Figure 3.

(a) Schematic representation of an eye showing representative AB pairs of photoreceptor cells (red discs) distributed around the retina, modelled as a hemisphere with the pupil (black spot) at its centre. The white line through of each of the red discs indicates the direction of the TDM (y_m axes) in the two cells (figure 1c). Three rays of light are shown entering the eye through the pupil and hitting the retina at points 1, 2 and 3. The photoreceptor cells are arranged with the symmetry (z_c) axis of each cell pointing towards the pupil and the x_c axes following the lines of longitude of the sphere, taking the pupil and the centre of the retina as points on the sphere's equatorial plane. (b) An equal-angle projection of the ratio (A/B) of the outputs of the array of cell pairs showing the reference points 1, 2 and 3. The eye looks horizontally to magnetic north. For this simulation, the sun was on the horizon in the west and the maximum polarization was 75%. This is essentially the same pattern as shown in figure 2e.