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\ D z 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 visible 

 rarely reaches 0.5 mm. Within this the fringes grow from the fine hair-lines, 

 usually oblique, to their maximum coarseness. Apart from the small range 

 of displacement, these fringes are available for measurement. If both mirrors 

 M 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 slightly 

 on an axis parallel to LT, figure 14. The fringes then pass through a maximum 

 in the vertical direction (linear phenomenon). Fore-and-aft motion of P 

 rotates 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 vanish. 

 Just before this (the spectra separating), the strip within which the fringes 

 lie widens enormously. In other words, the breadth of the phenomenon 

 depends on diffraction, not on dispersion, so that even though the prism P 

 scarcely separates the V lines, the striated strip has about the same width as 

 when it is produced by highly resolving gratings. 



It is preferable to use sunlight directly (without a long-focus condensing 

 lens), as there is a superabundance of light. The best results are attained 

 with a large collimator. A spectacle lens with a focal distance of i meter is 

 excellent. The range of displacement of M is not increased, but the spectra 

 and fringes become very sharp. If, with the large collimator, the spectra 

 are 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. With 

 further separation confocal elliptic fringes often cross the gap, as in figure 15. 

 Here a and /3 are graphs suggesting the wave-lengths of the two spectra, g 

 being the gap or deficient overlapping. The appearance in the telescope is 

 shown at 7, 5 and S' being the spectra. When the fringes are erect, huge 

 vertical furrows may lie in the gap. When the gap is closed, the linear phe- 

 nomenon reappears. These enlarged fringes vanish, however, within 0.25 

 mm. of displacement at M . On the other hand, when the spectra are made 



