REVERSED AND NON-REVERSED SPECTRA. 23 



the black longitudinal lines of the two spectra together. The question thus 

 arises how close this coincidence is to be. When the phenomenon is sharp, 

 it has been found possible to displace the two black lines so that a fine, 

 bright strip of spectrum may just be seen between, without quite destroying 

 the interferences. Naturally they are then much weaker. This result is in 

 harmony with the observations made on rotating one spectrum, on a longi- 

 tudinal axis, 1 80 with reference to the other. 



Since the phenomenon was originally produced with sunlight, it might be 

 supposed that the edges of the Fraunhofer line, under conditions of tremor, 

 would interfere with each other as indicated in figure 14, where A is one and 

 B the other of the two superposed spectra. The change of wave-length is 

 suggested by the slant of lines on the 

 diagram. In such a case, whereas the 

 conditions a and c would show bright 

 overlapping spectra, the dark line would 

 appear under condition b. But even 

 in this case, lines of slightly different wave-length would have to interfere 

 with each other. The crucial test was made by using an arc-lamp spectrum, 

 and it was then found that the phenomenon appeared as well as with sunlight. 



A further question at issue is the breadth of spectrum needed 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 coincidence, and it was found that for none of the secondary 

 lines in the orange-yellow spectrum was it extinguished or even modified. 

 If, however, the corresponding D lines of the spectra (D\D\; D 2 D 2 ') were 

 superposed, the phenomenon in these experiments played like a wavy strip at 

 their edges only. Sometimes a bright line flashed through the middle of the 

 coincident lines. One would conclude, therefore, that the part of the spectrum 

 used in producing these interferences is not much broader than either the 

 DI or D z 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, figure 12, or the corresponding case in 

 figure 1 1 . 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. Gradu- 

 ally they became finer and eventually reached the normal appearance of the 

 DI, D 2 lines. In order to facilitate adjustment and with the object of obtain- 

 ing cases correlative with the results for the dark-line spectrum, a beam of 

 sunlight (as at L, figure 12) was introduced between the carbons and the phe- 

 nomenon established 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 DI or D 2 lines 

 were placed in coincidence, the thread-like phenomenon fails to appear with 



