362 G. Hinrichs on Spectral Lines. 
By having more accurate data, the intervals become more sim- 
plified, especially in regard to the consolidating of groups of 
lines, as has been shown in the preceding paragraph. 
In the synopsis of the calcium spectrum given on page 35 of 
our first article (this Jour., vol. xxxvili) the deviations amount 
to fully 
a=0"" 0000030, 
while in our present article the greatest deviation is only (§ 18) 
a’=0™"-0000001 
or a: a’=30: 1, only one-thirtieth of the previous one. 
This comparison might be considerably enlarged—but the 
preceding is enough to show that far from disappearing by 
closer approximation and more accurate data, the laws there. , 
enunciated are becoming even more prominent and unmistak- 
able in proportion as the data of observation have become more 
rigorous and reliable. It may therefore be hoped that contin- 
ued investigation will tend to the better establishment of the 
laws referred to. 
And it is in view of this greater confidence we are permitted 
to bestow on the regular distribution of the dark lines that we 
will venture upon the dangerous ground of a theoretical expla- 
nation of these wonderful facts and mysterious laws. e im- 
portance of spectral a in a practical point of view is es- 
my first article on the dark lines. 
little detail on this difficult and obscure, but most important. 
int. 
_ $26. Origin of the dark lines—The mere aspect of the dark 
lines in any spectroscope conveys almost a moral conviction to 
the mind that they are allied in their origin to the dark lines of 
interference produced by thin plates and the like; for example, 
Talbot's interference phenomena produced by the interposition 
of a little’ piece of glass or mica between the eye and the ocular 
of a telescope directed to a spectrum, or Baden Powell’s lines 
produced by a glass plate immersed in a prismatic trough fill 
with cassia-oil, ete. : 
This idea is already old, and I know of no definite refutation 
thereof. The lines produced by the vapors of iodine, bromine, 
hypochlorous acid and others, are closely allied to the regular 
spectral lines and the above lines of interference. Bottger 
- This Journal, 1863, vol. xxxy, p. 414, 
