C. Barus — Interference of Reversed Spectra. 427 



of G' is very rapid, so that relatively wide nearly vertical forms 

 may be obtained. All these effects may be produced by solar 

 or by arc light around the line of symmetry of the overlapping 

 spectra, or with sodium light when either D X D{ or DJ)^ coin- 

 cide. 



The fine vertical or inclined lines appear as such when the 

 slit is widened, either in case of white or of sodium light. 

 These are the interferometer fringes seen above (§6), coarse 

 fine. With sodium light any width of slit or no slit at all 



or 



Figs. 9, 12, 13, 14, 15. 



Illllll ^ " ^ I ^ 



abode ^^_s__^ 

 cfig- cj ~\ 



d o a> 





A. 75 



a 



$IaA4-. 



tod e -f 



are equally admissible. The same is true for the narrow 

 maxima. 



Finally (and this is the important result) the extremely large 

 horizontal maxima, when a single fringe fills the field, cannot 

 be seen apart from pulsations, in the case of a wide slit. With 

 a very narrow slit such as is suited for the Fraunhofer lines, 

 these horizontal fringes appear as intensely bright or very 

 dark images of the slit. In other words, the normal phenom- 

 enon of overlapping symmetrical spectra as described in the 

 earlier paper, is merely the vertical strip of an enormous hor- 

 izontal interference fringe, made sharp and differentiated by 

 its narrowness. This case occurs at once when the rulings of 

 the two gratings G and G' are all but parallel and hence it is 

 the regular phenomenon when but a single grating is used 

 for the two diffractions, as in figure 1. 



