On the authority of Clark, even a six-foot objective would not necessitate a combined thickness of its glasses in excess of six inches. Present disks are vastly superior to the early ones in transparency, and there is reason to expect still greater improvement. The engineering troubles incident to execution of the mechanical side of the scheme need not stand in the way; they never have, indeed the astronomer has but just begun to invoke the fertile resources of the modern engineer. Not long before his death the younger Clark who had just finished the great lenses of the 40-inch Yerkes telescope, ventured this prevision, already in part come true: "The new astronomy, as well as the old, demands more power. Problems wait for their solution, and theories to be substantiated or disproved. The horizon of science has been greatly broadened within the last few years, but out upon the borderland I see the glimmer of new lights that await for their interpretation, and the great telescopes of the future must be their interpreters." Practically all the great telescopes of the world have in turn signalized the new accession of power by some significant astronomical discovery: to specify, one of Herschel's reflectors first revealed the planet Uranus; Lord Rosse's "Leviathan" the spiral nebulæ; the 15-inch Cambridge lens the crape, or dusky ring of Saturn; the 18-1/2-inch Chicago refractor the companion of Sirius; the Washington 26-inch telescope the satellites of Mars; the 30-inch Pulkowa glass the nebulosities of the Pleiades; and the 36-inch Lick telescope brought to light a fifth satellite of Jupiter. At the time these discoveries were made, each of these great telescopes was the only instrument then in existence with power enough to have made the discovery possible. So we may advance to still farther accessions of power with the expectation that greater discoveries will continue to gratify our confidence. CHAPTER XX THE STORY OF THE SPECTROSCOPE Sir Isaac Newton ought really to have been the inventor of the spectroscope, because he began by analyzing light in the rough with prisms, was very expert in optics, and was certainly enough of a philosopher to have laid the foundations of the science.