Distance Perception

20. I do not individually explain what gives the soul the opportunity to conceive all the differences in color

We know that:

• overly intense light can harm vision
• moderate light can refresh it, especially the color green

Green consists of the proportion of 1:2 and is related to the octave in musical consonances.

Green is like daily bread, as opposed to other types of food which give more pleasure.

The novelty of the non-green colors are endearing and often provide greater delight to the sight than green.

• They are like excellent and unusual musical modulations or the delicate food of a wise cook.

They initially stimulate the senses like a pleasant itch.

• But they quickly cause weariness and nausea much faster than simple and ordinary objects.

How does the soul perceive the position, shape, distance, size, and other similar qualities?

These are perceived by many of our senses.

For example, when hand A touches object C, the parts of the brain B, from which the filaments of those nerves originate, are arranged differently than if it were touching something else, whether in shape, size, or location.

Therefore, the soul can perceive the position, shape, size, and similar qualities of the object in a peculiar way through the manner in which it is affected.

Similarly, if the eye D looks at the object E, the soul will be able to perceive the position of that object because the nerves of that eye will be arranged differently than if it were looking elsewhere.

• It will recognize its shape because the rays from point 1 converging at point 2, to the optic nerve, and the rays from point 3 at point 4, and so on for the others, will paint a figure exactly similar to its own shape.

It will also be able to perceive the distance of point 1 because the arrangement of the crystalline humor will be different for the other figure, so that all rays coming from this point will precisely converge at point 2 on the fundus of the eye, which, as previously stated, I assume to be the medium between distant and near.

The soul will be able to perceive the shapes of point 3 and at the same time of all the others from which rays will enter the eye simultaneously because with the crystalline humor arranged in that way, the rays from point 3 will not converge as precisely at point 4 as those coming from point 1 do at point 2.

But it does not preserve the same proportion as to the forces acting, as at other times.

Finally, the soul can perceive the size of objects affecting sight, and other similar qualities, solely by the knowledge it has of the distance and position of all the points that make them up. Likewise, it will judge the distance of those points solely by the opinion it forms of their size.

If these two hands F and G grasp different ends of the stick H and I, with which they touch the object K, although the soul may not know the length of these sticks in any other way, yet knowing the distance between the two points F and G, and the magnitude of the angles FGH and GFI, it can be more certain, as if by some natural geometry, where the object K is.

Similarly, if both eyes L and M directly look at the object N, the length of the line LM, and at the same time the angles LMN and MLN, in Figure XXIV, will show the soul where the point N is.

However, in all these cases, the soul can often be deceived. First, if the position of the fingers or the eyes is distorted against nature by an external cause, the brain particles, from which the nerves originate, will not respond as accurately as they would if they depended only on naturally acting muscles; thus, the soul, which feels only through the intervention of those brain particles, may then be deceived as shown in Figure XXV.

For example, if the hand F, when placed by itself to turn towards O, is distorted by some external force towards K, the parts of the brain from which these nerves originate will not be arranged in the same way as if the hand were turned towards K by spontaneous muscle movement, nor even in the same way as if it were truly directed towards O. But rather in a manner between those two, namely in the same way as if it were turned towards P.

Hence, the soul will take the opportunity, as depicted in Figure XXVI, to judge that the object K is at point P and is different from what the hand G touched.

Similarly, if the eye M is twisted away from the object N by force and placed as if it were looking towards Q, the soul will judge it to be directed towards R.

And because with the eye so positioned, the rays of the object N incident into the eye just as they would if coming from point S, if the eye were truly turned towards R, it will judge the object N to be at point S, and therefore different from what is seen by the eye M.

Likewise, the two fingers T and R, touching the sphere X, will give the soul the opportunity, as shown in Figure XXVII, to think that two different bodies are being touched because the crossed fingers are held against their natural position.

Furthermore, if rays or any other lines, due to the action of any distant objects, are curved when they reach the senses, the soul, which commonly assumes them to be straight, will take the opportunity to deceive itself.

Just as, if the stick H is bent towards K, the object K that it touches will appear to the soul towards R.

Figure XXIX.

If the eye L receives the rays of the object N through the glass 3, which refracts or bends them, that object will appear to the soul to be at point A. Figure XXX.

Similarly, if the eye B receives the rays of the point D through the glass C, which I suppose are all refracted or bent in the same way, as if they were coming from point E, and as if those from point F were coming from point G; and so on with the others: the object DFH will appear to the soul with the same magnitude and distance as the object EGI is seen.

To conclude, it must be noted: all the methods by which the soul endeavors to discern the distance of objects by sight are doubtful and uncertain.

For as for the angles LMN and MLN, and those similar to them, they hardly change sensibly anymore when the object is farther away than 15 or 20 feet. See above Fig. XXIV.

As for the disposition of the crystalline humor, it also changes less sensibly as soon as the object is more than three or four feet away from the eye.

Finally, the judgment regarding distance, which we form either by opinion based on the size of the objects we conceive, or from the fact that the rays emanating from their different points do not unite so accurately on the fundus of the eye, shows how easily we can be deceived by arguments based on perspective.

For whenever figures are painted smaller in perspective than we imagine they should be, and with darker colors and more confused lines, they always appear more distant and larger than they really are.