A text-book of physics, largely experimental. On the Harvard college "Descriptive list of elementary physical experiments." (1891)
Hall, Edwin Herbert, 1855-1938, Bergen, Joseph Y. (Joseph Young), 1851-1917 · New York, H. Holt and Company · leaf 189
i66 PHYSICS. 141. Principal Focus of Concave Mirror.—The principal focus of a concave mirror is the point to which rays, comingto the mirror parallel to the principal axis, converge afterreflection. In other words, it is the point which marks theimage (real) of a very distant point on the principal…
Physics
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