Threshold sensitivity was studied in two observers along lines running vertically and horizontally through the region of the optic disk. When the test field fell entirely on the disk the luminance of test flashes of 0.04-sec duration could be raised to a level high enough to cause an excitation. Under these conditions the luminance may be 1 to 2 log units higher than that required for excitation of retinal regions adjacent to the disk. It is assumed that the excitation was brought about by scatter of light in the ocular media. This assumption was supported by the fact that (1) differences in sensitivity were found between the two observers due to changes in transmittance of the dioptric media with age, and (2) that when test fields of various sizes (1°, 2°, 5°, and 10°) were projected into the disk area and dark adaptation was studied, bipartite dark-adaptation curves were obtained which indicated the participation of rods and cones. The size of the disk area as determined by excitability measurements corresponds to that found in tangent screen tests. The nasal edge of the disk is more precisely defined than the temporal edge, a fact which is accounted for by the relatively greater sensitivity of the retinal elements in the nasal area.
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