Contradictory additivity data of previous investigators, defined as the summated response of the retina to light entering different parts of the pupil, have led to an intensive study of variables inherent in these measurements. The experimental results have demonstrated that the Maxwellian and the ordinary type of viewing may be used interchangeably in Stiles-Crawford and additivity effect experiments. The experiments conducted in this research have revealed that additivity data are affected by at least the following two factors: the Stiles-Crawford effect and the blur of the retinal image. When blur is eliminated in additivity studies, the Stiles-Crawford effect does not reduce perceived brightness as much as might be predicted. The blur effect, caused by ocular aberrations, spherical and chromatic aberrations in particular, is a variable dependent upon the observer. This variability is thought to account for the several discrepancies in the data found in the literature. Experiments concerning the effect of blur on perceived brightness have been completed. It was shown that blur reduces perceived brightness of a field, and the degree of the reduction is dependent upon the test method.
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