The optic axis of the globe only exceptionally coincides with the visual axis. The angle formed by these two axes has had several denominations, according to the points of reference. Thus the angle formed by these two axes when their intersection is at the nodal point of the eye had been called angle alpha by Helmholtz.1 The angle formed by the line uniting the rotation center of the eye to the point of fixation with the optic axis is the angle gamma (Donders2). The angle formed by the central pupillary line, that is, a line perpendicular to the cornea and passing through the center of the pupil and the visual axis, is the angle kappa (Landolt3 ). Helmholtz' angle alpha was called beta by Brubaker,4 and Landolt's angle kappa was christened delta by Howe.5 In 1866 Helmholtz6 himself gave a definition of angle alpha, according
Alvaro ME. IMPORTANCE OF THE ANGLE DELTA IN LOCALIZING INTRAOCULAR FOREIGN BODIES: Report of Illustrative Case. Arch Ophthalmol. 1939;22(6):1078–1082. doi:10.1001/archopht.1939.00860120150011
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