C. Barns — Interference of Reversed Spectra. 419 



this seems to be its character ; i. e. the enhanced doublet cuts 

 vertically across the breadth of the spectrum. It is not to be 

 overlooked, however, that in certain adjustments, particularly 

 in the non-symmetrical case of figure 3, more than two black 

 lines frequently occur. Again, the presence of many oblique 

 lines will be shown in § 4 and 8. These would be quite 

 unaccounted for. 



Finally, many attempts were made to find whether the phe- 

 nomenon would occur again beyond its normal range of about 

 2 X '5 cm of displacement. But though the micrometer screw 

 actuating the mirror M was effectively 2 X 3 cm long, no recur- 

 rence could be found. At the ends of its range the phenom- 

 enon drops off rather abruptly. 



Hence none of the inferences put forward adequately 

 account for the phenomenon (even as seen with a single grat- 

 ing) as a whole. One feature, it is true, has been left without 

 comment and that is the width of the slit image since within 

 the limits of the present method of experiment, with but one 

 grating, this circumstance seems to offer no clue. If, for 

 instance, the spectra actually coincide in color throughout their 

 extent, as in ordinary interferences, the interference patterns 

 should be enormous, for the path difference may be zero. The 

 invariability of the present phenomena as to size within its 

 long range of presence, the occurrence of intensely sharp and 

 bright or dark single lines, within a breadth of spectrum much 

 less than the distance apart of the D y D^ lines, is in no way 

 suggested by the width of slit image. Moreover in spite of its 

 persistence, the interference phenomenon of reversed spectra 

 has the sensitiveness of all interferences. Slight tapping on 

 the massive table throws it out altogether. Clearly therefore, 

 a modification of method is essential if new light is to be 

 thrown on the phenomenon, and from this viewpoint, a separa- 

 tion of the two diffractions seems most promising. 



4. Apparatus with two gratings. — All the varied experi- 

 ments described in the preceding paragraph failed to show any 

 essential modification of the linear interference pattern obtained. 

 It therefore seemed promising to modify this limitation of the 

 experiments, although the difficulty of finding the phenomenon 

 would obviouslj 7 be greatly increased. 



In the present method the glass grating G, fig. 5, receives 

 the white beam L from the collimator. This light is then dif- 

 fracted to the opaque mirrors M (on a micrometer slide) and 

 JV, thence reflected to the reflecting grating G\ plane or 

 curved. Here the two beams of the identically colored light 

 selected are again diffracted to the telescope or lens at T. 

 Since the gratings G, G' rarely have the same grating con- 

 stant, their proper position must be found by computation and 



