Non-reversed Spectrum Interferometry . 419 



(i. e., one with weak objective) is advantageous, as the blade of 

 parallel rajs issuing is proportionately wide and tbe range of 

 displacement at M or JV, larger. Similarly divergence subse- 

 quently imparted by dispersion (prism, grating), before the 

 rays reach the mirrors, M, JV, should have the same effect. 

 The results obtained for dispersion bear this out, bat 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 dispersion has a deeper significance, bear- 

 ing directly on the structure of the interfering wave trains : 

 i. e., the length of the coordinated, uniform wave train is 

 greater, as the dispersion to which the wave train has been sub- 

 jected is greater. Two parts of it will therefore fit over a cor- 

 respondingly longer range of path difference. 



A number of otlier results point in the same direction. Thus 

 I may instance the impossibility of obtaining fringes with 

 homogeneous light and a wide slit, whereas two identical 

 sodium lines (D 1 and DJ), superposed, show the interferences 

 strongly. The lines actually become helical in shape and 

 much broader. The range of displacement of JV may be 

 decreased from *25 cm to •10 cm , by narrowing the beam emerg- 

 ing 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 appar- 

 ently they may be seen to slowly move laterally across the fidu- 

 cial 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. 



10. Prismatic Refraction, with grating. — The method 

 indicated in fig. 14 was next tested for small distances and the 

 experiments begun in the third order of spectra of the grating 

 G. The refracting prism, P, was a small right-angled sample, 

 with faces only about l 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°, 8 = 34° and a- = 90°. 



Hence on displacing the micrometer mirrors, M or JV, the 

 illuminating strips move relatively rapidly across the face of 

 the grating, (Jr. .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 millimeters, passing from hairlike 



