With Bradley, in 1741, a new period commenced in instrumental astronomy, not so much of discovery as of measurement. The superior accuracy and minuteness with which the motions and distances of the heavenly bodies were now observed, resulted in the accumulation of a mass of new materials, both for tabular comparison and theoretical speculation. These materials formed the enlarged basis of astronomical science between Newton and Sir William Herschell. His gigantic reflectors introduced the astronomer to regions of space before unvisited--extended beyond all previous conception the range of the observed phenomena, and with it proportionably enlarged the range of constructive theory. The discovery of a new primary planet and its attendant satellites was but the first step of his progress into the labyrinth of the heavens. Cotemporaneously with his observations, the French astronomers, and especially La Place, with a geometrical skill scarcely, if at all, inferior to that of its great author, resumed the whole system of Newton, and brought every phenomenon observed since his time within his laws. Difficulties of fact, with which he struggled in vain, gave way to more accurate observations; and problems that defied the power of his analysis, yielded to the modern improvements of the calculus. HERSCHELL'S NEBULAR THEORY. But there is no _Ultima Thule_ in the progress of science. With the recent augmentations of telescopic power, the details of the nebular theory, proposed by Sir W. Herschell with such courage and ingenuity, have been drawn in question. Many--most--of those milky patches in which he beheld what he regarded as cosmical matter, as yet in an unformed state,--the rudimental material of worlds not yet condensed,--have been resolved into stars, as bright and distinct as any in the firmament. I well recall the glow of satisfaction with which, on the 22d of September, 1847, being then connected with the University at Cambridge, I received a letter from the venerable director of the Observatory there, beginning with these memorable words:--"You will rejoice with me that the great nebula in Orion has yielded to the powers of our incomparable telescope! * * * It should be borne in mind that this nebula, and that of Andromeda [which has been also resolved at Cambridge], are the last strongholds of the nebular theory."[A] [Footnote A: _Annals of the Observatory of Harvard College_, p. 121.]