REVERSED AND NON-REVERSED SPECTRA. 43 



To resume the evidence thus far obtained, we may therefore assert that 

 in the case of homogeneous light and a wide slit, or the absence of a slit, the 

 field would either be bright or dark, as a whole. There is a single enormous 

 horizontal fringe in the field. Hence the pronounced flickering with half 

 wave-length displacements of any part of the apparatus. With the slit 

 narrowed until the Fraunhofer lines are seen sharply, the linear phenomenon 

 in question (Chapter I) appears. This may become ladder-like, but it always 

 remains very narrow (-3- DiD^) when the rulings of the two gratings are not 

 quite parallel. 



17. Subsidiary diffractions. The behavior of the linear phenomena some- 

 times suggests probable relations to the Fresnellian interferences, produced, 

 however, not within the telescope, as in 27, 28, Chapter III (for the inter- 

 ferences are seen together with the Fraunhofer lines in the principal focal 

 plane), but outside of it, at the grating, as suggested by figure 26. If the 

 concave grating G' is screened off, until a width of strip parallel to the rulings 

 and not more than 5 mm. wide is used, the linear phenomenon is much en- 

 hanced, being both broader and stronger, without losing its general character. 

 Here the D lines are still visible. The ladder-like patterns show an equally 

 pronounced coarsening. So far as these phenomena go, it is obvious that the 

 resolving power of the grating must be in question, seeing that the total 

 number of rulings has been greatly reduced. The use of screens with narrower 

 slits carries the process farther; but after the opening is less than 2 mm. in 

 width the available light is insufficient for further observation. If a small 

 lens is used, the phenomena can still be seen over 2 meters beyond the principal 

 focus of the grating. 



A screen was now made as in figure 280, with two slits about 2 mm. wide 

 and 2 mm. apart (6), and placed over the effective part of the grating. The 

 result, after careful trial as to position, was noteworthy. Oblique fringes 

 were widened to many times the DiD z space and coarsened, showing a definite 

 grid-like design, as in figure 286, whereas, on removing the screen, the original 

 pattern of a regular succession of brilliant dots (fig. 28 c) again appeared. 



It was with the linear fringes, however, that the evidence obtained was 

 most striking; for these now showed all the Fresnellian interferences (fig. 28 d). 

 On removing the screen, the brilliant linear phenomenon (fig. 28 e), which 

 in all the experiments made had thus far resisted manifolding, appeared at 

 once. The pattern, d, moreover, when viewed with a small lens, within a 

 meter in the direction of the rays, showed very definite enlargement with 

 distance. Though a fine slit was needed, the resolving power of the grating 

 was now too small to show any Fraunhofer lines. Similar results were obtained 

 for a wire i or 2 mm. in diameter. With the screen, figure 28 a, and a bar, b, 

 i mm. wide, the fine interference grid due to the bar, and the coarse grid due 

 to the spaces (the fine lines being about twice as narrow as the coarse, but 

 all of the same inclination) were often obtained together (fig. 28/). A space 

 i cm. wide intersected by a bar 2 mm. wide gave similar results, fine grids or 



