446 C. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



fourth triple from the third; and that is again about half that of the 

 second triple from the first. And finally, to carry the principle still further 

 into details, but these details supposed to be more nearly eternal than worlds, 

 and suns, and stars, — in each of the six triples the third line is about half 

 the brightness of the second, and at half the distance from it, that the second 

 line is from the first line. 



Indeed the characteristics of an Oxygen triplet are so peculiar, and so 

 closely adhered to in every instance, that I have been able in some tubes 

 swarming with lines of impurities to pick out an Oxygen triplet, as easily as 

 one would distinguish in a crowd of civilians, a soldier with cross-belts and 

 scarlet coat. 



Although then there is so little show of general light in a pure Oxygen 

 tube, geometrical order is preserved there amongst such lines as it does show, 

 most rigidly. So that while Hydrogen, with its multitudinous, brilliant, varied 

 lines dancing or vibrating through the whole length of the spectrum, may be 

 likened to a big, curly-haired, Newfoundland dog, bounding about and barking 

 at its own free will, — Oxygen is a Bull dog which, without any show, runs 

 straight to his quarry and holds him fast with an iron grip. 



This view, moreover, trifling as it may appear, comes out more notably still, 

 when we attend to the many single lines which there are, after all, in the 

 Oxygen spectrum, and are shown both in my Index Map, and the larger plates, 

 Nos. LXVL, LXVIL, LXVIIL, and LXIX. For, in spite of its faintness of 

 light, Oxygen in the spectrum actually outflanks every other gas. That is to 

 say, it begins with a very well marked and sharply defined line, further away 

 into the ultra-red than any line, band, or haze of any known elemental gas. 

 This same lowest line too of the O tubes appears to be identical with the most 

 red-ward line in the jar-discharge in the open air, as described in my recent 

 paper to this Society on Brewster's Solar line Y. 



Subject 3. — N or Nitrogen. 



This is the last gas I have observed on the present occasion, and its 

 spectrum is in many respects the most mysterious, and most multitudinously 

 lined of all, when seen with great dispersive power; for otherwise, it is an affair 

 of hazy bands alone. 



Just as it was with O, so here, mutatis mutandis is it with N, that a 

 condensed induction spark, or jar-discharge, discloses the high-temperature, or 

 line, spectrum of the gas; and if the two gases be mixed together as they arc 

 in the atmosphere, the same discharge shows the line spectra of both gases 

 overlying or multiplying each other ; as may be seen in the upper portions of 

 of the Index Map; forming hazy lines when in a dense, sharper lines in a 



