Defects Of The Eye

The eye has defects. It is unable to focus the incoming rays at a precise point.

The best it can do is to simply gather all the rays coming from a single point, such as X, in another point, such as S, in the middle of the back of the eye.

Only a few of the rays:

• from point V assemble exactly at point R
• from point Y assemble exactly at point T

The others must deviate somewhat from these points.

This is why:

• this painting is never as clear at its extremities as it is in the middle
• vision occurs mainly along a straight line

This line passes through the centers of the crystalline humor and the pupil, such as the line XK2S, which is called the axis of vision.

The rays coming from point V deviate around point R.

• They deviate more if the opening of the pupil is larger.

The larger pupil will make the colors of this painting stronger and more vivid.

• But it also prevents the shapes in the painting from being as distinct.
• This is why it can only be mediocre.

These rays would deviate even more around point R if V were:

• much closer to the eye at 10, or
• much farther away at 11

If V is the ideal distance to the eye, those new locations would make the painting less distinct.

Transparent bodies invert the images that pass through them as they make the rays coming from a single point assemble in some other point.

• T on the left represents Y which is on the right.
• R on the right represents V which is on the left.

At point R, object V should not be:

• larger than object 10 even if object 10 is smaller but closer
• smaller than object 11 even if object 11 is larger but farther away, though object V would be a little more distinct

The straight line VXY is represented by the curve RST.