Nonius alignment apparatus for measuring vergence

The present paper describes an apparatus designed for the assessment of vergence. Based on the Nonius alignment system of viewing dichoptic stimuli, this apparatus provides a compact, portable, easily constructed, and easily maintained piece of equipment. It can be used for the assessment of vergence under a wide variety of viewing conditions and laboratory demands. Also included are an example of one way in which the apparatus may be set up, the equations for finding vergence angle using that set-up, and data concerning the validity and reliability of measurements made with the apparatus.