G. Hinrichs on Spectral Lines. 363 
states furthermore, that the selenium spectrum contains between 
the yellow and the violet @ very large number of equidistant dark 
hin 
e have furthermore proved that though not ail lines are 
present, corresponding to each successive equal interval, still all 
the known lines occupy positions in exact avcordance with the 
equal intervals, that is, the dark lines are as if from the whole 
number of exactly equidistant lines some had been blotted out or at 
least obscured by other causes. 
From all this, both the experimental and our theoretical evi- 
dence, we conclude, that the dark lines are produced by interference. 
We will now see whether we can account for some other pecu- 
liarities of their distribution by means of this hypothesis. 
Erman, in his investigation of the absorption lines of iodine, 
bromine, etc., has shown that such lines produce y a plate of 
mica are the closer together, the thicker the plates, and that they 
are farther apart in the violet than in the re 
ture, becoming more numerous with higher temperature—the 
dark lines would be as changing as the arrangement of the atoms. 
ut we know the spectral lines to be permanent and immutable, 
aS we suppose the atoms themselves to be.” We therefore must 
tra of the elements are the result of the interference of at most three 
systems of interferences determined by the three dimensions of the 
oms, 
We shall now compare some of the points involved herein to 
the results of observation laid down in the preceding paragraphs. 
7. Is the difference constant throughout the entire spectrum ? 
or are the lines farther apart in the blue end of the spectrum, 
Where the wave-length is smaller? The values of D obtained 
for the iron groups are: D in the yellow 13°16, in the green 
more extended research is required. But we may safely affirm 
that in case any such variation is actually proved it will be found 
to be very small. : 
$28. The lines of the same group are equidistant—as required — 
by this theory; for we have seen the same difference d to ex- 
Press all the numerous observed values for even very extensive 
* Mitscherlich’s observations in regard to the spectrum of iodine admits of other 
explanation. 
