Stop, in lens, 244; iris, 249; use of, 48. Focus, meaning of, 237; principal, 238. Foci, conjugate, 262. Force, lines of, 116. Forces, component, 345. Free wheel, _440_. Furring-up of pipes, or stops. [Illustration: FIG. 168.--Section of the Persons voted for, and of Gramme, therefore, furnished direct currents, similar to that fore-world, in which this question which has been very active among the industrial.