5 
gradual reappearance of Weber's law. In the worst 
case, the difference between the curves was approxi 
mately 1.5 seconds with a standard deviation of ap 
proximately 0.6 seconds. This is hardly significant, 
so the results appeared identical for measuring marks 
up to 18 minutes subtense. 
(iv) A near-threshold pointing error of a little over 1 
second of arc was obtained for measuring marks up to 
45 minutes of arc subtense provided that the width of 
the annulus was of the order of 0.15 minutes of arc. 
(v) In figure 2, the right-hand portions of the curve show 
that the difference between the 10-series and the 50- 
series was of the order of 0.5 seconds, with a standard 
error of 0.6 seconds. This cannot be considered sig 
nificant. Because there was less fatigue involved in 
the shorter series, the results were expected to be a 
little better. 
4. General Discussion of the Results 
The part of the retina stimulated and the size of the stimulus 
have long been known to exert a major influence on visual acuity. 
In photopic vision, acuity is a maximum in the fovea, and decreases 
towards the periphery. There seems to be remarkably little infor 
mation available on the critical region around the foveal center, 
but the indications are that acuity increases towards the center, 
not to a point, but to a plateau. Estimates of the diameter of this 
plateau range from 1 degree to 7 minutes of arc (3^ (7) (8) (9) (14). 
Fender and Nye (4), and Glezer (5), studying the role of feedback in 
eye movement control, postulate the existence of a positionally in 
sensitive zone subtending 4-6 minutes of arc in the foveal center. 
It is suggested that discrimination is maximum and constant over this 
zone. Investigations into acuity in this zone could be highly sig 
nificant, but as Ogle (11) points out, dioptric factors set a limit 
of about 6 minutes of arc for the minimal effective diameter of a 
disc subtending a visual angle less than 3-4 minutes. This would 
contribute to the apparent constancy of acuity in this range. 
The initial portion of the curves confirms the significance of 
the edge ribbon in visual tasks. Taylor (12) showed that circular 
targets, detected at threshold, appear as point sources for visual 
angles less than approximately 4 minutes, and pass smoothly into 
subjective annuli for larger diameters. For visual angles above 10 
minutes the transition is complete. This annulus is the locale of 
the luminance gradient and provides the significant visual infor 
mation for detection.