C. Barus — Interference of Reversed Spectra. 433 



the interferences are seen together with the Fraunhofer lines 

 in the principal focal plane), but outside of it, at the grating, 

 as suggested by fig. 12. If the concave grating G' is screened 

 off, until a width of strip parallel to the rulings and not more 

 than five millimeters wide is used, the linear phenomena is much 

 enhanced, 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. 



It is thus obvious that diffraction and 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 further ; but after the open- 

 ing is less than two millimeters in width the available light is 

 insufficient for further observation. If a small lens is used, 

 the phenomena can still be seen over 3 meters beyond the 

 principal focus of the grating. 



A screen was now made as in fig. 14 a, with two slits about 

 two millimeters wide and two millimeters apart (b) 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 D^D n space and coarsened, showing 

 a definite grid-like design, as in fig. 14 5, whereas on removing 

 the screen, the original pattern of a regular succession of bril- 

 liant dots, fig. 14 c, again appeared. 



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

 obtained was most striking ; for these now showed the regular 

 Fresnellian interference design, fig. 14 d. On removing the 

 screen, the brilliant linear phenomenon, fig. 14 e, which in all 

 the experiments made had thus far resisted manifolding, 

 appeared at once. The pattern, moreover, when viewed with 

 a smaller 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 diffrac- 

 tions were obtained for a wire one millimeter or two milli- 

 meters in diameter. With the screen, fig. 14 «, and a bar, b, 

 one millimeter 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. 14/". A space l cm wide 

 intersected by a bar three millimeters wide gave similar results, 

 fine grids or thick lines, according as one or both spaces were 

 used. If either mirror, M or JY, is screened the whole phe- 

 nomenon vanishes. 



It follows then, that if rv and -yV, fig. 15, represent the 

 traces of the planes of the two reversed, overlapping spectra at 

 the grating, b the focus and aa'b the direction of the homo- 



