another way of thinking of the maximum visual field is to think of all of the retina that can be reached by light from the external environment. the visual field in this case is all of the external environment that can project light onto the retina. meridians correspond to sections of great circles passing through the centre of the fovea. in an analogy to meridian (geography), in which meridians are lines of longitude, the north pole might correspond to the fovea, greenwich would correspond to a retinal location about 39 degrees to the left of the fovea (because the retinal image is inverted, this corresponds to a location in the visual field to the observer's right), and the south pole would correspond to the centre of the pupil.