The amazing pupil reflex:
Pupil reflex
You can discover how your pupils work with the help of a friend. Turn out the lights in a room so that it is dim but light enough that you can still see. After a few minutes, look into the eyes of your friend and see how large the pupils are. Now turn on the lights or shine a flashlight into your friend's eyes. Notice how quickly the pupils close. When you remove the light, do the pupils get large again? This automatic adjustment is called a reflex. A reflex is an instant response to something, and it happens without thinking. The pupil reflex was discovered in 1751. It was the first of many reflexes all over the human body to be discovered.
This is from

Open Your Eyes: Discover Your Sense of Sight.

Illustrated by Cynthia C. Lewis.

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