. Carnegie Institution of Washington publication. s edge. REVERSED AND NON-REVERSED SPECTRA. 27 The fringes are strong and large and lie within a remarkably wide trans-verse strip. This may be 10 or 20 times as wide as the D\ Dz doublets, which,in view of the small dispersion, are hardly separated. For the same reason,moreover, the range of displacement of M within which fringes are visiblerarely reaches mm. Within this the fringes grow from the fine hair-lines,usually oblique, to their maximum coarseness. Apart from the small rangeof displacement, these fringes are available for measureme
. Carnegie Institution of Washington publication. s edge. REVERSED AND NON-REVERSED SPECTRA. 27 The fringes are strong and large and lie within a remarkably wide trans-verse strip. This may be 10 or 20 times as wide as the D\ Dz doublets, which,in view of the small dispersion, are hardly separated. For the same reason,moreover, the range of displacement of M within which fringes are visiblerarely reaches mm. Within this the fringes grow from the fine hair-lines,usually oblique, to their maximum coarseness. Apart from the small rangeof displacement, these fringes are available for measurement. If both mirrorsM and N are on micrometers, they may be brought forward or the reverse,alternately, and the range increased 5 or 10 times. To change the form of the fringes, the first prism, P, may be tilted slightlyon an axis parallel to LT, figure 14. The fringes then pass through a maximumin the vertical direction (linear phenomenon). Fore-and-aft motion of Protates the fringes, partially, toward the horizontal; but, as a rule, the com- 14. 16 15 ponent beams b and b pass beyond the edge of P and the fringes before this (the spectra separating), the strip within which the fringeslie widens enormously. In other words, the breadth of the phenomenondepends on diffraction, not on dispersion, so that even though the prism Pscarcely separates the V lines, the striated strip has about the same width aswhen it is produced by highly resolving gratings. It is preferable to use sunlight directly (without a long-focus condensinglens), as there is a superabundance of light. The best results are attainedwith a large collimator. A spectacle lens with a focal distance of i meter isexcellent. The range of displacement of M is not increased, but the spectraand fringes become very sharp. If, with the large collimator, the spectraare just separated in the field of the telescope, by fore-and-aft motion of P,a magnificent display appears, resembling a thick, twisted golden cord. Wi
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