450 J. C. Draper—Dark Lines of Oxygen in the Solar Spectrum. 
divide this, H. Draper places the two branches at 44817 and 
A4319; Pliicker at A4317 and A4820._ My own readings are at 
2481650 and 44819°75. Those of Angstrém’s chart are at 
A4316°57 and 44319°15. 
In the above measurements, those for the more refrangible 
line are all within one-half of a wave length. The readings 
of the other line are within one wave length. In these posi- 
tions there are, therefore, oxygen lines, and in the same regions 
of the solar spectrum, Dr. Rutherfurd, Mr. Christie, and my- 
self, agree in finding faint dark lines. More than this; grant- 
ing that the forked symbols of Angstrém’s chart indicate, (and 
I cannot understand what else they mean) that the observer 
believed that the line 44816°57 is divisible into two lines, 
placed at 14316-20 and 1 4316-95, and the line 14319°15, into 
two, placed at 14318°85 and 24319°45, we find that the three 
charts of the solar spectrum gives lines which are very nearly 
coincident with the readings of these forked symbols. It is 
not often that the results obtained by so many independent 
investigators agree as closely regarding a problem to be solved 
only by observation and experiment, and considering the small- 
ness of a wave length, and the difficulty of making such meas- 
urements, the agreements must be regarded as favoring strongly 
the identity of these feeble lines of the solar spectrum with 
the lines of the electric spectrum of oxygen. 
mong the obstacles in the way of comparing the electric 
lines of oxygen with those of the solar spectrum, and which in 
part explains the discrepancies that exist in the measurements 
of the lines of oxygen, is the difficulty in making exact meas- 
urements of these lines. Of this difficulty Pliicker speaks: in 
a memoir on the oxygen spectrum in the Phil. Trans. for 1865. 
In the same memoir he also discusses the variation in the 
breadth of the lines, and the variation in this variation, in 
different parts of the electric spectrum of oxygen. e posi- 
tions given by Pliicker and myself agree closely, although he 
gives no fractions. We both used two prisms of flint-glass, 
though at times he used four. Pliicker’s results, and my own, 
were obtained in pure oxygen gas, while those of H. Draper, and 
of Angstrém’s chart, were from the electric spark in air. This 
may possibly explain why they agree in differing slightly from 
Pliicker me myself. 
in the measurement of the lines of the solar spectrum made 
by Mr. Christie, the spectroscope . was, he says, equal in dis- 
* 
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