Abstract
Hecht and Pirenne 1 attempted to measure the minimum visual threshold of the nocturnal long-eared owl by comparing the intensity of monochromatic light necessary to cause a pupillary constriction of 0.5 mm in the dark adapted owl with the intensity necessary to do the same in the human, thus assuming that the relationship between thresholds for intensity discrimination and pupillary constriction is the same in both species. Their data for the human pupil were taken from measurements on 6 subjects made by Reeves in 1918 and 1920. 2
Reeves'data, although unique, cannot be taken as too accurate at low intensities because of too few measurements. Reeves took as the first point on his curve the size of the pupil after 15 minutes in the dark. He assumed this to be its size at the visual threshold which was assigned an arbitrary value of -6 log intensity units. At his next point, -3.8 log intensity units, the pupil already shows a distinct contraction. One cannot be sure at what point in the curve the pupil starts to contract, and consequently any reasonably accurate judgment cannot be made as to the relationship between visual threshold and threshold for pupillary contraction.
Reeves'work is here repeated more carefully, using as subjects both the human and the rabbit, and comparison is made between their pupillary thresholds. If Hecht and Pirenne are justified in their assumptions, then the data here obtained may offer information regarding the visual threshold in the rabbit gained by an objective method.
The method used is to be described in detail elsewhere. It is a modification of the infrared photographic method described by Gullberg, Olmsted, and Wagman, 3 and enables the pupil diameter to be measured under any condition of light or dark adaptation, as well as the subjective visual threshold.
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