Note 523 — Chapter XV · [512]
Reading edition
Ibid. p. 407. When the sparks fell so close together that their irradiation-circles overlapped, they appeared like one spark moving from the position of the first to that of the second; and they might then follow each other as close as 0.015'' without the direction of the movement ceasing to be clear. When one spark fell on the centre, the other on the margin, of the retina, the time-interval for successive apprehension had to be raised to 0.076''.