Please note that the content of this book mainly consists of articles that are available online from Wikipedia or other free sources. The adaptation of the eye (aptare, Latin: to adapt) refers to the processes of adjustment to the predominant light intensities in the visual field. In the first step, during the pupillary light reflex (or simply: pupillary reflex), the size of the iris (the colored part of the eye) changes (in photographic terms, the aperture), which causes a rapid adjustment of the relative amount of light entering the eye.