Sight

18. The sense of sight depends on two nerves composed of a vast number of extremely mobile capillaries

These convey those various actions of the air-aether to the brain. This provides the soul with the opportunity to conceive various ideas of colors and light.

The structure of the eye is very useful and necessary for the air-aether.

A, B, C in Fig. XV is a membrane that is quite thick and hard, forming a kind of round vessel and constituting the receptacle of the other parts of the eye.

D, E, F is a thinner membrane spread within the former like the covering of a room.

G, H, I is the nerve commonly called the optic nerve, whose capillaries H, I, HI spread throughout the entire space from A, B, H to I, covering the entire back of the eye.

K, L, M are 3 very translucent fluids, stretching these tunics from all sides, in the shape depicted here.

In the first membrane, the part BCB is translucent and more convex, and refraction of incident rays occurs towards the perpendicular.

The inner surface of the second membrane, part EF facing the back of the eye, is entirely dark and black.

It has a small round hole in the middle of the front part, appearing darkest to observers, which we call the pupil.

However, this gap is not always the same size. But EF, the second little membrane, is attached to a very clear fluid, resembling a small muscle: which, directed by the brain, contracts or dilates as required by usage.

The figure of humor L is called crystalline. It is similar to the glasses described by us in the second book (Dioptrics), by which all rays coming from one point are collected at another point.

It is made of a less soft and more consistent material, and consequently causes greater refraction than the other two humors that surround it.

EN are numerous black filaments protruding inward from the membrane DEF, which surrounds the crystalline humor on all sides.

They are like small tendons, by the operation of which the crystalline humor becomes immediately more convex or flatter as our vision is directed to nearby or distant objects, thus slightly altering the entire shape of the eye.

Finally, OO are 6-7 muscles attached externally to the eye, by which it can be easily moved in any direction.

However, the membrane BCB and the 3 humors KLM are very translucent. They do not hinder the rays entering through the pupil from penetrating to the back of the eye where the nerve is.

They also affect the nerve no less easily than if it were completely bare and not covered by any such tunics.

Moreover, they protect the nerve against injuries from air and other bodies, which, if they acted upon it, would injure it with little effort.

Furthermore, they preserve it so tender and accurate that it is less surprising that it can be moved by actions as slightly perceptible as those we call colors.

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

For example, if the ray PBKQ were not refracted at point B, it could not pass through points FF, and through them 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, with the crystalline humor so arranged as to look at point R, the rays RN S and RL S etc. are accurately gathered at point S.

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

For it collects all the rays of point T around point V and all the rays of point X around point Y. The same reasoning applies to the others.

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

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

Object R can act more strongly on the part of the nerve that is at point S, by sending a large number of rays to it, than if it only affected it with a single ray.

That part of the nerve S will convey the action of that object R 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 EN also renders vision more distinct.

For, as is evident from the above about the nature of this color, it dulls, darkens, and prevents the power of the rays reflected from the back of the eye to the front, thus preventing them from causing confusion of rays there.

For example, the rays of object X tending to point Y in the whitish nerve are reflected from there to N and F in all directions; from where they could be reflected to S and V and thereby disturb the actions of R and T, if bodies N 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, as it was said in the second book (Dioptrics):

If, for example, the humor LN is of such a figure that it can precisely direct all the rays of point R to point S, it cannot, with the same unchanged figure, cause the rays of point T, which is nearer, or of point X, which is more distant, to impinge on it: But it will be the cause that the ray Tl tends 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. But the change in size that occurs in the pupil moderates the powers of vision.

For when the light is more intense, it decreases; so that the nerve is not offended by an excessive number of rays entering the eyes. But when the light is weaker, it dilates so that sufficient rays enter to stimulate the senses.

Moreover, 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, and hence the pupil must be larger.

The smallness of the pupil also renders vision more distinct. It must be understood that no matter what shape the crystalline humor may have, it is not possible for rays emanating from different points of the object to all be accurately collected in different points; but as soon as, for example, the rays of point R are accurately gathered at point S, none will be from point T except those that pass through the circumference and center of a circle that can be described on the surface of the crystalline humor, which can be perfectly collected at point V.

Consequently, others, which are in a much smaller number, as the pupil is smaller, touching other points of the nerve, cannot but cause confusion. Hence, if the vision of the same eye is currently weaker than before, objects will also be less distinctly perceived; whether this arises from the distance of the object or from the weakness of the light.

Therefore, it happens that the soul can perceive only one single point distinctly in each vision; namely, that to which the whole eye turns. And therefore, other things will appear to it more confused, the farther they are from this.

For if, for example, all the rays of point R are accurately gathered at point S, the rays of point X will be less accurately collected at point Y than the rays of point T at point V. And the same judgment holds for the others, namely, in proportion to their distance from point R.

However, the muscles O O, turning the eyes in all directions very readily, compensate for this deficiency; namely, 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.