An instrument for measuring the distance between the centres of rotation of the two eyes

A knowledge of the distance between the two eyes being required in dealing with the problems of binocular vision, the instruments described in this paper may be of interest to physiologists. All those methods which involve measurement of the distance between the pupils (or other external parts of the eyes) are liable to errors, for the following reasons:— 1. It is difficult to eliminate parallax between the scale or index of the measuring instrument and the eyes. 2. The distance between the pupils is affected by the convergence of the optical axes, and still more by the abnormal direction of either. 3. It is difficult to ensure the eyes being kept still during the process of measuring.