of the explanation. 



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 the 

 direction /, normal to the line ab. Now suppose a is moved toward the right 

 or b toward the left, or both, parallel to the edge of the prism, as the arrows 

 in the figure suggest. Then the fringes will successively take the trends of 

 which cases i, 2, 3, 4 are typical examples. In other words, they will be 

 markedly accelerated and retarded in passing through the cases 2 and 3 re- 

 spectively. This is precisely what takes place and suggests why the case 

 between 2 and 3 may be used as a fiducial mark in interferometry. If a and 

 b also move vertically, in figure 49 there will be no essential difference, unless 

 the 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 the 

 mirror M, will be, as above, figure 17, if 5 = 6'0 = go9 



s = 2e sin 8/2 



