Interference patterns observed in the back-focal plane of the objective at different distances d between the top surface of the coverslip and the bottom of the bead: (a) d = 5D, (b) d = 10D, (c) d = 15D. D is the period between two dark fringes at the center of the interference pattern. Image (d) was obtained at d = 5D with a phase contrast objective, showing the phase ring in its back focal plane. The white circles show the edge of the objective pupil, with a diameter of 5.2 mm.