Journal - Franklin Institute of the State of Pennsylvania (1826)
Franklin Institute, Philadelphia, Franklin Institute (Philadelphia, Pa.) · [Philadelphia : Franklin Institute] · leaf 401
e. The piano convex lensis therefore the only lens of which the focal length can be measureddirectly. If the plane side is placed towards a very distant object,the distance of the curved side to the image is the principal focus.It is often necessary to know the focal length of a lens or a… … Zentmayer—A Lecture on Lenses. 343 the lens for a very distant object, on a screen, and mark the positionof the screen. Do not move the lens but place a bright object, abouttwice the focus of the lens, in front of it as near as you can suppose ;now move the screen about the same distance from the…
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