At midsummer, 1774, Herschel removed from No. 7, New King Street, to a house situated near Walcot Turnpike, Bath. A grass-plot was attached to the new residence, and it afforded convenient space for workshops. For already he designed to “carry improvements in telescopes to their utmost extent,” and “to leave no spot of the heavens unvisited.” An unprecedented ambition! No son of Adam had ever before entertained the like. To search into the recesses of space, to sound its depths, to dredge up from them their shining contents, to classify these, to investigate their nature, and trace their mutual relations, was what he proposed to do, having first provided the requisite optical means. All this in the intervals of professional toils, with no resources except those supplied by his genius and ardour, with no experience beyond that painfully gained during the progress of his gigantic task.
Since the time of Huygens, no systematic attempt had been made to add to the power of the telescope. For the study of the planetary surfaces, upon which he and his contemporaries were mainly intent, such addition was highly desirable. But Newton’s discovery profoundly modified the aims of astronomers. Their essential business then became that of perfecting the theories of the heavenly bodies. Whether or not they moved in perfect accordance with the law of gravitation was the crucial question of the time. Newton’s generalisation was on its trial. Now and again it almost seemed as if about to fail. But difficulties arose only to be overcome, and before the eighteenth century closed the superb mechanism of the planetary system was elucidated. Working flexibly under the control of a single dominant force, it was shown to possess a self-righting power which secured its indefinite duration. Imperishable as the temple of Poseidon, it might be swayed by disturbances, but could not be overthrown.
The two fundamental conclusions—that the Newtonian law is universally valid, and that the solar system is a stable structure—were reached by immense and sustained labours. Their establishment was due, in the main, to the mathematical genius of Clairaut, D’Alembert, Lagrange, and Laplace. But refined analysis demands refined data; hence the need for increased accuracy of observation grew continually more urgent. Attention was accordingly concentrated upon measuring, with the utmost exactitude, the places at determinate epochs of the heavenly bodies. The one thing needful was to learn the “when” and “where” of each of them—that is, to obtain such information as the transit-instrument is adapted to give. In this way the deviations of the moon and planets from their calculated courses became known; and upon the basis of these “errors” improved theories were built, then again compared with corrected observations.
For these ends, large telescopes would have been useless. They were not, however, those that Herschel had in view. The nature of the orbs around us, not their motions, formed the subject of his inquiries, with which modern descriptive astronomy virtually originated. He was, moreover, the founder of sidereal astronomy. The stars had, until his career began, received little primary attention. They were regarded and observed simply as reference-points by which to track the movements of planets, comets, and the moon. Indispensable for fiducial purposes, they almost escaped consideration for themselves. They were, indeed, thought to lie beyond the reach of effective investigation. Only the outbursts of temporary stars, and the fluctuations of two or three periodical ones, had roused special interest, and seemed deserving of particular inquiry.
Of the dim objects called “nebulæ,” Halley had counted up half a dozen in 1714; Lacaille compiled a list of forty-two at the Cape, in 1752–55; and Messier published at Paris, in 1771, a catalogue of forty-five, enlarged to one hundred and three in 1781. He tabulated, only to rid himself of embarrassments from them. For he was by trade a comet-hunter, and, until he hit upon this expedient, had been much harassed in its exercise by mistakes of identity.
But Herschel did not merely “pick up;” he explored. This was what no one before him had thought of doing. A “review of the heavens” was a complete novelty. The magnificence of the idea, which was rooted in his mind from the start, places him apart from, and above, all preceding observers.
To its effective execution telescopic development was essential. The two projects of optical improvement and of sidereal scrutiny went together. The skies could be fathomed, if at all, only by means of light-collecting engines of unexampled power. Rays enfeebled by distance should be rendered effective by concentration. Stratum after stratum of bodies—
“Clusters and beds of worlds, and bee-like swarms
Of suns and starry streams,”
previously unseen, and even unsuspected, might, by the strong focussing of their feebly-surviving rays, be brought to human cognisance. The contemplated “reviews” would then be complete just in proportion to the grasp of the instrument used in making them.