Luminosity functions have been measured for four normal trichromats for three field sizes (1°, 12′, 1.5′) at various levels of retinal illuminance. As the illuminance decreases the reverse Purkinje effect is observed, i.e., there is a decrease in spectral sensitivity of the eye to blue relative to red. The dependence of the spectral sensitivity on field size is not pronounced for the experimental conditions used here. Considerable differences are found between the luminosity functions of the observers. These differences are several times larger than those obtained on repeated curves by any observer. It is shown that the use of an average luminosity function may give very misleading results in some cases.

A comparison is made with the data obtained by previous workers.

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