Broca's Brain - Carl Sagan [142]
There is preliminary photographic spectroscopy of the Andromeda nebula by Julius Scheiner at Potsdam, Germany, who concludes correctly that “the previous suspicion that the spiral nebulae are star clusters is now raised to a certainty.” As an example of the level of personal vituperation tolerated at this time, the following is an extract from a paper by Scheiner in which W. W. Campbell is criticized: “In the November number of the Astrophysical Journal, Professor Campbell attacks, with much indignation, some remarks of mine criticizing his discoveries … Such sensitiveness is somewhat surprising on the part of one who is himself given to severely taking others to task. Further, an astronomer who frequently observes phenomena which others cannot see, and fails to see those which others can, must be prepared to have his opinions contested. If, as Professor Campbell complains, I have only supported my views by a single example, I was only withheld by courteous motives from adding another. Namely, the fact that Professor Campbell cannot perceive the lines of aqueous vapor in the spectrum of Mars which were seen by Huggins and Vogel in the first place, and, after Mr. Campbell had called their existence in question, were again seen and identified with certainty by Professor Wilsing and myself.” The amount of water vapor in the Martian atmosphere that is now known to exist would have been entirely indetectable by the spectroscopic methods then in use.
Spectroscopy was a dominant element in late-nineteenth-century science. Ap. J. was busily publishing Rowland’s solar spectrum, which ran to 20,000 wavelengths, each to seven significant figures. It published a major obituary of Bunsen. Occasionally the astronomers took note of the extraordinary nature of their discoveries: “It is simply amazing that the feeble twinkling light of a star can be made to produce such an autographic record of substance and condition of the inconceivably distant luminary.” A major topic of debate for the Astrophysical Journal was whether spectra should be shown with red to the left or to the right. Those who favored red to the left cited the analogy of the piano (where high frequencies are to the right), but Ap. J. opted gamely for red to the right. Some room for compromise was available on whether, in lists of wavelengths, red should be at the top or at the bottom. Feelings ran high, and Huggins wrote to say that “any change … would be little less than intolerable.” But the Ap. J. won anyway.
Another major discussion in this period was on the nature of sunspots. G. Johnstone Stoney proposed that they were caused by a layer of condensed clouds in the photosphere of the Sun. But Wilson and FitzGerald objected to this on the ground that no conceivable condensates could exist at these high temperatures, with the possible exception of carbon. They suggested instead and very vaguely that sunspots are due to “reflection by convection streams of gas.” Evershed had a more ingenious idea. He thought that sunspots were holes in the outer photosphere of the Sun, permitting us to see to much greater and hotter depths. But why are they dark? He proposed that all the radiation would be moved from the visible to the inaccessible ultraviolet. This, of course, was before the Planck distribution of radiation from a hot object was understood. It was not at this time thought impossible that the spectral distributions of black bodies of different temperatures should cross; and some experimental curves of this period indeed showed such crossing—due, as we now know, to emissivity and absorptivity differences.
Ramsay had recently discovered the element krypton, which was said to have, among fourteen detectable spectral lines, one at 5570 Å, coincident with “the principal line of the aurora.” E. B. Frost concluded: “Thus