C. Barus — Interferences of Grossed Spectra. 489 



interferences quiet. Shutting off the light from either mirror, 

 M or iV, naturally quenches the interferences, but leaves the 

 yellow part of the spectrum behind. 



Obviously coincidence of the longitudinal axes of the spectra 

 alone is needed. Therefore upon moving the two double D 

 lines apart, by aid of the adjustment screws on the mirror M 

 and N, symmetrically to the ends of the yellow field in the 

 telescope, the interferences were isolated and located midway 

 between the D doublets of each spectrum ; i. e., in the center 

 of the field of the telescope. They could now be observed to 

 better advantage. In the small telescope there is apparently 

 but one dark line, and if stationary, its character when centered 

 would be surmised to be given by the intersection of a vertical 

 diameter with a series of con-focal ellipses, successively bright 

 and dark, as indicated in figure la. The light and dark parts 

 alternate or flicker. On moving the micrometer, the vertical 

 intersector A takes a more and more lateral position like B, 

 so that the trembling interferences would soon be invisible as 

 they rapidly become finer (not shown). 



On using higher magnification (larger telescope), two black 

 lines bordering a bright, or a black line between two bright 

 lines, seemed to be visible; but the interferences would have to 

 be stationary to be definitely described, since the width of the 

 pattern is not more than 1/3 to 1/2 of the distance between 

 the sodium lines. The interferences, moreover, did not now 

 conform to the design B, fig. la, anticipated, but were more 

 of type G, with the long dark lines very slightly oblique to 

 the vertical, and vibrating within a vividly yellow band. 

 Sometimes these were heavier with two or three faint lines on 

 one side. 



Farther experiment showed that the phenomenon is not 

 influenced by the width of the slit, except that it is clearest 

 and sharpest with the narrowest slit possible and vanishes 

 when the slit is made so wide that the Fraunhofer lines disap- 

 pear. It may easily be produced by the modified method fol- 

 lowing in any wave length, red, yellow, green, etc., with no 

 essential difference except in size. It is present moreover in 

 all focal planes, i. e. the ocular of the telescope may be inserted 

 or pulled out to any distance, yet the same phenomena persist 

 on the vague, colored background. A number of observations 

 were made to detect the change of the pattern of the interfer- 

 ence between its entrance into the field and its eventual evan- 

 escence in case of the continuous displacement of the mirror 

 M. In figure la this would be equivalent to a passage of B 

 into B' through A and the fringes for a distant center should 

 therefore rotate, as they actually do in the experiments of 

 paragraph 4. But in the present case the type G persists, 



