...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」
...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」
...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」
...—The amount by which a giventelescope magnifies depends upon the focal length of the objective(or mirror) and the focal length of the eyepiece, andis equal to the ratio of these two quantities...
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」
...telescope having a focal length of8½ feet:...
Richard A. Proctor 「Half-hours with the Telescope」
...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」
...No subliminal nor fringe of consciousness can rank in theintellectual life beside the burning focal center where the rays ofknowledge converge...
Dr. D.M. Brooks 「The Necessity of Atheism」
...Admitting the light of the sun through a narrowslit in a window-shutter, an inverted image of the slit can be thrown, by alens of suitable focal length, on the wall opposite...
George Forbes 「History of Astronomy」
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