REVERSED AND NON-REVERSED SPECTRA. 25 



ment at M or N larger. Similarly, divergence subsequently imparted by 

 dispersion (prism, grating), before the rays reach the mirrors, M, N, should 

 have the same effect. The results obtained for dispersion bear this out, but 

 not those for a long collimator. Moreover, the width of the slit, so long as 

 the Fraunhofer lines do not vanish, is of no consequence. It thus seems 

 tenable (to be carefully investigated below) that the positive effect of dis- 

 persion has a deeper significance, bearing directly on the structure of the 

 interfering wave- trains i.e., the length of the coordinated, uniform wave- 

 train is possibly greater as the dispersion to which the wave-train has been 

 subjected is greater. Two parts of it will therefore fit over a correspondingly 

 longer range of path-difference. 



A number of other results point in the same direction. Thus, I may again 

 point to the impossibility of obtaining fringes with homogeneous light and a 

 wide slit, whereas two identical sodium lines (D\ and D'i), superposed, show 

 the interferences strongly. The lines actually become helical in shape and 

 much broader. The range of displacement of N may be decreased from 0.25 

 cm. to o.io cm. by narrowing the beam emerging from the collimator with a 

 slotted screen, while the fringes themselves are coarsened by this process. 

 With the screen removed the fringes are not only sharper and finer, but 

 apparently they may be seen to slowly move laterally across the fiducial 

 sodium lines. This is in accord with the increased range of displacement of 

 the mirror. The observation, however, is complicated by the fact that the 

 sodium doublets are not quite in the same focal plane. The fringes must, 

 in a reduced case, lie midway between them, in the line of symmetry of the 

 spectra. 



13. Prismatic refraction. The method indicated in figure 12 (P, prism; 

 M,N, mirrors; G, grating; T, telescope) was next tested for small distances and 

 the experiments begun in the third order of spectra of the grating G. The 

 prism was a small right-angled sample, with faces only about i cm. square; 

 but it sufficed very well. Its distance from the grating being about 13 cm. 

 and the illuminated spots on the mirrors 18.8 cm. apart, the mirrors were 

 nearly normal to each other. In fact, as 6 in the third order is about 62 

 and i' about 28, 5 = 34 and a = 90. Hence, on displacing the micrometer 

 mirrors M or N, the illuminated strips move relatively rapidly across the face 

 of the grating. Nevertheless, the fringes are easily found and controlled. 

 Their range of visibility is larger than in the cases of the preceding paragraph. 

 They remain in view for normal displacement of M of 3 to 4 mm., passing 

 from hair-like striations, through sharp arrows, back to the hair-like forms. 

 The range has thus been increased by the dispersion. The arrows are of 

 the type shown in figure 13, with reentrant sides and part of the outline 

 accentuated. 



In the second order of spectra from G, the phenomena were much the same, 

 but far more brilliant. The arrows were now evenly wedge-shaped and very 

 slender. The fringes entered as nearly vertical hair-like striations, and, after 



cx 





L 



