A high-school astronomy: in which the descriptive, physical, and practical are combined .. (1859)
Mattison, Hiram, 1811-1868 · New York, Mason brothers · leaf 221
nhour of twilight both morning and evening. In the arctic regions, the sun is never more than 18° below the horizon; so that thetwilight continues during the whole night. 497. In making astronomical observations, for the pur-poses of navigation, &c, allowance has to be made forrefraction,… … REFRACTION BY GLASS LENSES. 498. A lens is a piece of glass or other transparentsubstance, of such a form as to collect or disperse therays of light that are passed through it, by refractingthem out of a direct course. They are of different forms,and have different powers. V LENSES OP DIFFERENT…
Astronomy
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