Strange to say, Herschel’s original ideas regarding the Universe were accepted for many years by astronomical writers. Arago accepted Herschel’s original theory, unaware that he had in reality abandoned it, and he was followed by a host of French and English writers who did not take the trouble to read each of Herschel’s papers, merely quoting that of 1785, and believing that it represented his final ideas on the subject. Even Sir John Herschel seems to have been unaware that his father gave up the disc theory of the Universe. The famous German astronomer, Wilhelm Struve, after an exhaustive study of Herschel’s papers, was enabled to prove in 1847 that the theory had been abandoned by Herschel; and in England the late R. A. Proctor independently demonstrated the same thing. Meanwhile, supposing Herschel had not given up his theory, it would be quite untenable. After considering the fact that the brighter stars, down to the ninth magnitude, aggregate on the Milky Way, Mr Gore says: “As the stars are by hypothesis supposed to be uniformly distributed throughout every part of the disc, and as the limiting circles for stars to the eighth and ninth magnitudes fall well within the thickness of the disc, there is no reason why stars of these magnitudes should not be quite as numerous in the direction of the galactic poles as in that of the Milky Way itself. We see, therefore, that the disc theory fails to represent the observed facts, and that Struve and Proctor were amply justified in their opinion that the theory is wholly untenable, and should be abandoned.”
The observations made by Herschel himself eventually proved fatal to the disc theory—a hypothesis which he had all along held very lightly. His ideas about subordinate clusters within the Milky Way were soon confirmed, and though in 1799 he still adhered to the disc theory, he wrote in 1802, “I am now convinced, by a long inspection and continued examination of it, that the Milky Way itself consists of stars very differently scattered from those which are immediately about us. This immense starry aggregation is by no means uniform. The stars of which it is composed are very unequally scattered”—a conclusion quite opposed to the disc theory, where the Milky Way was supposed to be merely an extended portion of the Universe.
In 1811 Herschel wrote as follows: “I must freely confess that by continuing my sweeps of the heavens, my opinion of the arrangement of the stars, and their magnitudes, and some other particulars, has undergone a gradual change; and, indeed, when the novelty of the subject is considered we cannot be surprised that many things formerly taken for granted should on examination prove to be different from what they were generally but incautiously supposed to be. For instance, an equal scattering of the stars may be admitted in certain calculations; but when we examine the Milky Way, or the closely compressed clusters of stars, of which my catalogues have recorded so many instances, this supposed equality of scattering must be given up.”
This was the virtual abandonment of the disc theory. Six years later Herschel announced that in six cases he had failed to resolve the Milky Way, stating that his telescopes could not fathom it. This was the abandonment of his second assumption—namely, that his telescope was sufficiently powerful to penetrate to the limits of the Universe. Yet he still thought that some of the star-clusters might be external galaxies, although he could not even dogmatically assert our Universe to be limited. In an error of translation, Struve left the impression that Herschel believed our Universe to be unfathomable or infinite, and was obliged to devise a most artificial theory of the extinction of light to account for the fact that the sky did not shine with the brilliance of the Sun, which it would do were the stars infinite in number. Of course, Herschel did not actually believe the Universe to be infinite, and, had he lived, he would probably have shown that all the star-clusters which we see are included within the bounds of our finite Galaxy.
In 1814 Herschel was “still engaged in a series of observations for ascertaining a scale whereby the extent of the Universe, as far as it is possible for us to penetrate into space, may be fathomed.” In 1817 he described another method of star-gauging, which Arago and other writers have confused with that which he devised in 1785. The two methods, however, were quite distinct from each other. In the first system, one telescope was used on different regions of the heavens; whereas in the second method, various telescopes were used on identical regions. The principle was that the telescopic power necessary to resolve groups of stars indicates the distance at which the stars of the groups lie. This, however, also assumed an equal distribution of stars, and as the late Mr Proctor says, “I conceive that no question can exist that the principle is unsound, and that Herschel would himself have abandoned it had he tested it earlier in his observing career.... In applying it, Sir W. Herschel found regions of the heavens very limited in extent, where the brighter stars (clustered like the fainter) were easily resolved with low powers, but where his largest telescopes could not resolve the faintest. These regions, if the principle were true, must be long, spike-shaped star groups, whose length is directed exactly towards the astronomer on Earth,—an utterly incredible arrangement.”
Herschel, at the time of his death, left unsolved the problem of the construction of the heavens. It is still unsolved, and will doubtless remain so until astronomers know more about the distances and motions of the stars. His last observation of the Galaxy showed that even with his 40-foot reflector he could not fathom it. Consequently, as we have mentioned, Struve and his successors regarded the Universe as infinite—a theory which has now received its death-blow. Herschel was undoubtedly correct when he stated his belief in a limited Universe.
Herschel’s star-gauges, and those of his son, still remain of immense value to astronomers in any discussion of the construction of the heavens. Thus, although they failed to reveal to Herschel the structure of the Universe, they have been of much use to his successors. Herschel’s discussion of the supreme problem—the ultimate object of his observations—constitutes one of the most interesting chapters in the history of science, and marks a new era in human thought. In the words of Miss Clerke: “One cannot reflect without amazement that the special life-task set himself by this struggling musician—originally a penniless deserter from the Hanoverian Guard—was nothing less than to search out the ‘construction of the heavens.’ He did not accomplish it, for that was impossible; but he never relinquished, and, in grappling with it, laid deep and sure the foundations of sidereal science.”
CHAPTER III.
THE SUN.
Four years after the death of Herschel, an apothecary in the little German town of Dessau procured a small telescope, with which he began to observe the Sun. The name of this apothecary was Samuel Heinrich Schwabe (1789-1875). In 1826 he commenced to observe the spots on the Sun’s disc, counting them from day to day, more for self-amusement than from any hope of discovery; for previous astronomers had agreed that no law regulated the number of the sun-spots. Every clear day Schwabe pointed his telescope at the Sun and took his record of the spots; this he continued for forty-three years, until within a few years of his death on April 11, 1875. As early as 1843 Schwabe hinted that a possible period of ten years regulated the distribution of the spots on the Sun, but no attention was given to his idea. In 1851, however, the result of his twenty-six years of observation was published in Humboldt’s ‘Cosmos,’ and Schwabe was able to show that the spots increased and decreased in a period of about ten years. Astronomers at once recognised the importance of Schwabe’s work, and in 1857 he was rewarded by the Gold Medal of the Royal Astronomical Society of London.
Rudolf Wolf (1813-1892) of the Zürich Observatory now undertook to search through the records of sun-spot observation, from the days of Galileo and Scheiner, to find traces of the solar cycle discovered by Schwabe. He was successful, and was enabled to correct Schwabe’s estimate of the length of the period, fixing it as on the average 11·11 years. Additional interest, however, was given to Schwabe’s and Wolf’s investigations by the remarkable discoveries which followed. In September 1851 John Lamont (1805-1879), a Scottish astronomer,—born at Braemar in Aberdeenshire, but employed as director of the Munich Observatory,—after searching through the magnetic records collected at Göttingen and Munich, discovered that the magnetic variations indicated a period of 10⅓ years. Soon after this Sir Edward Sabine (1788-1883), the English physicist, from a discussion of an entirely different set of observations, independently demonstrated the same thing, proving conclusively that once in about ten years magnetic disturbances reached their height of violence; and Sabine was not slow to notice the correspondence between the magnetic period and the sun-spot period. In the same year (1852) Wolf and Alfred Gautier (1793-1881) independently made the same discovery, which had thus been made by four separate investigators.