FOCUS n.
as from a near object; -- so called because the positions of the object and its image are interchangeable. -- Focus tube (Phys.), a vacuum tube for Roentgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect. -- Principal, or Solar, focus (Opt.), the focus for parallel rays.…
IN prep.
t. In its different applications, it approaches some of the meanings of, and sometimes is interchangeable with, within, into, on, at, of, and among. It is used: --