No image is carried to the eye, passing through uniform air, except along a straight line. And no body can wholly send its images along a single line. And when many lines converge on one single point, since that point is indivisible, the images there must be confused among themselves; and the eye does not discern where there is confusion. Therefore the images pass with an intersection within the point. Every intersection of the lines of the images turns the objects upside down. The receptive faculty, set behind the spherical crystalline humour, sets the images upright again. If the eye does not make a point, no smallest thing will ever diminish over any distance; and if it makes a point, that point is indivisible, and in the indivisible parts no image can be contained, but they are confused among themselves. And if the objects are seen well without confusion, and every smallest thing diminishes with distance, it is necessary that the lines of the images make an intersection at the point and, having passed it, begin to separate again. This being so, and were the receptive faculty at that separation, every object would appear upside down; since it does not appear so, it is necessary that these images pass through a spherical and transparent body, which turns behind itself, to the contrary, that which is given to it in front; which by its nature turns all things given to it upright on one side to the opposite side upside down; and if the receptive faculty is there, it will indeed be able to see the objects. Now turn your mind within the eye and you will see that necessity, mistress of all things, has provided itself well; you will see the eye have within itself the instruments necessary to its office.

This proposition appears manifest also, because if you look at one and the same thing with two eyes and, pushing the lower lid somewhat upward with your finger, you will see the thing divide into two: the one remaining still and the other coming downward. The one that seems to remain in its place is seen by the eye that is not moved; the one that seems to descend from high to low is seen by the eye that is pushed from low to high. Now here one sees contrary movements, because if I push the eye upward the thing seen seems to descend, and if I push it downward the thing makes the contrary movement, which could not happen if the eye were not as I describe it. We know the eye to have something soft, and pushing the front part toward the spherical crystalline humour, those images of the thing seen that lie in that part - which cannot be moved so little that it will not appear much - being moved upward by the receptive faculty on the opposite side of the spherical body, seem to come downward.

It is necessary to see with a single eye, because the two eyes make two things appear as three, and three ...

a c - b d; a n r - b - c m S. That image of the object which falls upon the surface of the eye between equal angles will be well seen, as b. And that which falls between less equal angles will be less seen, as a c. If the image of a body is placed at m, the receptive faculty takes it at r; and if you push m upward, r will go downward, and it is necessary that this reasoning have effect.