REVERSED AND NON-REVERSED SPECTRA. 35 



the coarsest possible for a given fore-and-aft position of the grating G'. In 

 the figure, the sequence a, b, c, d, e is obtained for a continuous rotation of 

 the grating (in one direction around a normal axis). 



It now became interesting to ascertain how the vertical set c, figure 22, 

 would behave with the fore-and-aft motion. The experiments showed that 

 there was no further rotation, but that, while G' passes normally to itself 

 over about 1.5 cm. on the slide, the vertical fringes pass from extreme fine- 

 ness at the limit of visibility, through an infinite vertical maximum (a single 

 vague shadow pulsating in the field), back to extreme fineness again, without 

 any rotation. If the edges of the corresponding yellow strips (superposed 

 DI, D\ lines) did not quite coincide, the fringes were seen outside of the prin- 

 cipal focal plane, as usual. Probably the vertical and horizontal maxima are 

 identical in occurrence and appear in case of parallelism in the rulings of the 

 two gratings G and G', and the absence of path-difference. Hence if a single 

 grating is used, as in the original method, the interferometer fringes are not 

 obtainable. This is an important and apparently final result, remembering 

 that fore-and-aft motion is probably equivalent to a rotation around a vertical 

 axis, parallel to the grating. 



With regard to the rotation in case of fore-and-aft motion of G', it is well 

 to remark that in approaching the position c, figure 24, it is apt to be very 

 rapid as compared with the displacement, precisely as in the case of the picket- 

 fence analogy. 



Hence the original phenomenon, consisting of single lines, can not be mani- 

 folded by increasing the width of slit. It vanishes for a wide slit into an 

 indiscernible shadow. The phenomenon is a strip cut across an enormous 

 black or bright horizontal fringe, by the occurrence of a narrow slit. More- 

 over, the scintillations variously interpreted above are now seen to be due to 

 tremors, however different from such an effect they at first appear; i.e., the 

 enormously broad, horizontal fringe changes from dark to bright, as a whole, 

 by any half wave-length displacement of any part of the apparatus. It is 

 thus peculiarly sensitive to tremors. On the other hand, oblique or fine 

 vertical fringes are always recognizable for any size of slit. The inquiry is 

 finally pertinent as to why the phenomenon is so remarkably sharpened by 

 a narrow slit; but this must be left to the following experiments. 



To be quite sure that the concave grating G' had no fundamental bearing 

 on the phenomenon, I again replaced it by the Michelson plane reflecting 

 grating (fig. 16, G transmitting, G' reflecting). In the same way I was able 

 to rotate the fringes, continuously, through a horizontal maximum of size 

 by fore-and-aft motion of G' . Rotation of G' in its own plane increased or 

 decreased the breadth and distance apart of the fringes through a maximum, 

 coinciding with the parallelism of the rulings of the two gratings. Here I 

 also showed decisively that as the rungs of the interference ladder (fig. 2 1 c) 

 thickened and receded from each other, the design passed, in the transitional 

 case, through the original phenomenon of the single vertical line dark or 

 brilliant yellow, for a slit showing the Fraunhofer lines clearly. The phenome- 



