. Carnegie Institution of Washington publication. 22. brilliantly illuminated. When the path-difference is sufficiently small, thefringes appear and cover the whole length of superposed spectra are displaced with rotation if M is moved normally to itself. As first obtained, the fringes were too closely packed for accurate measure-ment. But the following example of the displacement e of the mirror M,for successions of 40 fringes replacing each other at the sodium lines, showsthe order of value of results: 10^=, , , cm., so that per fringe o~6 cm. cm. The computed
. Carnegie Institution of Washington publication. 22. brilliantly illuminated. When the path-difference is sufficiently small, thefringes appear and cover the whole length of superposed spectra are displaced with rotation if M is moved normally to itself. As first obtained, the fringes were too closely packed for accurate measure-ment. But the following example of the displacement e of the mirror M,for successions of 40 fringes replacing each other at the sodium lines, showsthe order of value of results: 10^=, , , cm., so that per fringe o~6 cm. cm. The computed value would be (<p, the prism angle) X 58-93 __, oe = 2 COS 5/2 assuming 6 = 90° — <p. The difference is due both to the small fringes, whichare difficult to count, and to the rough value of <5. The range of measurementis small (if M only moves), not exceeding mm. for a moderately strong 36 THE INTERFEROMETRY OF telescope. But one-half of this displacement is available, as the fringesincrease in size (usually with rotation) fro
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