...The magnifying power of a compound microscopedepends upon the focal length of the object-glass (orglass nearest the object), upon the length of the tube,and the power of the eye-piece... 
Henry J. Slack 「Marvels of Pond-life」
... The outer lens is a meniscus--that is bounded by aconcave and convex spherical surface which meet--having a focal lengthof 18 inches...
Henry H. Snelling 「The History and Practice of the Art of Photography」
...But Lord Rosse’s 6-foot reflector, 54feet focal length, constructed in 1845, is still the largest ever made...
George Forbes 「History of Astronomy」
...By moving it to and fro a little you will be ableto find a distance, corresponding to the focal length of thelens, at which a picture of the window is formed on thepaper...
Garrett Serviss 「Pleasures of the telescope」
...With such a lens the magnifying power of the telescopeis measured by the ratio of the focal length of the objectiveto that of the eye lens...
Garrett Serviss 「Pleasures of the telescope」
...Next let us return to the focus and then movethe eyepiece gradually inside the focal point or plane...
Garrett Serviss 「Pleasures of the telescope」
...—All our astronomical telescope have objectives of thestandard focal length (focus = to about 15 times diameter of aperture)...
Wm. Gaertner & Co. 「Astronomical Instruments and Accessories」
...5 feet focal length, and another, of 12 inches diameter and 18 feetfocal length, for Lamont, of Munich...
George Forbes 「History of Astronomy」
...These he fixed in apiece of organ pipe, which served the purpose of a tube, the glassesbeing distant from each other by the difference of their focal lengths...
Thomas Orchard 「The Astronomy of Milton's 'Paradise Lost'」
...As a camera obscura is actually atelescope, magnifying objects in proportion to the focal length of thelens employed, he may be said to have first seen these spots with theaid of an optical instrument...
David Brewster 「The Martyrs of Science, or, The lives of Galileo, Tycho Brahe, and Kepler」
...It is clear that theshorter the focal length of ab, the nearer must ab be placed to theimage, and the smaller will the emergent pencils be, but the greater themagnifying power of the eye-glass...
Richard A. Proctor 「Half-hours with the Telescope」
...If no new adjustment is required for distinct vision, the sphericalaberration is corrected, since the mean focal length and the focallength of the central rays are equal...
Richard A. Proctor 「Half-hours with the Telescope」
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