420 PROF, PIAZZI SMYTH ON THE CONSTITUTION OF THE LINES 
cipally intended to show the physical circumstances under, or temperature level 
on, which it is produced,—might perhaps privately have been intended to 
indicate that he had somehow an idea that if the lines were not compound, 
they ought to have been, and would one day be found so to be. 
That, so far as I can ascertain, is the furthest point the subject has even yet 
reached elsewhere ; and it was at that point that I had taken it up in 1879 and 
1880, in the paper already printed by this Society. In the regular course of 
that paper, going through many gases, with single prism power only, I abun- 
dantly confirmed Messrs. PLUcCKER’s and ScuustTer’s four wide apart lines, as 
constituting in themselves alone, almost the whole and entire low-temperature 
spectrum of oxygen ; I did indeed also find a strong line in the scarlet-red, 
besides two in the red or ultra-red, and two in the citron-green, of extreme 
faintness, and only probably belonging to the same spectrum. But they did 
not in any way alter the apparent anomaly of the Doctor’s name; for each of 
these new lines was also solitary : and what are after all nine, or to keep within 
more certain bounds five, simple lines standing separate, and along a length 
where 10,000 such could take their places without interfering with each other ! 
But when I looked last November with the improved apparatus, what a 
change was there! for, of the five certain lines, no less than four were found to 
be triple ; after a fluted fashion too. These four truly compound lines then 
were,—mine in the scarlet-red, and PLUCKER and ScHuSsTER’s three lines in the 
orange, the citron and the green respectively ; but their last line in the indigo- 
violet remained persistently and positively single. Still, with four-fifths of this 
most scanty spectrum, now proved to consist of triplets instead of single lines, 
Dr. ScuusTer’s original and really most happy name for it, of “ the compound- 
line spectrum” of oxygen was fully justified, at the same time that the minia- 
ture scale of the triplets seemed to make the physical nature of oxygen more 
markedly different than ever, from all other known gases ; for they, at the 
same temperature level, generally make their bands or compound line arrange- 
ments, on a comparatively enormous scale, and in multitudinous groupings. 
To those who are engaged in chemical spectroscopy it will at once convey 
an idea of the small-sized triplicity of these oxygen lines to be told, that from 
the first to the second of each triplet the distance is one-fifth that of the well- 
known salt-line double ; and from the second to the third is between one-eighth 
and one-ninth of the same space ; while the salt-line double itself is only one- 
eighth of the average distance apart of the stronger flutings of the citron-band 
of the carbo-hydrogen blow-pipe flame, which band has some six or seven of 
such flutings within its easily perceivable breadth. 
Or, again, if we should on the black board represent the separation of the 
first and second of any oxygen triplet by a tenth of an inch, and from the second 
