If the eye is required to look at an object placed too near to it, it cannot judge of it well--as happens to a man who tries to see the tip of his nose. Hence, as a general rule, Nature teaches us that an object can never be seen perfectly unless the space between it and the eye is equal, at least, to the length of the face.
Differences of perception by one eye and by both eyes (26-29).
When both eyes direct the pyramid of sight to an object, that object becomes clearly seen and comprehended by the eyes.
Objects seen by one and the same eye appear sometimes large, and sometimes small.
The motion of a spectator who sees an object at rest often makes it seem as though the object at rest had acquired the motion of the moving body, while the moving person appears to be at rest.
Objects in relief, when seen from a short distance with one eye, look like a perfect picture. If you look with the eye _a_, _b_ at the spot _c_, this point _c_ will appear to be at _d_, _f_, and if you look at it with the eye _g_, _h_ will appear to be at _m_. A picture can never contain in itself both aspects.
Let the object in relief _t_ be seen by both eyes; if you will look at the object with the right eye _m_, keeping the left eye _n_ shut, the object will appear, or fill up the space, at _a_; and if you shut the right eye and open the left, the object (will occupy the) space _b_; and if you open both eyes, the object will no longer appear at _a_ or _b_, but at _e_, _r_, _f_. Why will not a picture seen by both eyes produce the effect of relief, as [real] relief does when seen by both eyes; and why should a picture seen with one eye give the same effect of relief as real relief would under the same conditions of light and shade?
[Footnote: In the sketch, _m_ is the left eye and _n_ the right, while the text reverses this lettering. We must therefore suppose that the face in which the eyes _m_ and _n_ are placed is opposite to the spectator.]
The comparative size of the image depends on the amount of light (30-39).
The eye will hold and retain in itself the image of a luminous body better than that of a shaded object. The reason is that the eye is in itself perfectly dark and since two things that are alike cannot be distinguished, therefore the night, and other dark objects cannot be seen or recognised by the eye. Light is totally contrary and gives more distinctness, and counteracts and differs from the usual darkness of the eye, hence it leaves the impression of its image.