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 33
istance FA, the principal focal distance. Fig. 3,788 shows the case in which the luminous source is outsidethe principal focus, but so near that all incident rays form a divergentpencil. MOTION PICTURES 2,687 Virtual Foci.—A double convex lens has a virtual focus when theluminous object is placed… … Fig. 3,788.—Principal focus in double curve lenses. CASE II: Divergent rays from luminoussource. In the figure the luminous source being at L, by comparing the path of a divergingray LB, with that of a ray, SB, parallel to the axis, the former is found to make with thenormal, an angle LBN, greater…
Electrical engineering
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