...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」
...A pond, a little more than an acre in size, was a focal area foropossums and more were caught there than on any other part ofthe Reservation...
Henry S. Fitch 「Ecology of the Opossum on a Natural Area in Northeastern Kansas」
...The diameter and focal length of a lens must depend in a great measureon the distance of the object, and also on the superficies of the plateor paper to be covered...
Henry H. Snelling 「The History and Practice of the Art of Photography」
...The lines A B C represent parallel rays which pass through the lens, D D,and meet at F; this point being the principal focus, its distance from thelens is called the focal length...
Samuel D. Humphrey 「American Handbook of the Daguerrotype」
...With that apparatusI could throw a beam of negativeheat with a focal point which Icould adjust at will...
Various 「Astounding Stories of Super-Science, March 1930」
...And, dominating it, in the center of that flabby forehead, a focal pointfor the gaze of the horrified observers, was a single glassy and lidlesseye...
Charles Willard Diffin 「Brood of the Dark Moon」
...5 feet focal length, and another, of 12 inches diameter and 18 feetfocal length, for Lamont, of Munich...
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」
...The first was made with a Newtonian telescope something less than seven feet focal length, a power of 222, and an aperture of four and a half inches...
Edward Singleton Holden 「Sir William Herschel: His Life and Works」
...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」
...The Huygens eyepiece consists of twoplano-convex lenses whose focal lengths are in the ratioof three to one...
Garrett Serviss 「Pleasures of the telescope」
...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 diameter of this object-glass isthirty-six inches, and its focal length is fifty-six feet two inches...
Robert Stawell Ball 「The Story of the Heavens」
...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」
...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」
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