The concave part of Hertel's exophthalmometer which contacts the orbita functions like an inclined plane. Therefore, the measurement is sensitive against tilting and changing of the basic distance. We modified this contact surface into a right angle. The tangents at the contact points form a common line at right angles to the measurement direction. Reproducibility of measurements was studied on 10 patients who were measured by 5 to 12 different examiners with both instruments. Accuracy improved significantly by 57%.