Figure 3: Illustration of a Fermat surface for a source (red symbol) close to the line of sight to
a galaxy (green symbol). In each case the appearance of the images to the observer is shown by a
greyscale, and the contours of the Fermat surface are given by green contours. Note that images
form at stationary points of the surface defined by the contours. In the three panels, the mass of the
galaxy, and thus the distortion of the Fermat surface, increases, resulting in an increasingly visible
secondary image at the position of the saddle point. At the same time, the primary image moves
further from the line of sight to the source. In each case the third image, at the position of the Fermat
maximum, is too faint to see.