Darkening of Red in Protanopes

THE degree of darkening of the Ilford Spectrum Red colour-filter was measured for twelve protanopes and compared with its brightness for eighteen deuteranopes and several hundred normal subjects. The light transmitted by the red filter from a daylight source was mixed with light transmitted by the Ilford Spectrum Green filter and matched for brightness against light transmitted by the Ilford Spectrum Yellow filter by varying the intensity of the yellow. The mean brightness-level for the eighteen deuteranopes was the same as for normal subjects for all red-green mixtures. For the twelve protanopes the red light was only 15·5 per cent as bright on the average as for normal subjects ; but the brightness-level steadily increased as green was added, until it reached the normal level at the pure green end.