The relation between foveal intensity threshold and length of an illuminated slit.

Abstract

The authors describe a method of determining accurately for the human fovea intensity thresholds of narrow illuminated slits for comparison with similarly obtained thresholds of circular areas. The threshold is high for short slits and decreases as the slit increases in length. The rate with which the threshold decreases is markedly less as a function of length of slit than as a function of diameter of illuminated circle. Astigmatic effects which might appear in the expt. actually do not influence the decrease of threshold with increase of slit length. Further decrease in width of the narrow slit apparently operates to increase the threshold uniformly, irrespective of length of slit. The results are interpreted quantitatively in terms of a spatial gradient of visual excitation.