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A scale for measuring the angle of heterophoria and heterotropia consisting of one horizontal and one vertical line in the form of a cross with a light source placed at the centre of intersection. The lines are graduated in prism dioptres or degrees and calibrated for use at a given distance (usually 6 metres). Syn. Maddox tangent scale.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann