2 
Draper. j 76 (July 2), 
parisons. In order to be certain that a line belongs to Oxygen, Ihave com- 
pared, under various pressures, the spectra of Air, Oxygen, Nitrogen, Car- 
bonic Acid, Carburetted Hydrogen, Hydrogen and Cyanogen. Where 
these gases were in Plicker’s tubes a double series of photographs has been 
needed, one set taken with, and the other without Leyden jars. 
As to the spectrum of Nitrogen and the existence of this element in the © 
sun there is not yet certainty. Nevertheless, even by comparing the dif- 
fused Nitrogen lines of this particular photograph, in which Nitrogen has 
been sacrificed to get the best effect for Oxygen, the character of the evi- 
dence appears. The triple band between 4240. 4227. if traced upward into 
the Sun has approximate representatives. Again at 4041. the same thing is 
seen, the solar bright line being especially marked. In another photo- 
graph the heavy line at 3995. which in this picture is opposite an insuffi- 
ciently exposed part ofthe Solar Spectrum shows a comparison band in the 
Sun. 
The reason I did not use air in an exhausted Pliicker’s tube for the pro- 
duction of a photograph to illustrate this paper, and thus get both Oxygen 
and Nitrogen lines well defined at the same time, was partly because a 
brighter light can be obtained with the open air spark on account of the 
stronger current that can be used. This permits the slit to be more closed 
and of course gives a sharper picture. Besides the open air spark enabled 
me to employ an iron terminal, and thus avoid any error arising from acci- 
dental displacement of the reference spectrum. In Pliicker’s tubes with a 
Leyden spark the Nitrogen lines are as plain as those of Oxygenhere. As 
far as I have seen Oxygen does not exhibit the change in the character of 
its lines that is so remarkable in Hydrogen under the influence of pressure 
as shown by Frankland and Lockyer. 
The bright lines of Oxygen in the spectrum of the solar disc have not 
been hitherto perceived probably from the fact that in eye observation 
bright lines on a less bright background do not make the impression on 
the mind that dark lines do. When attention is called to their presence 
they are readily enough seen, even without the aid of a reference spectrum. 
The photograph, however, brings them into a greater prominence. From 
purely theoretical considerations derived from terrestrial chemistry, and 
the nebular hypothesis, the presence of Oxygen in thesun might have been 
strongly suspected, for this element is currently stated to form eight-ninths 
of the water of the globe, one-third of the crust of the earth, and one-fifth 
of the air, and should therefore probably be a large constituent of every 
member of the solar system. On the other hand the discovery of Oxygen 
and probably other non-metals in the Sun gives increased strength to the 
nebular hypothesis, because to many persons the absence of this import- 
ant group has presented a considerable difficulty. 
At first sight it seems rather difficult to believe that an ignited gas in the 
solar envelope should not be indicated by dark lines in the solar spectrum, 
and should appear not to act under the law ‘‘a gas when ignited absorbs 
rays of the same refrangibility as those it emits.’’ But in fact the sub- 
