OPTICS, BOOK I

through specific directions original: "verticationibus"; this term refers to the orientation or the "turning" of a line of travel: and these are the directions of straight lines whose endpoints meet only at the center of sight. These lines converge at the center because they are the diameters of the eye itself and are perpendicular to the sensitive surface of the eye. Thus, sensation arises from the forms coming from visible things, and these lines act as a kind of instrument for the eye, by which visible things are distinguished by the sight, and by which the parts of any visible thing are ordered by the sight. The fact that the existence of sight is restricted to certain directions only has parallels in natural things. For light arises from luminous bodies and extends only along straight directions, and does not extend along arched or twisting lines. Heavy bodies move downward by natural motion along straight lines—not along curved, arched, or twisting lines—yet they do not move along just any straight lines between themselves and the earth’s surface, but along their own proper lines, which are perpendicular to the surface of the earth and its diameter. Likewise, celestial bodies move along spherical lines, and not along straight lines nor lines of a different order. When we observe natural motions, we find that each of them is restricted to certain directions only. It is therefore not impossible that sight is restricted in its reception of the operations of light and color to certain straight directions which meet only at its center and are perpendicular to its surface. Furthermore, the understanding that sight perceives visible things from the directions of straight lines whose endpoints meet at the center of sight is accepted by mathematicians, and there is no disagreement among them on this; these lines are called by them "radial lines" original: "lineae radiales". Since this is possible, and the forms of light and color reach the eye and pass through the transparency of the eye's layers—yet vision is not completed by the reception of these forms except when the eye receives them from these directions alone—it follows that the eye does not perceive the light and colors of visible things except from the forms coming to it from the surfaces of the visible things. It does not perceive these forms except from the directions of straight lines whose endpoints meet only at the center of sight. Let us now summarize what can be gathered from everything we have said: that sight perceives the light and colors on the surface of a visible thing from the extended form, and from the light and color on the surface of the visible thing, through the transparent body that is the medium between the eye and the thing seen. Sight perceives nothing from the forms of visible things except from the directions of lines extended between the visible thing and the center of sight alone. It has been declared that this is possible.

21. A visible object is seen when it is opposite the eye. [Proposition] 2, Part 3.

We shall now explain the question of why vision occurs in this manner, by saying that vision cannot exist except in this way. For when the eye perceives a visible thing after not having perceived it, something happens to it that was not there before; and nothing happens that was not there before except by some cause. We find that when the eye is opposite a visible thing, it perceives it; and when it is removed from that opposition, it does not perceive it; and when it returns to that opposition, the sight returns. Similarly, we find that when the eye perceives a visible thing and then closes the eyelids, the sensation is destroyed; and when it opens the eyelids and the visible thing is in opposition, the sensation returns. The cause is this: when the cause is destroyed, the effect is destroyed, and when the cause returns, the effect returns. Therefore, the cause that makes that thing happen in the eye is the visible thing when it is opposite the eye. Sight, therefore, does not perceive the visible thing except because of that which the visible things cause to happen in the eye—namely, when they are opposite the eye.

22. A visible object is seen through a transparent medium. [Proposition] 13, Part 3.

Furthermore, sight does not perceive a visible thing except when the body that is the medium between them is transparent diaphanum|a material that allows the passage of light, such as air, water, or glass. For the eye's perception of a visible thing through the air between them is not because of the air's "humidity," but because of its transparency. If some stone or any other transparent body were placed between the eye and the visible thing, the eye would then perceive the visible thing, and the perception would be according to the transparency of the mediating body; the more transparent the mediating body, the more manifest will be the eye's sensation of that thing. Similarly, when there is very thick water between the eye and the visible thing so that the transparency is destroyed—even though the humidity remains in it—the eye will not perceive the visible thing that is in the water. It is clear from these conditions that vision is not completed except through the transparency of the mediating body, and the sensation is not completed except through the transparency of the mediating body between the eye and the thing seen. Therefore, the light and color of a visible thing are not perceived by the eye except from something of that light and color in the eye; and that does not happen from the light and color in the eye except when the mediating body between the eye and the thing seen is transparent. Transparency is not distinguished from non-transparency by anything related to light and color, except that the form of light and color passes through the transparent and does not pass through the non-transparent. This is because a transparent body receives the form of light and color and gives it back to the parts opposite the light and color, whereas a non-transparent body does not have this property. And because the eye does not perceive the light and color of a visible thing except from something of that light and color happening in the eye—and that does not happen in the eye except when the mediating body between the eye and the thing seen is transparent—and a transparent body is distinguished from a non-transparent body by nothing else...