122 



THE INTERFEROMETRY OF 



up to the grating. Hence interference is produced in the telescope itself, where 

 rays are relatively very divergent, a condition which accounts for the smallness 

 of the interference pattern. This understanding of the case is tentatively 

 shown in figure 89, where is the objective of the telescope, M the larger image 

 from the mirror with the lens compensator, and N the image from the other 

 mirror (parallel rays) . If the corresponding rays be drawn through the extrem- 

 ity of M and N, their fields of interference, F and F', would begin in the plane 

 I and /'. For axial rays it would be at i. Thus the locus as a whole would not 

 be a plane, and this seems to be the case. If the telescope moves toward the 

 grating, II' moves toward the right in the figure, as though the virtual object 

 beyond the grating were fixed in position. At all events, the problem is to find 

 the interference diagram of two symmetrical plane parallel spectra, of different 

 areas and placed at definite distances apart. 







The appearance of the fringes is indicated in figure 90, where S is the height 

 of the spectrum, usually quite out of focus. There are many more lines than 

 could be drawn in the sketch. The ends a and a' seem to surround small ellipses, 

 but these are not quite closed on the outer edge. The center of symmetry 

 is at C. The demarcations are stronger and broader vertically if the distance 

 apart of the lenses C (fig. 87) is small; fainter, but nevertheless clear and nar- 

 rower, if this distance is large. Horizontally the fine lines thread the spectrum. 

 The best results were obtained when the lenses C are less than i cm. apart, the 

 middle band being about half as high as the spectrum. Two contiguous lenses 

 gave a design which nearly filled the spectrum vertically. For practical pur- 

 poses the lens compensator C is to be attached to the mirror M, just in front 

 of and moving with it. It makes little difference here whether the concave lens 

 or the convex lens of the doublet C is foremost. 



If the micrometer M is moved, or if the telescope is slid to the right or left, 

 or forward, so as to take in other parts of the spectrum, the nearly closed lines 

 at a and a' become finer and finer crescent-shaped lines, 

 always open outward, till they pass beyond the range of 

 vision. The whole phenomenon remains on the same level 

 of the spectrum. On moving the telescope forward as far 

 as G (fig. 87), the ocular has to be drawn outward (towards 

 the eye) till it is fully 2 cm. beyond the position of the 

 principal focal plane. The whole spectrum is now seen 

 with the interferences from red to violet (no ellipses), but 

 having the same relative position as before. The central 

 horizontal band measures about one-fifth the height of the spectrum, while 

 the fine parallel horizontal lines extend to the upper and lower edges. The 



