26 THE INTERFEROMETRY OF 



the non-symmetrical case of figure 13, more than two black lines frequently 

 occur. (Cf . 15.) These accessor}' lines are ordinarily very thin and crowded 

 on one side of the phenomenon only. It is thus merely the prevalent occur- 

 rence of paired dark and bright lines that are here brought to mind. Again, 

 the suggestion of many oblique lines has occurred in some of the observations. 

 These would be quite unaccounted for. 



Finally, many attempts were made to find whether the phenomenon would 

 occur again beyond its normal range of about 2X0.5 cm. of displacement. 

 But, though the micrometer screw actuating the mirror M was effectively 

 2X3 cm. long, no recurrence could be found. At the ends of its range the 

 phenomenon drops off rather abruptly. 



None of the inferences put forward adequately account for the phenome- 

 non as seen with a single grating, as a whole. In this dilemma I even went 

 so far as to suppose that a new property of light might be in evidence. One 

 feature, it is true, has been left without comment, and that is the width of 

 the slit-image. If ab, figure 15 b, is the angular width (d&) of this image, 

 the case of figure 150 should be additionally treated in terms of figure 156. 

 But 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 ordi- 

 nary interferences, the interference patterns ~ 8 

 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, with a distance (dff) much less than the distance apart of the DI, Z) 2 

 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 

 separation of the two diffractions seems most promising. 



10. Apparatus with two gratings. All the varied experiments described in 

 the preceding paragraph failed to show any essential modification of the linear 

 interference pattern obtained. In a measure this was to be anticipated, inas- 

 much as both diffractions take place at the same grating. It therefore seemed 

 promising to modify this limitation of the experiments, although the difficulty 

 of finding the phenomena would obviously be greatly increased. The separa- 

 tion of the two diffractions, however, seemed to be alone capable of resolving 

 the phenomenon into intelligible parts. 



In the present method the glass grating G, figure 16, receives the white 

 beam L from the collimator, which is then diffracted to the opaque mirror M 



