84 



THE INTERFEROMETRY OF 



again correspond and appear as a single striated field; but more frequently 

 the form figure 6oa is in evidence, though sometimes quite irregular. In fact, 

 there are many variations of this design. Families of curves, intersecting each 

 other nearly orthogonally, may even appear. 



If the fringes are originally quite vertical, there seems to be no rotation, 

 but two sets of vertical fringes apparently pass through each other as the 

 mirror M is rotated micrometrically on a vertical axis. These fringes at inter- 

 vals again unite into an apparently simple striation. One slit image may be 

 broader than the other. Fringes of different sizes then appear, so long as the 

 smaller is within the larger, and are most intense when the vertical edges meet. 

 In general, therefore, the interference patterns of originally nearly vertical 

 fringes consist of a succession of strands, nearly in parallel, which behave alike 

 but independently. 



60 



61 



a. 



If the grating is rotated on an axis normal to its face until the fringes are 

 nearly horizontal, a correlative series of interesting phenomena may be 

 observed. When the grating is normal to the incident pencil, the fringes are 

 usually arranged in parallel strands. They are equidistant in each strand; but 

 these strands are separated by a narrow band of even color, so that the phe- 

 nomenon looks as if thick, twisted, yellow cords were hanging apart, side by 

 side. Usually the central or the two central cords are more intense, and there 

 may be four to six in all, filling the whole of the wide-slit image. On rotating 

 the mirror, M or N, micrometrically, on a vertical axis, the fringes of the 

 strand may be made to correspond, so as to fill the field with uniform stria- 

 tions and without apparent vertical separation. This is particularly the case 

 when the fringes are very fine. 



On rotating the grating to the right or to the left about 20, on the vertical 

 axis from the symmetrical position of figure 58, the fringes reach a maximum 

 of size, after which (on further rotation to about 30) they diminish indefi- 

 nitely. These maximum cases are shown in figure 61, a and b, and their ap- 

 pearance is now that of a string of elongated beads, hung vertically and equi- 

 distant. On rotating N about a vertical axis, slightly, the nodules become 

 quite horizontal. They are continually in motion, up and down, and quiver 

 about the horizontal position like small disturbed magnetic needles. At times 

 the field appears reticulated (indicated in the figure) , as if two sets of nearly 

 horizontal fringes intersected at a small angle. It is now difficult to obtain 



