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Updated: June 28, 2025
He thought out a plan by which he could place pieces of glass in a tube, and bring the stars close to the earth. By daylight the whole plan was clear in his mind, and he hastened to the shop of the glassmakers. There, two lenses were made, one plano-convex, and the other plano-concave, and these were placed in a tube made of sheet copper.
Every part of the immense observatory and of the telescope is of American production, with the single important exception of the cast glass disc from which the two principal lenses, the one double convex and the other plano-concave, are produced. These were cast by Mantois, of Paris, whose superiority to the American manufacturers of optical glass is recognized.
Flint glass effects a greater dispersion than crown glass nearly in the ratio of three to two. The chromatic combination consists of a convex lens of crown backed by a concave, or plano-concave, lens of flint. When these two lenses are made of focal lengths which are directly proportional to their dispersions, they give a practically colorless image at their common focus.
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