REVERSED AND NON-REVERSED SPECTRA. 33 



13. Experiments continued. Contrast of methods. As these fringes were 

 produced with a concave reflecting grating, the question may be put whether 

 they would also appear in case of the plane reflecting grating, G', in the adjust- 

 ment of figure 1 6. The experiment was therefore repeated with a wide slit, 

 or with no slit at all, and there was no essential difference in the two classes 

 of results. 



On the contrary, when the method of but one grating and sodium light 

 was used (fig. n), the interferometer fringes, in case of a very wide slit or 

 the absence of a slit, could not be produced over the yellow field, as a whole. 

 There appeared, however, an obviously pulsating flicker in parts of the field, 

 on reducing the width of the slit till the sodium lines were each about the 

 width of a DiD z space, with either D\D'\ or D 2 D' 2 superposed. The sharply 

 outlined slit showed an irregular, rhythmic brightening and darkening over 

 certain parts of its length. These broad pulsations were very violent, very 

 much in character with the linear phenomenon above. This behavior is very 

 peculiar, recalling the appearance of a bright yellow ribbon undulating, or 

 flapping fore and aft, so as to darken parts of its length rhythmically. The 

 pulsations, moreover, were quite as active if seen at night, when the tremors 

 of the laboratory were certainly reduced to minimum. Nevertheless, I am 

 now convinced that such tremor only is in question. 



Regarding the phenomenon as a whole, one may argue that in case of the 

 wide slit and single grating, in which the lines for both diffractions are there- 

 fore rigorously parallel, the interference fringes are on so large a scale as to 

 cover the whole field of view and thus to escape detection; i.e., that a single 

 vague, quivering shadow of a flickering field is all that may be looked for, 

 in the limited field of view of the eyepiece. 



Returning to the case of two gratings and the wide vertical interference 

 fringes and, in turn, all but closing the slit (vertical interferences and sodium 

 arc light), the pulsating phenomenon simply narrowed in width. The two 

 or three sharp vibrating lines, alternating in black and yellow of the original 

 phenomenon (Chapter I), did not appear. The cause of this is now to be 

 investigated. 



14. Experiments continued. Rotation, etc., of grating. The method of 

 two gratings (fig. 16 or 20, plane transmitting and concave reflecting) was 

 first further improved by perfecting the fore-and-aft motion of the grating G' 

 (G' movable in the direction G'T on a slide), as well as the precision of the 

 independent rotation of G' normal to its face; i.e., around G'T. These adjust- 

 ments led to further elucidation of the phenomenon. To begin with the fore- 

 and-aft motion of the concave grating G' (i.e., displacements in the directions 

 G'T, fig. 20), it was found that the fringes, figure 21, a, b, c, d, e, in any good 

 adjustment, pass from extremely fine, sharp, vertical striations, which gradu- 

 ally thicken and incline to relatively coarse, horizontal lines, finally with 

 further inclination in the same direction into fine vertical lines again, while 

 G' continually moves (through about 5 cm.) on the slide normal to the face 



