Coddington, Henry, -1845 author · Cambridge : Printed by J. Smith, printer ot the University, for J. Deighton & Sons, and G.B. Whittaker, Ave Maria Lane · leaf 86
this we derive another useful proportion, Qf : QA :: QA : Qq. S From either the equations or the proportions it will be easy to prove that when the distance of Q from the lens is varied, that is, when the place of Q is changed, the lens remaining fixed, the two foci move in the same direction. The… … 90. The distance Qq between the foci is represented by A — A", or A + &"■> according as the lens is concave or convex,
Geometrical optics
Internet Archive · Getty Research Institute · Public domain (published before 1931). Free to use for anything.
