REVERSED AND NON-REVERSED SPECTRA. 29 



were then seen across the coincident lines, now no longer visible, quite inde- 

 pendent of the absence of light. This would seem to mean that the otherwise 

 quiet ether within the black line is stimulated into vibration by the identical 

 harmonic motions of the bright fields at and beyond the edges of the line 

 (diffraction). The question will presently be broached again in a different 

 way. Here I may note that in the above cases of transverse lines ( 8) it is 

 often possible to observe a very fine parallel yellow line within the coincident 

 D 2 , D' 2 , or DI, D\, doublets, excited, therefore, in the dark space and splitting 

 the line. 



The experiments were now repeated with the sodium arc, and these also 

 gave some striking results. Thus in the case of figure 17 d the lines were 

 separated, but the yellow striations seemed to show across the dark space 

 between D 2 and D' 2 . When the yellow light was too weak, cross-hatchings 

 were seen only across D' 2 , as in figure 17 e. Frequently the phenomenon 

 figure 17 / occurred on broadening the slit, in which D 2 and D' 2 interfered, 

 but only D' 2 was marked. Screening off D 2 (left mirror) at once removed the 

 fringes. I have interpreted this observation as the result of parallax, due to 

 the fact that the lines and the interferences are seen in different focal planes. 



A v , x It It If 



*' - g. ~ 

 , 8 



On the basis of these results one might with some plausibility adduce the 

 following remarks in explanation of the phenomenon: In figure 18 a, let Si 

 and Sz be the overlapping reversed spectra and let the line of symmetry be 

 at Xi, X 2 . Then if identical ether vibrations can react on each other across a 

 narrow ether gap, rays as far as X'i, X' 2 and X"i, X" 2 being of identical source 

 and wave-length, respectively, are still in a condition to interfere. There 

 would then be three groups of interferences, Xi X 2 , X'i X' 2) X"i X" 2 . If, figure 

 1 8 b, all are in phase, we should have a brilliant line; if all are in opposite 

 phases, a dark line on the principle of figure 18 c. Naturally, if wave-trains 

 react on each other across an ether gap, small as compared with the DI, D<t 

 interval, the assumption made above relative to interference of different 

 wave-lengths is superfluous. My misgiving in the matter arises from the 

 misfortune of having taken down the original apparatus, for modification, 

 and having since been unable to reproduce them with anything like the 

 decisiveness with which they were at first apparently observed. I can not 

 now be certain whether what occurred was actually what I seemed to see, 

 or whether the broad illumination of the sodium flash (broad individual 

 lines, DI to D 2 , virtually a continuous spectrum) may not have misled me. 

 The experiments were continued, as follows. 



