114 



THE INTERFEROMETRY OF 



Similarly, the silver deposit on plate-glass was removed in parallel strips, so 

 that the film had the appearance of a grid. The results when this plate was 

 placed normally in one of the component beams were the same. 



61. Vertical displacement of ellipses. If the fringes are too small when 

 horizontally centered by the micrometer, the center of ellipses may be brought 

 into the middle of the field of the telescope by sliding one component beam 

 vertically over the other without appreciably changing the direction of the 

 rays. In other words, one illuminated spot at d , figure 74, is to move vertically 

 relative to the other by a small amount. This may be done by placing a 

 thick plate-glass compensator, such as is shown in figure 77, in each of the 

 component beams abd and acd and suitably rotating one plate relative to the 

 other, each on a horizontal axis. Very little rotation is required. In the same 

 way elliptical fringes may be changed to nearly linear horizontal fringes 

 when desirable. If the fringes are to be sharp the slit must be very fine. When 

 sunlight 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, the 

 dark parts in step with the fringes of the spectrum. It looks like an optical 

 illusion of slanting lines or a shadow interference of two grids (fringes and 

 sodium lines respectively); but later experiments showed it to be an inde- 

 pendent phenomenon. (Cf. 63 et seq., 68, 70.) 



-~~ J "" 77 78 



The first result is particularly interesting, inasmuch as it is thus possible 

 to displace the centers of ellipses not only horizontally as usual relative to 

 the fixed sodium lines in the spectrum, but also vertically relatively to the 

 fixed horizontal shadow in the spectrum due to the fine wire across the slit. 

 The following experiment was made to coordinate the vertical displacement 

 of the component rays and centers of elliptic fringes: A glass plate d = 0.705 

 cm. thick was placed nearly normally in the beam ac, figure 74, and provided 

 with a horizontal axis and graduated arc. The amount (i) of rotation of the 

 plate, corresponding to the vertical displacement of one central fringe in the 

 telescope (i.e., passage of fringe a into b, into c, in the duplicate spectrum 5, 

 fig. 78), was then found to be, if i is the angle of incidence, 



i h 



No fringes 3.5 0.0149 cm - 



One fringe 5.0 .0214 



Two fringes 6.5 .0281 



where h is the corresponding vertical displacement of the rays ac, figure 74, 

 and computed from (n index, r angle of refraction) 



h = d(sin i cos i tan r} 



