90 



THE INTERFEROMETRY OF 



very soon afterward, in which the difference in size of fringes has become 

 enormous. Following e, the procession is reversed in g, h. 



Both systems (a and /3 systems, say) have passed through maxima, but 

 not at the same time, or not for the same fore-and-aft adjustment. Both sys- 

 tems have rotated, the rotation being very rapid near the maximum. The 

 reticulations quiver and look 

 precisely like capillary waves 

 in a rectangular trough of mer- 

 cury, except that they are usu- 

 ally at an angle to the bound- 

 ing edges of the superposed 

 slit images. 



In this quivering system of 

 two identically strong fringes 

 it is difficult to make out the 

 rotations, but after consider- 

 able revision the sequence in 

 figure 67 was definitely ascer- 

 tained. Beginning with the 

 extreme fore-and-aft position of G', and moving it successively forward in steps 

 of i or 2 millimeters, the apparently single grid, i changes to 2, where the 

 two systems a and /3 can be disentangled, a expanding and rotating more 

 rapidly, so that 3 and 4 follow. Here a is horizontal and probably of maxi- 

 mum size, /3 is still nearly vertical and but slightly expanded. Therefore, while 

 the a-effect wanes the /3-effect waxes, and the squared or orthogonal type, 5, is 

 produced. The lines are here equally strong and it is the symmetrical figure 

 of the series. Thereafter in 5, 6, 7, 8, and 9 the chief expansion and rotation 

 is transferred to the /3 system, with which the a. system has changed functions. 

 Hence both systems rotate nearly 180 in the same direction and pass through 

 maximum size; but the maximum is retarded in rotational phase for one as 

 compared with the other. Rotation and growth are accelerated near the 

 maximum. The total displacement of the grating G' between the cases i and 

 9 (fig. 67) was about 2 cm. ; but this depends upon the obliquity of the grating 

 and incidental conditions, as explained above. 



Suppose, in the second place, that the original fringes, i, figure 67, were 

 nearly horizontal ; in such a case the evolution is much the same, but the sym- 

 metrical form number 5 becomes smaller and more and more flatly rhomboidal 

 horizontally. Probably the scheme of rotation is the same, but is much harder 

 to ascertain in view of the flat forms. On the other hand, the field now abounds 

 in vertical strands of interferences, like those of the preceding paragraph, and 

 nodules are often in evidence, as before. 



If the original lines are quite vertical, they do not seem to rotate with fore- 

 and-aft motion of G', but form intersecting, vertical, apparently simple sys- 

 tems throughout the motion. Slight departure from the vertical produces 

 rhomboids very long vertically and often very coarse. 



