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To the Editor:
—More than 200 devices or methods for testing or measuring the color vision have been proposed, described or used. Some of these, such as after-image tests, are only of academic interest; some are very complicated, requiring elaborate apparatus and expert knowledge (luminosity curves, hue and chroma discrimination curves, etc.); some are simple screening devices aimed to separate observers with defective color vision from those with normal color vision (screening tests, e. g., the Ishihara, Rabkin, Stilling and American Optical Company polychromatic plates); some are designed to go further and to classify the type of defect (qualitative color vision tests, e. g., the Nagel anomaloscope, Farnsworth profile, Hardy-Rand-Rittler plates, Nagel cards, Trendelenburg dots, Schaaff mosaic), and some are intended not only to separate subjects with defective color vision from those with normal color vision and to diagnose the type of defect but also to yield an estimation of