The angle subtended by the consciously perceived image – the active angle – corresponds to a certain
area on the retina. Hypothetically, this section on the retina is displayed on an “internal visual memory
screen” that is always the same size. Several of the geometrical-optical illusions are attributed to a
changing active angle adapted to the extent of the stimulus. An example is given, and an algebraic
expression is derived from a model. In this way, the intensity of the illusion can be traced back to
the dimensions of the geometric elements of the stimulus. The visual sense is compared to a data
communication channel. Its limited capacity may occasionally lead to a reduction in the active angle,
enabling better resolution. This can make a celestial body appear larger. It occurs automatically close
to the horizon. Other reasons are given that can reduce the active angle. Depictions of celestial bodies
in paintings indicate that the active angle can vary greatly from case to case.
