Page "Optical aberration" Paragraph 23

If the object point be infinitely distant, all rays received by the first member of the system are parallel, and their intersections, after traversing the system, vary according to their perpendicular height of incidence, i. e. their distance from the axis.

This distance replaces the angle u in the preceding considerations ; and the aperture, i. e. the radius of the entrance pupil, is its maximum value.