The largest refracting telescope as yet actually completed has a light-gathering surface 27 inches in diameter. This is the great Vienna equatorial, admirably turned out by Mr. Grubb, of Dublin, in 1880, but still awaiting the commencement of its exploring career. It will, however, soon be surpassed by the Pulkowa telescope, ordered more than four years ago on behalf of the Russian Government from Alvan Clark and Sons, of Cambridgeport, Massachusetts. Still further will it be surpassed by the coming “Lick Refractor.” It is safe to predict that the optical championship of the world is, at least for the next few years, secured to this gigantic instrument, the completion of which may be looked for in the immediate future. It will have a clear aperture of _three feet_. A disc of flint-glass for the object-lens, 38·18 inches across, and 170 kilogrammes in weight, was cast at the establishment of M. Feil, in Paris, early in 1882. Four days were spent and eight tons of coal consumed in the casting of this vast mass of flawless crystal; it took a calendar month to cool, and cost 2,000_l._[3] It may be regarded as the highest triumph so far achieved in the art of optical glass-making. A refracting telescope three feet in aperture collects rather more light than a speculum of four feet.[4] In this quality, then, the Lick instrument will have—besides the Rosse leviathan, which, for many reasons, may be considered to be out of the running—but one rival. And over this rival—the 48-inch reflector of the Melbourne observatory—it will have all the advantages of agility and robustness (so to speak) which its system of construction affords; while the exquisite definition for which Alvan Clark is famous will, presumably, not be absent. Already preparations are being made for its reception at Mount Hamilton. The scabrous summit of “Observatory Peak” has been smoothed down to a suitable equality of surface by the removal of 40,000 tons of hard trap rock. Preliminary operations for the erection of a dome, 75 feet in diameter, to serve as its shelter, are in progress. The water-supply has been provided for by the excavation of great cisterns. Buildings are rapidly being pushed forward from designs prepared by Professors Holden and Newcomb. Most of the subsidiary instruments have for some time been in their places, constituting in themselves an equipment of no mean order. With their aid Professor Holden and Mr. Burnham observed the transit of Mercury of November 7th, 1881, and Professor Todd obtained, December 6th, 1882, a series of 147 photographs (of which seventy-one were of the highest excellence) recording the progress of Venus across the face of the sun.