In measurements of the photosensitivity of the eye the use of strong test stimuli is carefully avoided, because the state of adaptation of the eye is disturbed by such test stimuli. Little precaution has, however, been taken to avoid strong electric stimuli in measurements of the electrical sensitivity of the eye, because it has been believed that electric stimuli which cause only weak sensations of light have no disturbing effect upon the state of the eye.
1. It was found that electric stimuli of a few volts or a few hundred, μ2A have a strong depressing effect on the electrical sensitivity of the eye, and that this effect lasts as long as one hour.
2. After application of several electric stimuli of the mentioned order at an interval of7seconds, the determination of thresholds became extremely difficult, and in consequence the accuracy of measurements is strongly affected. For example, the standard deviation (S.D.) of10threshold values determined by the routine method was0.27% of the average. Prior to the determination of each threshold value, 10shocks of2volts were given at an interval of7seconds. The S.D. became7.9%, that is, about30times as high as the value obtained without the conditioning shocks.
3. Keeping this after-effect in mind, a method of equal logarithmic steps proposed by Riggs was criticized.