20 



THE INTERFEROMETRY OF 



The fringes at once sharpened, however, on readjustment of either mirror, 

 indicating a continuous small change of deviation, due to curvature, probably, 

 in the quartz wedge. In the preceding periodic case, no readjustment of 

 deviation sufficed to restore the fringes. The wedge was now detached and 

 used alone. In spite of the relatively large angle (i), no difficulty was ex- 

 perienced in adjusting or controlling the fringes ; but the face curvature just 

 suggested appeared as before, so that readjustment for varying wedge-angle 

 was required from time to time. 



11. Micrometer displacement of the second grating, G' (fig. 3). In the 

 preceding report (Carnegie Inst. Wash. Publication 249, Chapter III, 28) it 

 was shown that if the angle between the gratings G and G' is <p and the angle 

 between the mirrors M and N (which in a symmetrical adjustment would be 

 180 (0i+ #2), #i and 62 being the angles of diffraction at G and G' for normal 

 incidence at G) is decreased by a, so that the adjustment is non-symmetrical, 

 then the displacement de of the grating G' per fringe will be very nearly 



X cos 2 6 2 



5e= 



2 (a y) sin 2 



if a and <p are small. Here a. is effectively the angle between the mirrors M 

 and N, since, if M is rotated 180 on the line of symmetry (normal to the 

 grating G), the two mirrors would intersect at an angle a. The result of fore- 

 and-aft motion thus depends on the angle a <f, and if a = <f>, 8e= o per 

 fringe; i.e., fore-and-aft motion would produce no result. This is necessarily 



TABLE 7. 



the case when but a single grating is used, as in the earlier methods. In the 

 case of two gratings, however, it is not only difficult to make a perfectly 

 symmetrical adjustment of mirrors and grating, but it would not be of any 



