Pupil

Pupils were dark-coloured, light-receptive parts of an eye which could expand or contract, allowing more or less light into the eye. As the First Doctor noted, creatures with large pupils (such as Sensorites) had poor vision in the dark, and so needed to allow as much light into their eyes as possible. Cats on the other hand had strong night vision, and so smaller pupils. (TV: "The Unwilling Warriors")