On the Low- Temperature Spectrum of Oxygen. 331 



lines, arranged in peculiar bands or parcels, and known generally 

 as fluted spectra. Hence Dr. Schuster's name for what he saw 

 in oxygen, while principally intended to show the physical cir- 

 cumstances under (ortemperature-leyel 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 haye been, and would one day be found so to be. 



That, so far as I can ascertain, is the furthest point the sub- 

 ject has eyen 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 

 abundantly confirmed Messrs. Plucker's and Schuster's four 

 wide-apart lines, as constituting in themselyes 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 hues was 

 also solitary : and what are after all nine, or, to keep within 

 more certain bounds, fiye, 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 improyed 

 apparatus, what a change was there ! for of the five certain 

 fines 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 Pliicker and Schuster's three 

 lines in the orange, the citron, and 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. Schuster'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 miniature 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 arrangements 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 



