Page:Popular Science Monthly Volume 26.djvu/404

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390
THE POPULAR SCIENCE MONTHLY.

to the essential point—the quality of telescopic vision—the testimony of Mr. S. W. Burnham is in the highest degree encouraging. This well-known observer spent two months on the mountain in the autumn of 1879, and concluded, as the result of his experience during that time—with the full concurrence of Professor Newcomb—that "it is the finest observing location in the United States." Out of sixty nights he found forty-two as nearly perfect as nights can well be, seven of medium quality, and only eleven cloudy or foggy;[1] his stay, nevertheless, embraced the first half of October, by no means considered to belong to the choice part of the season. Nor was his trip barren of discovery, A list of forty-two new double stars gave an earnest of what may be expected from systematic work in such an unrivaled situation. Most of these are objects which never rise high enough in the sky to be examined with any profit through the grosser atmosphere of the plains east of the Rocky Mountains; some are well-known stars, not before seen clearly enough for the discernment of their composite character; yet Mr. Burnham used the lesser of two telescopes—a 6-inch and an 18-inch achromatic—with which he had been accustomed to observe at Chicago.

The largest refracting telescope as yet actually completed has a light-gathering surface twenty-seven 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 & 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 disk of flint-glass for the object-lens, 38·18 inches across, and one hundred and seventy 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 two thousand pounds sterling.[2] 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.[3] In this quality, then, the Lick

  1. "The Observatory," No. 43, p. 613.
  2. "Nature," vol. xxv, p. 537.
  3. Silvered glass is considerably more reflective than speculum-metal, and Mr. Common's 36-inch mirror can be but slightly inferior in luminous capacity to the Lick objective. It is, however, devoted almost exclusively to celestial photography, in which it has done splendid service. The Paris 4-foot mirror bent under its own weight when placed in the tube in 1875, and has not since been remounted.