constructor, by the discovery of the companion to Sirius. [Illustration: FIG. 3.--DORPAT REFRACTOR.] The next step was made on our side of the Atlantic, and proved to be a long and notable one, in a sense definitely marking out the boundary line of the modern era of giant refractors. This was the completion, by Thomas Cooke, of York, of a 25-inch instrument for the late Mr. Newall. It did not retain for long its pride of place. The palm was speedily taken back to America by Alvan Clark's construction of the 26-inch of the Washington Naval Observatory, with which Professor Asaph Hall discovered in 1877 the two satellites of Mars. Then came Grubb's 27-inch for Vienna; the pair of 30-inch instruments, by Clark and Henry respectively, for Pulkowa (Fig. 4) and Nice; and at last the instrument which has for a number of years been regarded as the finest example of optical skill in the world, the 36-inch Clark refractor of the Lick Observatory, California. Placed at an elevation of over 4,000 feet, and in a climate exceptionally well suited for astronomical work, this fine instrument has had the advantage of being handled by a very remarkable succession of brilliant observers, and has, since its completion, been looked to as a sort of court of final appeal in disputed questions. But America has not been satisfied even with such an instrument, and the 40-inch Clark refractor of the Yerkes Observatory is at present the last word of optical skill so far as achromatics are concerned (Frontispiece). It is not improbable that it may also be the last word so far as size goes, for the late Professor Keeler's report upon its performance implies that in this splendid telescope the limit of practicable size for object-glasses is being approached. The star images formed by the great lens show indications of slight flexure of the glass under its own weight as it is turned from one part of the sky to another. It would be rash, however, to say that even this difficulty will not be overcome. So many obstacles, seemingly insuperable, have vanished before the astronomer's imperious demand for 'more light,' and so many great telescopes, believed in their day to represent the absolute culmination of the optical art, are now mere commoners in the ranks where once they were supreme, that it may quite conceivably prove that the great Yerkes refractor, like so many of its predecessors, represents only a stage and not the end of the journey. [Illustration: FIG. 4.--30-INCH REFRACTOR, PULKOWA OBSERVATORY.]