GASEOUS SPECTRA UNDER HIGH DISPERSION. 449 



region ; the rest was therefore recorded by means of a 0'5" pressure tube, 

 compared occasionally with 10" and 2'5" pressure tubes. 



Of the gradual swellings and subsidences of brightness in each of the four 

 long groups of bands (the red, the orange, the yellow, and the green), and the 

 minute variations introduced into the composition and settings of the linelets 

 forming all the several bands of one group, compared with all those of another 

 group, — the Plates Nos. LXXI. to LXXIV. will give a better and quicker idea 

 than verbal description. And they will also indicate well the immense change 

 which comes on in the Glaucous region, making the rest of the Nitrogen 

 spectrum, through the Blue and Violet an utter contrast to its earlier appear- 

 ance from ultra Red to Green. 



This difference is marked strongly in Angstrom and Thalen's map, in so 

 far as those classic authors represent the Blue and Violet bands much broader 

 than the Red and Yellow. But they have wholly missed the club-like, or 

 fascicular, groups of lines with which each such Blue or Violet band com- 

 mences. 



I have had therefore to alter my Index Map considerably from theirs, in 

 order to represent this most innate and valuable distinction, as it appears to 

 me, of blue N, from blue CO, bands when in close neighbourhood. And the 

 minuter construction of these clubs of lines may be made out pretty well in 

 the larger plates of N, as Plates LXXV. and LXXVL, notwithstanding the 

 characteristic bad definition of any and all spectral lines in the Blue and Violet. 



There is another peculiarity, however, well worthy of note in these more 

 refrangible and very broad bands ; viz., that there is another class of bands 

 with sharper beginnings mixed up with them; and these additional, or smaller 

 featured bands (unnoticed I believe as yet by other spectroscopists), are more 

 constantly and certainly seen in tubes prepared for N, than those prepared for 

 N 2 0, or Nitrous Oxide. Or, in other words, the N 2 Spectrum is simpler 

 than that of N ; though the chemical notation as it stands now, is more 

 complicated. 



But I have no maps of the Nitrous Oxide spectrum to show, on account of 

 all the N 2 tubes, after a preliminary eye survey had been taken, having gone 

 wrong spontaneously, while I was observing those of N ; and I had no more 

 funds for further tube making. 



My present task therefore is finished, save a few words, or perhaps mere 

 conjectures respecting the possible chemical origin of the spectra just described 

 for 



The Three Elemental Gases, H, O, and N. 



