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Updated: June 19, 2025
Already there are plenty of people ready to lay down arbitrary assumptions about the lessons to be drawn from stellar spectra. Some say that they know with certainty that each star begins by being a nebula, and is condensed and heated by condensation until it begins to shine as a star; that it attains a climax of temperature, then cools down, and eventually becomes extinct.
The inconspicuousness of metallic lines in the spectra of the white stars he attributed, not to the paucity, but to the high temperature of the vapours producing them, and the consequent deficiency of contrast between their absorption-rays and the continuous light of the photospheric background.
This green area is the colour reverse to the red area, which had been previously inspected, as explained in the experiments on ocular spectra at the end of the work, and in Botanical Garden, P. 1. additional note, No. 1.
That in these experiments the colours of the spectra are the reverse of the colours which occasioned them, may be seen by examining the third figure in Sir Isaac Newton's Optics, L. II. p. 1, where those thin laminæ of air, which reflected yellow, transmitted violet; those which reflected red, transmitted a blue green; and so of the rest, agreeing with the experiments above related.
This peculiarity shows itself in the dreams of childhood, which exceed in the distinctness of their images those which come in later life. It shows itself, too, in the frequency with which, even when awake, the active organs perceive unreal sounds, or in the dark, at night, conjure up ocular spectra; and then not merely colours, but distinct shapes, which pass in long procession before the eyes.
He adverted also to the influential effects upon stellar types of varying surface gravity, which being a function of both mass and bulk necessarily gains strength with wasting heat and consequent shrinkage. The same leading ideas were more fully worked out in "An Atlas of Representative Stellar Spectra," published by Sir William and Lady Huggins in 1899.
It is obvious, from a mere glance at this threefold classification, that illusions of memory closely correspond to visual illusions. Thus, class may be likened to the optical illusions known as subjective sensations of light, or ocular spectra.
Now, the Martians would naturally resort to the same medium of communication as the most obvious, simple, and practicable. By producing a powerful light they might hope to attract our attention, and by imbuing it with characteristic spectra, easily recognised and changed at intervals, they would distinguish the light from every other, and show us that it must have had an intelligent origin."
You have never before observed in any classroom so intimate a concern about rainbows, prisms, spectra, and the scientific sources of aesthetic effects. Your mind runs back to your college days and returns almost alarmed to this unacademic display of genuine, spontaneous, unanimous enthusiasm toward a classroom study of a theoretical subject. At last the reason for it works into your mind.
Precisely what this implies and involves we do not know; but the symptoms of its occurrence are probably altogether different from those gathered by Sir Norman Lockyer from the collation of celestial spectra. A. J. Ångström of Upsala takes rank after Kirchhoff as a subordinate founder, so to speak, of solar spectroscopy.
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