. Carnegie Institution of Washington publication. 28 THE INTERFEROMETRY OF a slide carrying M is to be moved fore and aft until the interferences appear. These operations are difficult even to an experienced observer. The fringes are very susceptible to tremors, and only under quiet surroundings do they appear sharply. At other times they move, as a whole, up and down and intermittently vanish. The fringes so obtained, figure 17, were totally different from the preceding and consisted of short, black, equidistant, nearly horizontal lines across the active yellow strip of spectrum, at the axis
. Carnegie Institution of Washington publication. 28 THE INTERFEROMETRY OF a slide carrying M is to be moved fore and aft until the interferences appear. These operations are difficult even to an experienced observer. The fringes are very susceptible to tremors, and only under quiet surroundings do they appear sharply. At other times they move, as a whole, up and down and intermittently vanish. The fringes so obtained, figure 17, were totally different from the preceding and consisted of short, black, equidistant, nearly horizontal lines across the active yellow strip of spectrum, at the axis of coincidence. The strip was about of the same width as above. Thus the pattern presented the general appearance of a barber's pole in black and yellow, the width being less than the sodium interval, D\, D2, and the distance apart of fringes usually smaller. They were visually in motion up and down, rarely quiet, no doubt owing to tremor. Since the fringes were nearly horizonta1 or less than 30 degrees in inclination, it was possible to enlarge the width of the slit without destroying them, as in case of the hair-like vertical fringes in paragraph 2 above. In this way a breadth of strip greater than the distance Di, D2, could be obtained with sunlight or arc light, though a moderately fine slit was still desirable. 34 b d 17 e f. In general, the characteristics noted above were again observed. Thus on moving the micrometer screw controlling M, the interferences appeared rather abruptly. They vanished in a similar manner, after about cm. or more of the micrometer screw had been passed over. In other words, the fringes remain identical for a path-difference of about cm., or nearly 15,000 wave-lengths. If we call the four D lines available in the two solar spectra DI, D2, D\, D'2, respectively, a number of curious results were obtained on placing them variously in approximate coincidence. Thus figure 17 a, when each D line of one spectrum coincides with the mate of the
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