Depth Perception Apparatus

The three needle apparatus has been widely used in studies of
Binocular Depth Perception. It was devised by Howard and
Dolman. This consists of an elongated box which conceals inside
three vertical rods or pins.These pins can be seen by a and the
inside of the box is illuminated. subject through a small window
Two of these rods are fixed and the one in the middle is movable,
resting on a pulley bottom wheeled on a road running completely
across the length of the box. A meter-scale is fixed to the groove
like opening on the top so that the exact position of the movable
rod can be read.