C. Barus — Interference of Reversed Spectra. 421 



nearly in coincidence to an eye placed at the telescope T may 

 be found. The grating G' is next to be slowly rotated on 

 a line (parallel to LT) normal to its surface, to the effect that 

 the black axes of both spectra (i. e., the spectra, as a whole) 

 may coincide. This must be done accurately and the last 

 adjustments may be made at the screws controlling M and JV. 

 Finally the micrometer slide carrying M is to be moved fore 

 and aft, until the interferences appear. These operations 

 are difficult even to an experienced observer. The new fringes 

 are very susceptible to tremor and only under quiet surroundings 

 do they appear sharply. At other times they move, as a whole, 

 up and down and intermittently vanish. 



The fringes so obtained, figure 6, were totally different from 

 the preceding and consisted of short black equidistant nearly 

 horizontal lines across the active yellow strip of spectrum at 

 the axis of coincidence. The strip was about of the same 

 width as above. Thus the pattern presented the appearance 

 of a barber's pole in black and yellow, the width being less 

 than D X D^ and the distance apart of fringes usually smaller. 

 Since the fringes were as a rule nearly horizontal, it was 

 possible to enlarge the width of the slit without destroying 

 them as in case of the hair-like vertical fringes in paragraph 2 

 above. In this way a breadth of strip greater than the 

 distance DJ}„ could be obtained with sunlight or arc light, 

 though a moderately fine slit was still desirable. 



In general the characteristics noted above were again ob- 

 served. Thus on moving the micrometer screw controlling M, 

 the interferences appeared rather abruptly and vanished in a 

 similar manner after about - 4 CIT or more of the micrometer 

 screw (15,000 wave lengths) had been passed over. 



If we called the four D lines available in the two solar 

 spectra D^D^D/D/, respectively, a number of curious results 

 were obtained on placing them variously in approximate 

 coincidence. Thus figure 6 a, when each D line of one 

 spectrum coincides with the mate of the other {D X D^' ; D/D z ), 

 equidistant dots, surrounded apparently by yellow luminous 

 circles, appeared between the two doublets. On widening the 

 slit the dots changed to a grid of nearly horizontal lines 

 covering the strip D^D^ figure 6 b. Often the lines in part 

 of the slit seemed to slope upward and in another to slope 

 downward. 



The phenomenon of chief interest, however, was observed 

 (fig. 6 c) in placing two identical D lines of the solar spectrum 

 in coincidence Z>,; D„D 7 ' ; D\). The fringes were then seen 

 across the coincident lines, now no longer visible as such, 

 quite independent of the absence of light. This would seem 

 to mean that the otherwise quiet ether within the black line 



