424 C. Bams — Interference of Reversed Spectra. 



lines are self -re versed, superposition of 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 D^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. 



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 phenomenon at the grating. Screen- 

 ing the transmitting grating G was without consequence ; but 

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

 about live millimeters 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, fig. 6, was now 

 very common, both with sunlight and with sodium light. For 

 a given slit the phenomenon began with a strong burr c, fig. 6, 

 completely obliterating and widening the superposed D„D/ 

 lines. When these lines were moved apart, the striations fol- 

 lowed them, as in fig. 6, A and i, to a limit depending on the 

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

 fig. 6, k, in which the interferences proper are strong and 

 marked between the D X D( doublets, but much fainter stria- 

 tions 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. Several days after, however, with 

 another adjustment, it in turn failed. Clearly there is some 

 variable element involved that escaped me, and it will hardly 

 be worth while to pursue the question further with the given 

 end in view without a radical change of method. 



A word may be added in relation to Fresnellian interferences 

 in the present work. These would be liable to occur if the 

 observations had been made outside of the principal focus with 

 the sodium lines blurred. In all the experiments on the excita- 

 tion of a narrow ether gap, however, the D lines were clearly 

 in sight and sharp, so that the phenomena of non-reversed 

 spectra and homogeneous light (discussed elsewhere) are not 

 here in question. True, such interferences may often be found 

 in the case of reversed spectra, when the sodium lines are pur- 

 posely blurred by pushing the ocular toward the front or to the 

 rear. 



6a. Experiments continued. — To turn to a second-class of 

 experiments ; important results were obtained with homo- 

 geneous light (sodium arc) on placing the D^DJ orZ> 2 Z>/ lines 

 in coincidence and then broadening the slit indefinitely, or 



