...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」
...Such microscopesmay be good enough for the generality of purposes, butthey do not, with glasses of given focal length, affordthe same magnifying power as is done by instrumentsof better construction...
Henry J. Slack 「Marvels of Pond-life」
...The focal attention iscentred on the Infanta, with the figures on either side more orless out of focus, those on the extreme right being quite blurred...
Harold Speed 「The Practice and Science Of Drawing」
...Icould vary the focal point from a fewfeet to several miles...
Various 「Astounding Stories of Super-Science, March 1930」
...In 1665 Cassini, with a fine telescope, 35-feet focal length, observedmany spots moving from east to west, whence he concluded that Jupiter rotateson an axis like the earth...
George Forbes 「History of Astronomy」
...'The whole immense powerof the telescope was applied, and to its focal image about one half ofthe power of the microscope...
Richard A. Proctor 「Myths and Marvels 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'」
...The distance of Ffrom AB is called the focal length of the lens, and F itselfis called the focus...
George C. Comstock 「A Text-Book of Astronomy」
...On the other hand, the eye-glass mustnot be of such small focal length that the image appears indistinct andcontorted, or dull for want of light...
Richard A. Proctor 「Half-hours with 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」
...The Huygens eyepiece consists of twoplano-convex lenses whose focal lengths are in the ratioof three to one...
Garrett Serviss 「Pleasures of the telescope」
...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」
...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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