330 On the Relative Intensity of the Spectral Lines of Gases. 



of a more complicated spectrum, concealed partly by want of 

 brightness in the aurora itself, and partly by the medium, 

 more or less dense, in which that phenomenon probably occurs. 

 As Geissler's tubes were at the time under examination, they 

 were found convenient for this purpose. Nitrogen- (in the 

 shape of " air "-) tubes were also used; but I do not find they 

 were specially tested; at least they are not recorded. I have, 

 however, a recollection that the bright double line in the green 

 survived all others. The other results are taken from notes 

 made at the time. 



The spectroscope was a direct-vision one, by Browning, 

 giving a dispersion equal to about two white glass prisms of 

 60°. Collimator and telescope 6 inches long respectively, and 

 carrying 1-inch aperture lenses. Slit about 3^ of an inch wide. 

 Source of light Geissler tubes excited by a ^-inch-spark coil. 



Hydrogen tube. — Tube showing ordinary hydrogen-lines, 

 with some others probably due to impurity. At 6 inches from 

 slit, a (solar F) was \ery bright ; the lines j3, 7, 8, e, and £ 

 also seen, but faint. At 12 inches from the slit, F and 7 were 

 alone seen; and at 24 inches, F stood by itself alone upon a 

 dark ground. 



Carbonic-acid tube. — At 18 inches from the slit the conti- 

 nuous spectrum and fainter lines disappeared, while the four 

 principal lines still shone out, that in the green being the 

 strongest. At 24 inches same lines visible, but faintly. 



Coal-gas tube. — At 24 inches whole spectrum quite brilliant, 

 the four principal lines very bright, and preserving their di- 

 stinctive colours. 



Oxygen tube (impure). — At 12 inches distance from the slit 

 the spectrum lost nearly all its light. F hydrogen and three 

 oxygen lines, a, /3, and y, alone remained. At 24 inches no 

 spectrum at all seen. 



It was noticed in the foregoing experiments that, on with- 

 drawing the tube from the slit, the colours of the spectrum 

 disappeared in the following consecutive order — red, yellow, 

 violet, and, lastly, green. It thus seems pretty clearly proved 

 that the brighter line or lines of a spectrum may be seen 

 singly as a matter of intensity. 



This, however, must be treated as independent of the exal- 

 tation or suppression of individual lines of a spectrum by such 

 causes as temperature, pressure, magnetism, &c. 



It is easy to conceive that the celestial bodies may be liable 

 to these influences to a very considerable extent, and that 

 modifications of their spectra may thus arise which we do not 

 produce in the laboratory and which are not dependent on the 

 question of intensity alone. 



Guildown, April 2, 1880. 



