Note 9 — Chapter III
Reading edition
This mode of finding the focus of a concave lens may be varied as follows:—let the lens be covered with paper, having two small circular holes; and on the paper for receiving the light, describe also two small circles, but with their centres at twice the distance from each other of the centres of the circles. Then move the paper to and from, till the middle of the sun’s light, coming through the holes, falls exactly on the middle of the circles; that distance of the paper from the lens will be the focal length required.