Peripheral visual acuity with special reference to scotopic illumination.

The capacity of the retina to discriminate form decreases rapidly just outside the fovea and continues to diminish peripherally. Wertheim's classical data^ show a steep drop in relative visual acuity from 1.00 at the fovea to 0.46 at 5 degrees , and then a slower drop to 0.20 at 10 degrees. An acuity of 0.20 could still be measured at 20 degrees. Ludvigh's meas-urements' roughly parallel these, the decline being even more precipitous in the paracentral region. Only 0.26 of the central acuity remained at S degrees, and O.lS at 15 degrees. His data showed a relationship between peripheral visual acu-ity and the population of cone cells in the retina as given by Østerberg.' The near parafoveal region was studied intensively by Weymouth and others,* who showed that acuity falls off immediately and continuously from the foveal fixation point. All these investigations were performed at high levels of illumination in order to demonstrate maximum acuity (Ludvigh used 26 to 36 foot-candles; Weymouth 18 millilamberts). No study of peripheral acuity under scotopic levels of illumination could be found. However, scotopic acuity measurements are of particular importance , because it is precisely under sco-topic illumination that the peripheral * The measurements reported in this communication were performed at the Army Air retina is called upon to take over the discriminatory functions. Only in the path-ologic eye with a central scotoma is eccentric photopic acuity of prime importance ; eccentric scotopic acuity is of importance in all eyes. It is quite plausible that with high illumination, peripheral ac-uity should parallel the cone population, as was pointed out by Ludvigh. Would scoto-pic measurements indicate a similar paral-lelism with the rod population? The rod population has its maximum density close to 20 degrees peripherally;' the sensitivity of the retina to light is also greatest in this region." However, there were several reasons to believe that peripheral scotopic acuity need not follow this pattern. In the first place, if night-blind subjects are excluded, there is little correlation between light sensitivity and sco-topic form discrimination.* Furthermore, it has been the experience of many subjects familiar with various methods of testing scotopic form discrimination, such as were employed by the Army Air Forces, that fixation at an angle much smaller than 20 degrees gives maximum acuity in these tests. Finally, there is the theoretic reason that the maximum in light sensitivity attained at about 20 degrees is caused …