This shows the result of an experiment where 10 subjects where kept in a darkened room and then brought into light, causing the pupils to dilate, and then the diameter in millimeters of the pupils was measured every 1/3 sec.  (The second column is the average eye diameter of the 10 subjects.)

Adapted from "Quantitative Methods in Psychology" by D. Lewis, McGraw-Hill, New York, 1960.