REVERSED AND NON-REVERSED SPECTRA. 31 



reaction of parallel ether wave-trains on each other across a very narrow 

 ether gap. The sodium arc lamp was used as a source of light. The results 

 as a whole were negative, or at least conflicting. Usually when strong inter- 

 ferences were observed for coincident positions of DZ, D' 2 , for instance, there 

 was no passage of fringes across the dark space when DZ and D'% were slightly 

 separated. At the beginning of the work (possibly as the result of lines 

 broadened by a flash of sodium light) the stretch of interference fringes across 

 the dark space was certain; but such evidence is not quite trustworthy, for 

 a continuous spectrum (i.e., lines broadened by the flash) would necessarily 

 produce the striations. With a very fine slit the coincident DI, D\ or D 2 , D' 2 

 was frequently much broadened by a sort of burr of fringed interferences. 

 When the lines are self -re versed, superposition of DI, D'\, etc., frequently 

 showed vivid interferences across the intensely black middle line. This and 

 the passage of the bright and dark lines across the superposed DI, D\ lines 

 of the solar spectrum are thus the only evidence of the reaction of separated 

 light-rays on each other across an ether gap observed in the new experiments, 

 and the above results could not be repeated. 



On introducing a refined mechanism to establish the sharpest possible 

 coincidence of the DI, D\ or D 2 , D' 2 lines, it seemed as if these lines could 

 at times be brought to overlap with precision, without the simultaneous 

 appearance of the interferences around then ; but on drawing out the ocular 

 of the telescope or the lens the cross-hatching invariably appears. If the 

 coincidence is not quite sharp, the phenomenon is usually very strong in the 

 isolated bright strip. Horizontal fringes are best for the test. 



An additional series of experiments was made some time later by screening 

 off parts of the concave grating G', in order to locate the seat of the phenom- 

 enon at the grating. Screening the transmitting grating G was without con- 

 sequence; but on reducing the area G' to all but the middle vertical strip 

 about 5 mm. wide, a very marked intensification of the phenomenon followed. 

 Although the spectrum as a whole was darker, the interferences stood out 

 from it, relatively much sharper, stronger, and broader than before. The 

 Fraunhofer lines were still quite clear. Thus the pattern, g, figure 17, was 

 now very common, both with sunlight and with sodium light. For a given 

 slit the phenomenon began with a strong burr c, figure 17, completely oblit- 

 erating and widening the superposed D 2 , D' z lines. When these lines were 

 moved apart, the striations followed them, as in figure 17, h and i, to a limit 

 depending on the width of the slit. A still more interesting pattern is shown 

 in figure 17 k, in which the interferences proper are strong and marked between 

 the two DI D'i doublets, but much fainter striations are also evident, reaching 

 obliquely across and obviously with the same period. 



With this improvement I again tested the ether-gap phenomenon, using 

 the sodium arc, and to my surprise again succeeded. DI D\ lines of half 

 the breadth of the doublets apart induced strong fringes between them, and 

 the experiments were continued with the same results for a long time. Several 

 days after, however, with another adjustment, it in turn failed. Clearly 



