Hawkins electrical guide ... : questions, answers & illustrations : a progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications ; a practical treatise (1914)
Hawkins, N. (Nehemiah), 1833-1928 · New York : T. Audel & Co. · leaf 31
rst three are thicker at the center than at theborders, and are called converging; the second three, which are thinner at the center arecalled diverging. In lenses whose two surfaces are spherical, the centers of these surfacesare called centers of curvature, and the right line which passes through… … Figs 3 784 and 3,785.—The principal focus. By definition, it is, that point where all the raysparallel to the principal axis meet after reflection, as, for instance, the rays from a source oflight at an infinite distance from the mirror. The sun is so far distant that its rays arepractically…
Electrical engineering
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