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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—