Treasury of Optics

directions: and these are the directions of straight lines whose extremities concur at the center of the eye alone. And these lines approach at the center because they are diameters of that eye, namely, and perpendicular to the surface of the sensing eye. And thus will be the sense from the forms arriving from the viewed objects, and these lines will be as if they were an instrument of the eye, through which viewed objects are distinguished by the eye, and through which the parts of any viewed objects are ordered by the eye. And that the essence of the eye is appropriated to some directions only has similarities in natural things. For light arises from luminous bodies and extends along straight directions only, and does not extend along arched or tortuous lines. And heavy bodies move to the lower part by natural motion along straight lines, not along curved, or arched, or tortuous lines. Nor yet will they move along all straight lines that are between them and the surface of the earth, but along proper straight lines, which are perpendicular to the surface of the earth and its diameter. And celestial bodies move along spherical lines, and not along straight lines, nor along lines of a different order. And when we have intuited natural motions, we will find that each of them is appropriated to some directions only. It is not, therefore, impossible that the eye be appropriated in the reception of the operations of light and color to some straight directions, which concur at its center alone, and are perpendicular to its surface. Moreover, the comprehension of the eye regarding viewed objects from the directions of straight lines whose extremities concur at the center of the eye is conceded by mathematicians, and there is no diversity among them in this. And these lines are called by them radial lines. And since this is possible, and the forms of light and color come to the eye and pass through the transparency of the coats of the eye, and vision is not completed from the reception of these forms except when the eye has received them from the directions alone, therefore the eye does not comprehend the lights and colors of viewed objects except from the forms arriving at it from the surfaces of the viewed objects, and it does not comprehend these forms except from the directions of straight lines whose extremities concur at the center of the eye alone. Let us aggregate, therefore, now those things which can be aggregated from all that we have said, and let us say that the eye senses the light and colors that are on the surface of the viewed object from the extended form, and from the light and color that are on the surface of the viewed object through the diaphanous body, which is the medium between the eye and the viewed object. And the eye comprehends nothing from the forms of viewed objects except from the directions of the lines extended between the viewed object and the center of the eye alone. And it is declared that this is possible.

21. A visible thing opposite to the eye is seen. 2 p 3.

We, however, now will explain the question of why vision happens according to this mode, by saying that vision cannot be except according to this mode. For when the eye has sensed a viewed object, after it was not sensing it, something happened to it that was not there before. And nothing will happen, which was not there before, except through some cause. And we find that when the eye is opposite to a viewed object, it will sense it, and when it is removed from its opposition, it will not sense it, and when it returns to the opposition, the sense will return. And similarly, we find that when the eye has sensed a viewed object, and then closes its eyelids, the sense is destroyed, and when it opens the eyelids, and the viewed object is in opposition, the sense will return. But the cause is this: that when the cause is destroyed, the caused is destroyed, and when the cause returns, the caused will return. The cause, therefore, that makes that thing happen in the eye is the viewed object when it is opposite to the eye. Therefore, the eye does not sense the viewed object except on account of that which makes the viewed object happen in the eye, namely, when they are opposite to the eye.

22. A visible thing is seen through a transparent medium. 13 p 3.

And again, the eye does not comprehend the viewed object except when the body, which is the medium between them, is diaphanum transparent/diaphanous. For the comprehension of the eye of a viewed object from the posterior of the air, which is the medium between them, is not because of the humidity of the air, but because of its diaphanitas transparency. For if there were between the eye and the viewed object some stone, or any other diaphanous body whatsoever, then the eye would comprehend the viewed object, and the comprehension would be according to the transparency of the mediating body. And the more transparent the mediating body is, the more manifest will be the sense of the eye of that thing. And similarly, when there is clear diaphanous water between the eye and the viewed object, the eye will comprehend the viewed object from the posterior of the water. And if that water were dyed with some strong dye, so that the transparency is destroyed, even if humidity remains in it, then the eye will not comprehend that viewed object which is in the water. It will be declared, therefore, from these dispositions, that vision is not completed except through the transparency of the medium body, and not through humidity. That, therefore, which the viewed object operates upon the eye upon its opposition against it, from which is the sense, is not completed except through the transparency of the body of the medium between the eye and the viewed object. Therefore, the light and color of the viewed object will not be comprehended by the eye except from something that is from that light and color in the eye. And that does not happen from light and color in the eye except when the body of the medium between the eye and the viewed object is diaphanous. Transparency, however, is not appropriated to any of those things that depend upon light and color, by which it is diversified from non-transparency, except because the form of light and color passes through a diaphanous body and does not pass through a non-diaphanous one, and because the diaphanous body receives the form of light and color and returns it to parts opposite to the light and color, while a body [that is not diaphanous] does not have that property. And because the eye does not sense the light and color that are in the viewed object except from something happening from the light and color in the eye, and that does not happen in the eye except when the body of the medium between the eye and the viewed object is diaphanous, and a diaphanous body is appropriated to nothing by which it is distinguished from a non-diaphanous body among those things...