William Horatio Bates, Perfect Sight Without Glasses… (1920)
“ Another and even better way to test the sight is to compare the blackness of the letter at the near-point and at the distance, in a dim light and in a good one. With perfect sight, black is not altered by illumination or distance. It appears just as black at the distance as at the near-point, and just as black in a dim light as in a good one. If it does not appear equally black to you under all these conditions, therefore, you may know that your sight is imperfect. ”
