Optometrist's Pupil Distance Measuring Rule

A British military device for measuring the distance between pupils. This is an aluminum ruler with special openings for each eye and the nose, including nose pads. The instrument was place on the nose bridge. The distance is measured by sliding the ruler until the moving cross- line is directly in front of the left pupil. The measurement is indicated in millimeters. Pupil distance is necessary for making prescription eyeglasses. Positioning lenses correctly, directly in front of each pupil is very important especially in high powered lenses. The inter-pupillary distance is a distance between centers of pupils of left and right eyes. It is measured while a person looks forward at a distant object. The ruler is marked Theodore Hamblin London W1. MKIII and has the wide arrow which attributes the ruler to the British military. Overall length 18 cm. Comes in original fitted case.