Draper. | 74. [July 20, 
Discovery of Oxygen in the Sun by Photography, and a new Theory of the 
Solar Spectrum. 
By Proressor Henry Draper, M. D. 
(Read before the American Philosophical Society, July 20, 1877). 
I propose in this preliminary paper to indicate the means by which I 
have discovered Oxygen and probably Nitrogen in the Sun, and also to 
present a new view of the constitution of the Solar Spectrum. 
Oxygen discloses itself by bright lines or bands in the Solar Spectrum 
and does not give dark absorption lines like the metals. We must there- 
fore change our theory of the Solar Spectrum and no longer regard it 
merely as a continuous spectrum with certain rays absorbed by a layer 
of ignited metallic vapors, but as having also bright lines and bands super- 
posed on the background of continuous spectrum. Such a conception not 
only opens the way to the discovery of others of the non-metals, sulphur, 
phosphorus, selenium, chlorine, bromine, iodine, fluorine, carbon, &c., 
but also may account for some of the so-called dark lines, by regarding 
them as intervals between bright lines. 
It must be distinctly understood that in speaking of the Solar Spectrum 
here, I do not mean the spectrum of any limited area upon the disc or 
margin of the Sun, but the spectrum of light from the whole disc. I have 
not used an image of the Sun upon the slit of the spectroscope, but have 
employed the beam reflected from the flat mirror of the heliostat without 
any condenser. 
In support of the above assertions the accompanying photograph of the 
Solar spectrum with a comparison spectrum of Air, and also with some of 
the lines of Iron and Aluminium is introduced. The photograph itself is 
absolutely free from handwork or retouching. It is difficult to bring out 
in asingle photograph the best points of these various substances, and I have 
therefore selected from the collection of original negatives that one which 
shows the Oxygen coincidences most plainly. There are so many variables 
among the conditions which conspire for the production of a spectrum 
that many photographs must be taken to exhaust the best combinations. 
The pressure of the gas, the strength of the original current, the number 
of Leyden jars, the separation and nature of the terminals, the number of 
sparks per minute, and the duration of the interruption in each spark, are 
examples of these variables. : 
In the photograph the upper spectrum is that of the Sun, and above 
it are the wave-lengths of some of the lines to serve as reference numbers. 
The wave-lengths used in this paper have been taken partly from Angstrom 
and partly from my photograph of the diffraction spectrum published in 
1872. The lower spectrum is that of the open air Leyden spark, the ter- 
minals being one of Iron and the other of Aluminium. I have photo- 
graphed Oxygen, Nitrogen, Hydrogen and Carbonic Acid as well as other 
