Clinical study of colour vision.

THE analysis of colour defects and individual differences in normal colour vision requires an apparatus in which spectral stimuli are used. It should be possible to determine the following: (1) Relative luminous efficiency of spectral light (luminosity curve). (2) Threshold of wavelength discrimination in spectrum (hue discrimination curve). (3) Threshold of saturation discrimination in spectrum (purity discrimination curve). (4) Trichromatic coefficients of spectral colours (colour mixture curves). In clinical practice these tests are too time-consuming, and the examination has therefore to be limited to the determination of a few points on every curve at various wavelengths, but it is desirable that in some special cases the analysis should be carried out more completely with the same apparatus. The Nagel anomaloscope, the only apparatus in general use for detecting defective colour vision, was designed to make one colour match: between monochromatic yellow and a mixture of red and green in the spectrum. It is a direct vision spectroscope with a circular field of about 2 degrees diameter, divided into two halves. The lower half of the field is illuminated by monochromatic yellow light from a slit of variable width, which can be controlled by the right-hand screw. The upper half of the field is illuminated by a mixture of monochromatic red and green. How the beams are made monochromatic will be clear from the diagram of the optical system of the anomaloscope (Fig. 1). The ratio between the widths of the slits for red