The optical illusion created by the line is caused by:
the direction that a line takes; in Fig. 4.11 all lines are of same length but the line direction creating a illusion of shortness (b), stopping the eyes with horizontal line (c) and producing line with greater height (d). Similarly in Fig.4.12, the angle between the lines is producing a broad or narrow effect when the angle is extended or lessened.
the division of space, in Fig. 4.13 (a) rectangle divided by vertical line looks narrower than the one by horizontal line, (b) similarly rectangle with close stripes look narrower than the rectangle with stripes far apart and (c) shows that diagonal breaking the rectangle starting from top make rectangle narrower than the one starting from the side.