The amount of light entering the eye is controlled by the pupil and the iris.

Explanation:
The iris is the coloured part of the eye, and it contains a circular opening called the pupil. The pupil can constrict or dilate to regulate the amount of light that reaches the retina at the back of the eye.

In bright conditions, the iris contracts the muscles around the pupil, causing it to constrict and become smaller. This reduces the amount of light entering the eye, protecting the sensitive retina from excessive light exposure.

Conversely, in dim lighting conditions, the iris relaxes the muscles around the pupil, causing it to dilate and become larger. This allows more light to enter the eye, improving visibility in low-light situations.

The regulation of pupil size is an involuntary process controlled by the autonomic nervous system, specifically the muscles of the iris.