86 THE INTERFEROMETRY OF 



result that not even the longitudinal axes of the spectra need be quite in coin- 

 cidence, though, of course, the phenomenon appears most intensely for the 

 case of precise superposition. The angle of admissible separation of longitu- 

 dinal axes is, however, much larger here than in the usual cases above, so that 

 one of the longitudinal guide-lines of the two spectra may be appreciably 

 above the other. 



The last result and the fact that the linear phenomenon appears here with 

 an indefinitely wide slit are new features. The cause of the latter has just 

 been referred to the exceptionally reduced width of spectrum resulting from 

 the double diffraction. If the dispersion were quite reduced to zero, all colors 

 in a definite narrow, transverse strip of the white slit image would be in a condi- 

 tion to interfere. This strip contains the superposed images of an indefinitely 

 fine slit. The slit in any other position, right or left, would have two non- 

 coincident images. Hence, when one wide-slit image moves over the other, 

 there is also a shift of the linear phenomena. 



To produce the linear phenomenon with sunlight is difficult. The inter- 

 ferences should first be produced with the sodium arc, strongly, and the arc 

 thereafter replaced with sunlight entering the slit at the same angle. Further- 

 more, the pencil leaving the collimator should be a narrow, vertical blade of 

 light, and at the mirrors, M and N, red and green light should be screened 

 off, retaining only a narrow strip of yellow light for each. Finally, to avoid 

 glare, the slit is not to be too broad nor too narrow to cut off the yellow field 

 of the telescope. 



Under these circumstances of completed adjustment, the linear phenomenon 

 usually appears strongly. Its form may be greatly modified by rotation of 

 either mirror, M or N, micrometrically, around the vertical axis, as already 

 suggested. The types are given in figure 62, quite fine, nearly vertical lines, 

 q, changing to moving, coarser forms, m, and these into the tumbling variety, 

 t, very coarse and nearly horizontal. The latter change by rotation and dimi- 

 nution into m' and q', while N is being continually rotated over a very small 

 angle, sliding one slit image continuously over the other. In the condition t, 

 the fringes rotate with astonishing rapidity, and this rotation is nearly 180; 

 i.e., if the angle between m and m' is a, the angle of rotation has been 180 a, 

 so that between q and q' there is about 180 of rotation. At the stage t, with 

 fine micrometric adjustment, the fringes may be made quite horizontal, and 

 they are then relatively large and square, or at times shaped like blunt arrow- 

 heads. This rapid rotation of fringes near / accounts for their turbulence, 

 since tremors have the effect not merely of raising and lowering them, but 

 also of producing the rotary motion in question. They may also be rotated, 

 of course, on slightly tilting the grating about an axis normal to its face. 

 Rotating the latter on an axis parallel to its face places the phenomenon in 

 different parts of the superposed yellow field. 



Since a preponderance of yellow homogeneous light is present in the whole 

 of the superposed wide-slit images in the telescope, it is not difficult to suggest 

 the cause for the variations of the interference pattern when one image passes 



