Fig. 4. (A) Results from experiments in which both walls are uniformly black and of equal height (h=0.2 m corresponding to 14.9° as seen from the midline of the channel). N=48. (B) Either the left-hand or the right-hand wall (left or right figure, respectively) is increased to double the linear height (h=0.4 m corresponding to 28.1° as seen from the midline of the channel). Orange line, position at which the heights of the left and the right walls appeared to subtend equal visual angles. N=22 (left figure) and N=21 (right figure). For further conventions, see Fig. 2A.