422 PROF. PIAZZI SMYTH ON THE CONSTITUTION OF THE LINES 
sharply confined to the width of opening of the slit for the time being. Not, 
however, until some months after, when the telescope power was also improved, 
and a new class of difficulties with the prisms had been overcome, could the 
desired trial on oxygen be made. Now, whereas I had, as mentioned already, 
on previous occasions with the best compound glass prisms I could procure, 
seen only an uncertain idea that some of the oxygen lines might be double, I 
now saw the real triplicity of four of the lines, and measured them micrometri- 
cally with a degree of certainty and satisfaction that I had never dreamt of with 
the older apparatus; and this triplicity of these lines never came out more 
remarkably than when the singularity of really single lines, such as those of 
hydrogen impurities in the same gas tube, was rendered most distinctly. I 
will, therefore, now only seek to conclude with a few words on the bearing of 
this tripleness of the oxygen lines,—first, on the disputed question of the 
existence of oxygen in the atmosphere of the sun ; and, second, on the absence 
of hitherto recognised oxygen manifestations, though oxygen is so well known 
to exist as a large part of the earth’s atmosphere, in the telluric rays that 
become visible in the solar spectrum at sunrise or sunset. 
Oxygen in the Sun. 
For many years it was a sort of crying wonder that the spectroscope showed 
no traces in the sun of so necessary a gas to combustion as oxygen. The 
expected test being, that we should see there as dark lines all the lines which 
are seen bright, when oxygen gas, on being rendered incandescent in a high- 
electric temperature, then shows what is called its ‘‘ elemental-line spectrum.” 
But not one of those lines could be detected by its dark counterpart in the 
sun. At length Professor Henry Draper, of New York, from a series of 
experiments made with extraordinary skill and power, announced that oxygen 
appears in the solar spectrum not in its dark but in its bright lines, outshining 
where they come the brightness of the sun’s continuous spectrum background. 
Yet though he gallantly made a voyage from New York to London especially 
to describe his experiments, and was honourably received and attentively 
heard, there are some persons there who are not convinced yet. 
Now Dr. Scuusrrr had already compared his four lines, though of the low- 
temperature oxygen spectrum, with ANGsTROm’s normal solar spectrum map, 
under the idea apparently that, though the solar oxygen might have been 
rendered incandescent in the very hottest central regions of the sun, it might 
have its Fraunhofer dark correspondences checked off by cooler oxygen vapour 
outside, But as his then knowledge of oxygen low temperature lines made 
them only four single, simple, thin lines, the comparison was not attended 
Ne 
