C. Barus — Interference of Reversed Spectra. 417 



to produce the phenomenon, for the observed breadth would 

 be influenced by the quiver of the apparatus. With this end 

 in view different lines of the spectrum were placed in full coin- 

 cidence. For none of the secondary lines in the orange-yellow 

 was it extinguished or even modified. If, however, D X D^ or 

 B 2 D./ were superposed, the phenomenon played like a wavy 

 strip at their edges only. Sometimes a bright line flashed 

 through the middle of the coincident lines. One may con- 

 clude, therefore, that the part of the mutually reversed spec- 

 tra used in producing these interferences, is not much broader 

 than either the D x or D 2 lines, while the other marked lines in 

 the orange-yellow are too narrow to appreciably influence it. 

 These results will be greatly amplified in the work done with 

 two gratings below. 



A corresponding experiment was now made with sodium 

 light. To obtain a sufficiently intense source, solid caustic 

 soda was volatilized between the carbons of the electric arc, A 

 and B, fig. 1 or 2. On drawing the carbons apart, strong D 

 lines were seen (in the entire absence of an arc spectrum), at 

 first so broad as to be self-reversing. Gradually they became 

 finer and eventually reached the normal appearance of the 

 D^ D r lines. In order to facilitate adjustment and with the 

 object of obtaining cases correlative with the results for the 

 dark line spectrum, a beam of sunlight (as at B, fig. 2) was 

 introduced between the carbons and the phenomenon estab- 

 lished faultlessly in the usual way. The pencil of sunlight 

 was then screened off and the arc light substituted, or the two 

 were used together. 



These observations seemed to show that when the normal 

 D 1 or Z> 2 lines were placed in coincidence, the thread-like phe- 

 nomenon does not appear with all the characteristics visible in 

 the case of sunlight. When the slit is broadened an alterna- 

 tion of brightness, or flicker of light, may be detected, vaguely. 

 With a slit of proper width to show the Fraunhofer lines all 

 this seemed to vanish. The actual phenomenon was therefore 

 apparently not reproduced or improved either by homogeneous 

 light or by widening the slit. Such experiments alternating 

 with sunlight were made at considerable length, but the adapta- 

 tion of methods for two gratings discussed in §4 will neverthe- 

 less modify this conclusion. 



If the narrow sodium line is broadened by adding fresh 

 sodium at the carbon, so that the yellow spectrum is again self- 

 reversed, the phenomenon plays with extreme vividness around 

 either of the reversed and coincident D x or B„ lines or even 

 within the black line in question, if narrow. But here the 

 light is no longer homogeneous. 



3. Inferences. — If the wave length of the two spectra is laid 

 off in terms of the angle of diffraction, 6, measured in the 



