Page "Pupil" Paragraph 3

The pupil gets wider in the dark but narrower in light.

When narrow, the diameter is 3 to 5 millimeters.

In the dark it will be the same at first, but will approach the maximum distance for a wide pupil 4 to 9 mm.

In any human age group there is however considerable variation in maximal pupil size.

For example, at the peak age of 15, the dark-adapted pupil can vary from 4 mm to 9 mm with different individuals.

After 25 years of age the average pupil size decreases, though not at a steady rate.

At this stage the pupils do not remain completely still, therefore may lead to oscillation, which may intensify and become known as hippus.

When only one eye is stimulated, both eyes contract equally.

The constriction of the pupil and near vision are closely tied.

In bright light, the pupils constrict to prevent aberrations of light rays and thus attain their expected acuity ; in the dark this is not necessary, so it is chiefly concerned with admitting sufficient light into the eye.