The ardour with which he pursued the inquiry betrays itself in the rapid succession of four masterly essays communicated to the Royal Society in 1800. They contained the first exposition worth mentioning of the properties of radiant heat. They gave the details of experiments demonstrating its obedience to the same laws of reflection, refraction, and dispersion as light; and showing the varieties in the absorptive action upon it of different substances. In the third memoir of the series, Professor Holden finds himself at a loss “which to admire most—the marvellous skill evinced in acquiring such accurate data with such inadequate means, and in varying and testing such a number of questions as were suggested in the course of the investigation—or the intellectual power shown in marshalling and reducing to a system such intricate, and apparently self-contradictory phenomena.” There is, indeed, scarcely one of Herschel’s researches in which his initiative vigour and insight are more brilliantly displayed than in this parergon—this task executed, as it were, out of hours. It is only a pity that he felt compelled, by the incompatibility of their distribution in the spectrum, to abandon his original opinion in favour of the essential identity of light and radiant heat. The erroneous impression left on the public mind by his recantation has hardly yet been altogether effaced.
CHAPTER V.
THE INFLUENCE OF HERSCHEL’S CAREER ON MODERN ASTRONOMY.
The powers of the telescope were so unexpectedly increased, that they may almost be said to have been discovered by William Herschel. No one before him had considered the advantages of large apertures. No one had seemed to remember that the primary function of an instrument designed to aid vision is to collect light. The elementary principle of space-penetration had not been adverted to. It devolved upon him to point out that the distances of similar objects are exactly proportional to the size of the telescopes barely sufficing to show them. The reason is obvious. Compare, for instance, a one-inch telescope with the naked eye. The telescope brings to a focus twenty-five times as much light as can enter the pupil, taken at one-fifth of an inch in diameter; therefore it will render visible a star twenty-five times fainter than the smallest seen without its help; or—what comes to the same thing—an intrinsically equal star at a five-fold distance. A one-inch glass hence actually quintuples the diameter of the visible universe, and gives access to seventy-five times the volume of space ranged through by the unassisted eye.
This simple law Herschel made the foundation-stone of his sidereal edifice. He was the first to notice it, because he was the first practically to concern himself with the star-depths. The possibility of gauging the heavens rose with him above the horizon of science. Because untiring in exploration, he was insatiable of light; and being insatiable of light, he built great telescopes.
His example was inevitably imitated and surpassed. Not through a vulgar ambition to “beat the record,” but because a realm had been thrown open which astronomers could not but desire to visit and search through for themselves. Lord Rosse’s six-foot reflector was the immediate successor of Herschel’s four-foot; Mr. Lassell’s beautiful specula followed; and the series of large metallic reflectors virtually closed with that of four-feet aperture erected at Melbourne in 1870. The reflecting surface in modern instruments is furnished by a thin film of silver deposited on glass. It has the advantage of returning about half as much again of the incident light as the old specula, so that equal power is obtained with less size. Dr. Common’s five-foot is the grand exemplar in this kind; and it is fully equivalent to the Parsonstown six-foot.
The improvement of refractors proceeded more slowly. Difficulties in the manufacture of glass stood in the way, and difficulties in the correction of colour. The splendid success, however, of the Lick thirty-six inch, and the fine promise of the Yerkes forty-inch, have turned the strongest current of hope for the future in the direction of this class of instrument. But all modern efforts to widen telescopic capacity primarily derive their impulse from Herschel’s passionate desire to see further, and to see better, than his predecessors.
His observations demonstrate the rare excellence of his instruments. Experiments made on the asteroid Juno, in 1805, for the purpose of establishing a valid distinction between real and fictitious star-discs, prove, in Professor Holden’s opinion, the reflector employed to have been of almost ideal perfection; and his following of Saturn’s inner satellites right up to the limb, with the twenty-foot and the forty-foot, was a tour de force in vision scarcely, if ever, surpassed.
In the ordinary telescopes of those days really good definition was unknown; they showed the stars with rays or tails, distorted into triangles, or bulged into “cocked hats;” clean-cut, circular images were out of the question. Sitting next Herschel one day at dinner, Henry Cavendish, the great chemist, a remarkably taciturn man, broke silence with the abrupt question—“Is it true, Dr. Herschel, that you see the stars round?” “Round as a button,” replied the Doctor; and no more was said until Cavendish, near the close of the repast, repeated interrogatively, “Round as a button?” “Round as a button,” Herschel briskly reiterated, and the conversation closed.
It seems probable that Herschel’s caput artis lost some of its fine qualities with time. Great specula are peculiarly liable to deterioration. Their figure tends to become impaired by the stress of their own weight; their lustre is necessarily more or less evanescent. Re-polishing, however, is a sort of re-making; and the last felicitous touches, upon which everything depends, can never be reckoned upon with certainty. Hence, the original faultlessness of the great mirror was, perhaps, never subsequently reproduced.