APA Dictionary of Psychology

a figure used to test simultaneously for visual acuity and astigmatism. Outlines of circles are interrupted by two small gaps, 90° apart, and the minimum gap size that can be detected is a measure of acuity (similar to Landolt circles). If one gap is more detectable than the other, it suggests that astigmatism is present. [Clarence Ferree (1877–1942) and Gertrude Rand (1886–1970), U.S. psychologists]