probably to be the result of an incandescent vapor or gas." Here it will be seen, is a first point of resemblance to the variable star of Corona Borealis.
A second study, made by Cornu on December 4th, enabled him to define the bright lines of the spectrum. Three of them are the lines C, F, and 434 of hydrogen; a fourth line seemed to correspond to the line D of sodium; another to the characteristic line b of magnesium. Finally, two lines with the wave-lengths 531 and 451 seem to coincide, the one with the famous line 1,474 (Kirchhoff's scale) observed in the solar corona during eclipse, the other with a line of the chromosphere. These results possess so high an interest that we will here transcribe a portion of the text of M. Cornu's note:
"The spectrum of this star," he writes, "consists of a certain number of detached bright lines on a sort of luminous ground, almost entirely interrupted between the green and the indigo, so that the spectrum appears as though made up of a number of separate parts. In order to study it qualitatively, I adopted a spectroscopic eye-piece of special construction, which utilizes most of the light, and allows the observer to vary its concentration. In measuring I used a direct vision Duboscq spectroscope. I observed only bright lines; the dark lines, if any there are, must be very minute, and escaped my observation, owing to the faintness of the star's light."
In this respect the spectrum of the star in the Swan differs from that of the star T of Corona Borealis, as may be seen from the figure here copied from Huggins.
We omit the details of the measurements of the positions of the lines, and pass to the results, which are stated in the following table. The bright lines, ranged in the order of their brightness, α, δ, γ, β, ζ, η, θ, ε, are eight in number, and their wave-lengths are here given in millionths of a millimetre:
| α | δ | γ | β | ζ | η | θ | ε | |
| Lines observed | 661 | 588 | 531 | 517 | 500 | 483 | 451 | 435 |
| Hydrogen | 665 (C) | 486 (F) | 434 | |||||
| Sodium | 589 (D) | |||||||
| Magnesium | 517 (b Mg.) | |||||||
| Corona line | 532 | |||||||
| Chromosphere line | 587 | 447 |
From this table it is seen that there is almost perfect coincidence as regards the lines-α, η, and ε, with three hydrogen-lines; as regards β, with the line b of magnesium; as regards δ, with the line of sodium, or, perhaps, as Cornu suggests, with the bright line D3 of the chromosphere. The γ corresponds with a bright line also belonging to the chromosphere and the solar corona; and, finally, θ corresponds with a line of the chromosphere. The line ζ alone stands unidentified with any known line.
