Star-land; being talks with young people about the wonders of the heavens (1899)
Ball, Robert Stawell, Sir, 1840- [from old catalog] · Boston and London, Ginn & company · leaf 248
ws the labor of giving it the true figure andpolish. It is not only more fragile than glass, but it isalso quite as hard, and therefore the grinding is a tedi-ous operation. First the surface has to be ground withcoarse sand, and then with emery, which is graduallymade finer and finer until the… … would perhaps be not more than the twentieth of aninch. Small though this depression is, yet it has to bemade with exactness. In fact, if it were wrong at anypoint by so much as the tenth of the thickness of thissheet of paper, the telescope would not perform accu-rately. The tool that is used in…
Astronomy
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