422 C. JBarus — Interference of Reversed Spectra. 



is stimulated into vibration by the identical harmonic motions 

 of the bright fields at and beyond the edges of the line. The 

 question will presently be broached again in a different way. 

 Here I may note that in the above cases of transverse lines 

 (§ 2), it is often possible to observe a very fine parallel yellow 

 line within the coincident D n D^ or D,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 6 d the lines were separated, but the yellow striations 

 seemed to play across the dark space between D^D^. When 

 the yellow light was too weak, cross hatchings were only seen 

 across D\ as in the figure 6, e. Frequently the phenomenon, 

 figure 6 f occurred on broadening the slit, in which D^ and 

 .2)/ interfered, but only J) J was marked. Screening off D^ 

 (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 

 plans. 



On the basis of these results one might argue with some 

 plausibility that identical wave trains react on each other across 

 an ether gap, small as compared with the D X D^ interval. In 

 such a case, moreover, the assumption made in the earlier 

 paper relative to interference of different wave lengths, is 

 superfluous. My misgiving in the matter is the misfortune 

 that having taken down the original apparatus, for modification, 

 I have since been unable to reproduce them with anything 

 like the decisiveness with which they were at first apparently 

 observed. I cannot now be certain whether what occurred was 

 actually what I seemed to see, or whether the broad illumi- 

 nation of the sodium flash (broadened individual lines, D 1 to 

 D„ virtually a continous spectrum) may not have misled me ; 

 or whether different focal planes were in question. 



6. Experiments continued: JVew Interferometer. — At the 

 outset it was necessary to ascertain the reason for the difference 

 of the phenomena, as obtained with one grating in paragraph 

 3, and with two gratings in paragraph 5. Since the probable 

 cause is a lack of parallelism of the rulings in the latter case, it 

 was necessary to remount the second grating G' in the manner 

 shown in fig. 7. Here AA is a base board, capable of sliding 

 right or left and of rotating on a horizontal axis parallel to the 

 grating. The latter (in a suitable frame) is held at the bottom 

 by the axle, e, normal to the grating and by the two set screws 

 a and o carried by the standards c and d. Thus the grating 

 could be rotated around an axis normal to its plane. At first 

 a Michelson plane reflecting grating G' and a telescope were 

 used as in fig. 5, but it was found preferable (fig. 8) to use a 



