An error affecting visual estimates of variable stars that arises when comparing stars of different colours, especially when one is red. It is caused by the differing sensitivity of the rods and cones in the retina to light of various wavelengths. It varies between observers, and also depends on the aperture of instrument used (i.e. on the amount of light gathered by the system), because the cones of the eye—unlike the rods—do not function at low light levels. It is named after the Czech physiologist Johannes Evangelista Purkinje (1787–1869).
Subjects: Astronomy and Astrophysics.