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This brilliant result determined him to make a “front-view” of the forty-foot. Its advance towards completion was not without vicissitudes. The first speculum, when put into the tube, February 19th 1787, was found too thin to maintain its shape. A second, cast early in 1788, cracked in cooling. The same metal having been recast February 16th, the artist tried it upon Saturn in October; but the effect disappointing his expectation, he wrought at it for ten months longer. At last, after a few days’ polishing with his new machine, he turned the great speculum towards Windsor Castle; when its high quality became at once manifest. And such was his impatience to make with it a crucial experiment, that—as he told Sir Joseph Banks—he directed it to the heavens (August 28th, 1789) before it had half come to its proper lustre. The stars came out well, and no sooner had he got hold of Saturn than a sixth satellite stood revealed to view! Its “younger brother” was detected September 17th; and the two could be seen, on favourable opportunities, threading their way, like beads of light, along the lucid line of the almost vanished ring. Herschel named them Enceladus and Mimas, and found, on looking up his former observations of Saturn, that Enceladus, the exterior and brighter object, had been unmistakably seen with the twenty-foot, August 19th, 1787. Mimas is a very delicate test of instrumental perfection.