The Nagel Anomaloscope and a simple filter anomaloscope were used in testing 103 subjects with deficient color perception for red and green. The filter anomaloscope differentiates the deuteranomalous (DA) and the protanomalous types (PA), but does not distinguish types of defect among dichromats.
A small number of subjects seemed to have a transitional defect between anomalous trichromasy and dichromasy. Evidence is presented suggesting that many if not all of these are actually dichromats, who give atypical responses because differences in saturation are mistaken for differences in hue.
The two instruments agreed closely in their classifications of subjects as to type and degree of defect. The filter anomaloscope is the simpler in procedure and in interpretation of results.
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