SUMMARY1. The ability of rhesus monkeys to detect the gap in Landolt ring testobjects that were presented against background luminances between 5 x 10-5 cd/M2 and 5 x 103 cd/M2 was compared with similar human data.2. At high luminance-levels the acuity of human observers is slightly better than that of rhesus, but rhesus have better acuity at scotopic luminance-levels. Both species have distinct photopic and scotopic acuity functions that cross at 6 x 103 cd/M2.3. The threshold for light detection is estimated to be the same for both species when specified in quanta incident on the retina.4. It is concluded that the receptor and neural mechanisms that mediate visual-acuity function similarly in rhesus and man, and that the differences in acuity that were measured in the two species may be attributed to optical rather than to physiological factors.

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Maximal visual acuity and normal color vision are closely associated with foveal receptor function. When the fovea is destroyed or altered by intense laser irradiation, severe and sometimes permanent losses in visual acuity ensue. In this paper we have shown that the threshold for long-term functional alterations in the fovea is determined not only by the wavelength and corneal exposure power of a laser flash but also by the types of performance criteria used to assess visual functioning. Chromatic acuity provides a much lower threshold for permanent alterations (Krypton — 6 mW; Argon — 3 mW) as opposed to achromatic targets (HeNe — 11 mW). Further, the use of chromatic acuity to assess post-exposure functioning can more accurately delineate the specific cone process most affected by the exposing source. The data from these studies also suggest that some relatively long-term (24 h) cumulative process within the retina is occurring which obviously will affect any threshold value determined.