Abstract
The electrical threshold of the eye was measured by the alternating current of 20 c. p. s. with flicker sensation caused by it as an index under various adaptation states.
1. The threshold-time curve during complete dark adaptation following light adaptation of 10 minutes to illumination of 9, 800 lux shows the following course: At first the threshold remained low for about 8 minutes, and then increased rapidly to attain a constant level in about 15 minutes. Thus, the curve showed an inflection at about 8 minutes.
2. The curve suffered from a modification when the retina was constantly exposed to a small point of light of 40' in visual angle following the same light adaptation as above.
The inflection at the middle stage became more obscure and the final level lower as local illumination was moved farther towards the periphery of the retina.
3. The effect of similar local illumination was investigated with colored lights.
When blue, green and red lights were used for local illumination, it was found that the red light showed an effect similar to white light confined to the fovea, but that the green and blue light acted as white light illuminating the periphery of the retina.
4. Next the effect of exposure to broad illumination was investigated upon the curve following the same light adaptation as above.
The curves by illumination of 0.1 lux, 1 lux and 10 lux were low and almost steady, but those by 100 and 1, 000 lux were generally much higher.
From these experiments it is likely that the threshold as determined by the A. C. of 20 c. p. s. is lowered when visual cells, especially rods are excited to some extent.
