Convexity and Refraction

19. Convexity and refraction is how the light coming from objects can enter through the pupil

If the ray YFT were not refracted at point B, it could not pass through section FF and go to the optic nerve.

Refraction occurs in the crystalline humor. It strengthens and clarifies vision.

The structure of that humor is composed in such a way that it corresponds exactly to:

• the refractions that occur in other parts of the eye, and
• the distances of the objects.

Hence, in direct vision to a certain point, all rays emanating from that point are accurately collected in a certain point of the retina or optic nerve of the eye.

By the same reasoning, it prevents any rays coming from elsewhere from falling on that part of the nerve.

For example, the crystalline humor looks at point X.

• The rays X1S and X3S etc. are accurately gathered at point S.

Similarly, this arrangement of the crystalline humor prevents rays coming from points V, Y, etc. from reaching it.

It collects all the rays of point V around point R and all the rays of point Y around point T. The same reasoning applies to the others.

But if there were no refraction in the eye, the object X would affect only one of its rays at point S, with the rest scattered throughout the space R and T.

Similarly, points V and Y, and all other intermediate points, would only send one of their rays to point S.

Object X can act more strongly on the nerve at point S by sending more rays to it.

That part of the nerve S will convey the action [light] of that object X more distinctly and faithfully to the brain, since it receives rays from it alone, than if it received them from many.

The black color of both the concave surface of the membrane EF and of the filaments E4 also renders vision more distinct.

Black dulls, darkens, and prevents the power of the rays reflected from the back of the eye to the front. This prevents them from causing confusion of rays there.

For example, the rays of object Y tend to point T in the whitish nerve.

• These are reflected from there to N and F in all directions.
• From there, they could be reflected to S and V and thereby disturb the actions of R and T, if bodies E and F were not black.

However, the change that occurs in the crystalline humor causes the images of more distant or nearer objects to be more distinctly painted on the back of the eye.

For example, assume that the humor LN has a shape that it can direct all the rays of point R to point S.

• At the same time, it cannot cause the rays of point T, which is nearer, or of point X, which is more distant, to impinge on it.
• But it will cause the ray Tl to go to K and Tn to G.

Conversely, that Xl tends towards G and Xn to K. And so on for the others.

So to represent the point distinctly, it is necessary to change the entire figure of the humor LN slightly and make it flatter or more obtuse.