Distance Perception and Optical Illusions

The soul can visually perceive the size of objects, and other similar qualities, solely by its knowledge 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.

Two hands F and G grasp the ends of the stick H and I and use it to touch the object K.

The soul may not know the length of these sticks. But they know:

• the distance between F and G
• the magnitude of the angles FGH and GFI

These let the soul know, as if by some natural geometry, where the object K is.

Similarly, eyes L and M look at the object N.

The soul will know where the point N is through:

• the length of the line LM
• the angles LMN and MLN

However, in all these cases, the soul can often be deceived.

If the position of the fingers or the eyes is distorted against nature by an external cause, the brain parts, from which the nerves originate, will not respond as accurately as they would if they depended only on naturally acting muscles.

Since the soul feels only through the intervention of those brain parts, it may then be deceived.

For example, the hand F by itself turns towards O.

• It is then 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, they will be arranged in a way between those two – in the same way as if it were turned towards P.

Hence, the soul will 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.

In such a position, 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,

Therefore is will be seen as different from what is seen by the eye M.

Likewise, the two fingers T and R touch the sphere X.

It will make the soul think that 2 different bodies are being touched because the crossed fingers are held against their natural position.

The soul commonly assumes rays and lines to be straight

If rays or lines become curved due to the action of any distant objects when they reach the senses, the soul will deceive itself.

For example, the stick H is bent towards K. The object that K touches will appear to the soul to be towards R.

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.

Similarly, if the eye B receives the rays of the point D through the glass C which 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.

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, we judge distances by opinion from either:

• the size of the objects we conceive or
• the rays emanating from them not uniting so accurately on the fundus of the eye

These show how easily we can be deceived by arguments based on perspective.

Shapes can be painted differently from what we imagine like when they:

• are smaller in perspective
• have darker colors and more confused lines

These make them appear more distant and larger than they really are.