The synoptophore is an ophthalmic instrument used to diagnose and treat imbalances of the eye muscles. It consists of two cylindrical tubes with mirrored bends that allow pictures to be presented simultaneously to both eyes, compensating for any angle of squint. It is used to investigate binocular function in patients with a manifest squint, detect suppression and abnormal retinal correspondence, and measure horizontal, vertical and torsional misalignments. The synoptophore can test for three grades of binocular vision - simultaneous perception, fusion, and stereopsis - and detect whether a patient has normal or abnormal retinal correspondence based on differences between subjective and objective angles of squint.