OPTICS, BOOK I.

opinion. To say, therefore, that there are [emitted] rays is to say nothing. Even all the mathematicians who say there are rays do not use anything in their demonstrations except imaginary lines, and they call these "radial lines." We have already declared that the sight perceives nothing of the things seen except through the directions verticationibus|the specific straight-line paths or orientations of these lines alone. Therefore, the opinion of those who think that radial lines are imaginary is a true opinion; and the opinion of those who think that something exits from the eye is a false opinion. It has now been declared from everything we have said that the sight does not sense the light and color which are on the surface of the visible object, except through a form extended from the surface of the visible object to the sight through the transparent medium diaphanum|a medium like air or water between the eye and the object; and that the sight perceives nothing of these forms except from the directions of the straight lines which are understood to be extended only between the visible object and the center of the eye, which are perpendicular to all the surfaces of the tunics tunicarun: the layers or membranes of the eye of the eye. And this is what we wished to declare. This, then, is the nature of vision in general: that the sight perceives nothing from the visible object by pure sensation alone except the light and color which are in the object. The remaining things which the sight perceives from visible objects, such as shape, magnitude, and the like, are not perceived by pure sensation, but through reason and signs This is a foundational claim of Alhazen: we see light/color physically, but we "see" size/shape through a mental process of inference.. And we shall declare this later in the second treatise, after a complete declaration in our discourse on the distinction of visible things which the sight perceives. And this which we have declared—namely, the quality of vision—is consistent with the opinion of those who verify both mathematics and physics.

24. Vision is seen to occur by synaugeia, that is, by rays received and emitted at the same time. original: "συναύγειαν" (synaugeian). This term refers to a classical theory where internal "visual fire" meets external light.

And it has been declared from this that both sects speak the truth: both opinions are correct and consistent with each other. But neither of them is complete without the other, nor can vision occur except through that which is joined from both sects. Sensation, therefore, does not exist except from the form and from the operation of the form upon the sight, and from the passion passione: in the philosophical sense of "being acted upon" or "undergoing an effect." of the sight from the form. The sight is prepared to be acted upon by this form according to its proper position, namely the position of the perpendicular directions upon its surface. The nature of the sight does not agree with this property except because visible things are not distinguished, nor are the parts of any of them arranged within the eye, except when its sensation comes from these directions alone. Therefore, the radial lines are imaginary lines, and by them is modeled the quality of the position upon which the sight is acted upon by the form. And it has already been declared [19 n] that when the eye is opposite a visible object, a pyramid In modern terms, a "cone of vision." will be formed between the object and the center of the eye, whose vertex is the center of the eye and whose base is the surface of the visible object. There will be a straight line between every point of the surface of the object and the center of the eye, understood to be perpendicular to the surfaces of the tunics of the eye. Thus the pyramid will contain all these lines, and the surface of the crystalline lens glacialis|literally "the icy part," referring to the lens will cut this pyramid, since the center of the eye, which is the vertex of the pyramid, is behind the surface of the lens. And when the air between the eye and the object is continuous, the form will be extended from that visible object according to the direction of that pyramid in the air (which the pyramid itself marks out) and in the transparent tunics of the eye, all the way to the part of the surface of the lens which is marked out by this pyramid. This pyramid will contain all the directions between the eye and the object from which the eye perceives the form of the visible thing. The form will be ordered just as it is ordered on the surface of the visible object and on that part of the surface of the crystalline lens. And it has already been declared [16 n] that sensation does not occur except through the crystalline lens. Therefore, the sensation of the eye from the light and color on the surface of the visible object comes only from the part of the lens marked out by the pyramid formed between that object and the center of the eye.

25. Vision is completed when the visible form, received by the crystalline humor, reaches the optic nerve. original: "20 p 3" — likely a reference to Proposition 20, part 3.

It has also been declared [4 n] that in this humor the crystalline humor or lens there is a certain amount of transparency and a certain amount of density spissitudo: thickness or relative opacity; and because of this, it is compared to ice. Because it has some transparency, it receives forms; and these pass through it according to its degree of transparency. Because it has some density, it prevents forms from passing [completely] through it according to its degree of density; thus forms are fixed in its surface and body, though weakly. Similarly, in any transparent body in which there is some density, when light shines upon it, the light will pass through it according to its transparency and will be fixed on its surface according to its density. Furthermore, the crystalline lens is prepared to receive these forms and to sense them. Therefore, the forms pass through it by means of the sensitive faculty that receives them. When the form reaches the surface of the lens, it acts upon it, and the lens is affected by it; for it is the property of light to act upon the sight, and the property of the sight to be affected by light. This operation which light performs on the lens passes through the body of the lens along the rectitude of the radial lines only, because the lens is prepared to receive forms of light from the directions of the radial lines. When light passes into the body of the lens, color passes with it—for color is mixed with light—and the lens receives this operation and this passage. From this operation and passion being acted upon arises the sensation of the lens from the forms of visible things which are on its surface and which pass through its entire body; and from the arrangement of the parts of the form on its surface and throughout its body arises its sensation of the arrangement of the parts of the source.

26. Vision belongs to the class of things that cause pain. original: "16 p 3"

And this operation which light performs on the lens is of the genus of pain, although some pains are endurable and do not harm the limb because of them; such pains are not manifest to the sense, nor does the sufferer judge them to be pain. The evidence for this is that light induces pain, for strong lights offend the eye and clearly harm it—such as the light of the sun when a viewer looks directly at its body, or the light of the sun reflected from polished bodies to the eye—since these lights induce manifest pain in the sight. The operation of all light on the sight is of this same genus and does not differ except by degree, according to more or less.