The Popular recreator. A key to in-door and out-door amusements .. (1873)
London, New York [etc.] Cassell, Petter & Galpin · leaf 726
opticians Fig- 1- or mechanicsworkshops, assuch an arrange-ment makes themindependent ofextraneous aid. Next we have toconsider the ques-tion of the bestfocal length for acondenser. In oldforms they may befound as long asfour inches backfocus, that is tosay, the distanceat which parallelrays (as… … take three condensers of equal diameter, say 4 inches—of therespective focal lengths of 4, 3, and 2^-inch focus, that, whilethe first will only pick up 50°, and the second 66°, the third will collect 80° of the rays emittedfrom the source of light placed atthe focal point. Now, as the office of the…
Amusements · Games · Sports
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