Some ray diagrams may also show a third ray. Peep holes contain concave lenses. They are set into doors so the occupant can identify a visitor before opening the door.

In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.

A DISCUS SIGN on the determination of equilibrium diagrams by X-ray methods arranged by the Institute of Physics was held at the Royal Institution, London, on September 22. The chairman ...