426 C. Barus — Interference of Reversed Spectra. 



terference 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 aud yellow, of the phenomenon 

 described in the earlier paper, did not appear. 



la. Experiments continued. — The method of two gratings 

 (figure 5 or 8, 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 adjustments lead 

 to a further interpretation of the phenomenon. To begin with, 

 the fore and aft motion of the concave grating G' (i. e., dis- 

 placements in the directions G' T, figure 8), it was found that 

 the fringes, figure 9, abcde, in any good adjustment, pass from 

 extremely fine sharp vertical striations, which gradually 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 of the grating. It was not at all dif- 

 ficult to follow the continuous tilt of these lines through the 

 horizontal, occurring on careful and continuous front and rear 

 motions of the grating G' through the limiting positions. The 

 fringes usually vanish vertically merely because of their small- 

 ness. 



Again on rotating the grating G' around an axis normal to 

 its face, the fringes merely vary in size, without changing their 

 inclination. Thus if the horizontal fringes (which were here 

 always closer than the inclined set) are in view, these will pass 

 from extremely small size, fine hair-like striations, through a 

 maximum (which is a mere shadow as a single fringe probably 

 fills the field), back into fine lines again. Only a few degrees 

 of rotation of the grating suffice for the complete transforma- 

 tion. The maximum is frequently discernible only in con- 

 sequence of a flickering field. An oblique set of fringes is 

 equally available, remaining oblique as they grow continually 

 coarser and in turn finer with the continuous rotation of the 

 oratine-.* 



When the very large horizontal fringes are produced by this 

 method, the change into vertical fringes by fore and aft motion 



* Recent work snowed that rotations of fringes with passage through 

 maximum size are also producible here. The subject will be resumed. 



