Pupil Size and Successive Vision

The change in size that occurs in the pupil moderates the powers of vision.

• This is because when the light is more intense, it decreases.
• The pupil limits the number of rays entering the eyes so that the nerve is not offended
• But when the light is weaker, it dilates so more rays can enter

If the light remains constant, the pupil must become larger when the object being viewed is more distant than when it is closer.

For example, if more rays of point R do not enter through the pupil of eye 7 than are required for perception, it is necessary for so many rays to enter the eye 8. Hence the pupil must be larger.

The smallness of the pupil also renders vision more distinct.

No matter what shape the crystalline humor may have, it is not possible for the rays emanating from the object to be accurately collected in different points.

But as soon as the rays of point X are accurately gathered at point S, the only rays from point V will be those that pass through the crystalline humor, which can be perfectly collected at point R.

Consequently, the other fewer rays coming through the smaller pupil will touch the other points of the nerve causing confusion.

Hence, if the vision of the same eye is weaker than before, objects will be seen less clearly whether from:

• the distance of the object or
• the weakness of the light.

Therefore, the soul can perceive only one single point distinctly in each vision – that point to which the whole eye turns.

Therefore, other things will appear to it more confused, the farther they are from this.

For example, if all the rays of point X are accurately gathered at point S, then the rays of point Y will be less accurately collected at point T than the rays of point V at point R.

The same judgment holds for the others, namely, in proportion to their distance from point R.

However, the muscles O O turn the eyes in all directions very readily.

They compensate for this deficiency by applying them successively to all points of the object in the smallest possible time, so that the soul sees one thing after another, yet all very distinctly.