Made by Norton & Gregory in the early 20th century, this large optical square for use by surveyors is engraved on the brass drum 'NORTON & GREGORY Ld, LONDON & GLASGOW’ who were instrument makers active from the latter part of the 19th century and well into the 20th century. There are also the initials W.R.M who was presumably a previous owner.

Viewing is through the pinhole viewer, and unusually for this type of instrument there is a separate brass key for adjusting the mirror angles, which unscrews when required. There is a twist action to close all apertures when not in use, and the two drums can be dismantled for cleaning and adjusting the mirrors by simply unscrewing the retaining screw on the side and pulling the two halves apart.

In excellent condition including the mirrors, it comes with its original leather case, which is also in very good sound condition with all stitching intact.

Measuring 75mm in diameter and 28mm in depth, it is a well made instrument with a substantial feel to it, it weighs just under 400 grams in its case.