BOOK I OF OPTICS

...only of its surface, and it does not perceive that point of the visible object from the rest of the form reaching its surface from the rest of the surface of sight: vision will be completed, and the parts of the visible object will be ordered, and things will be distinguished in themselves by the sight, and vision will not be completed except in this way. And this cannot be so, unless there is a single point among those on the surface of the sight through which the form of one point of the surface of the visible object passes, distinct from the remaining points on the surface of the sight; and there must be a line along which the form comes to that point of the surface of sight, distinct from the other lines along which the form comes. Because of this, the crystalline lens Original: glacialis—literally "icy," referring to the crystalline lens of the eye, which medieval scholars believed was the primary organ of sight can perceive the form coming along that line, and from the point of the surface of sight which is upon that line, and it cannot perceive it through any other.

17. Perpendicular light penetrates through any various media; oblique light is refracted. Original references: 42. 43. 44. 45. 47 p 2.

And when lights are introduced, and the quality of their passage and their extension in transparent diaphanous|transparent or light-permeable bodies is tested, it is found that light extends through a transparent body along straight lines, as long as the transparent body is of uniform transparency. But when it meets another body of a different transparency from the previous body in which it was traveling, it does not pass through in a straight continuation of the lines along which it was traveling before, except when those lines are perpendicular to the surface of the second transparent body. If those lines are oblique hitting at an angle to the surface of the second body and not perpendicular, the light will be deflected refracted at the surface of the second body and will not extend straight. When it is deflected, it will extend into the second body along those straight lines onto which it was deflected; and the lines along which the light was deflected in the second body will also be slanting relative to the surface of the second body and not perpendicular. And if some lines along which the light comes in the first body are perpendicular to the surface of the second body and others are slanting, the light that was on the perpendicular lines will extend into the second body in a straight line, while that which was on the slanting lines will be deflected at the surface of the second body according to those slanting lines, and will extend in it along the straight path of those slanting lines onto which it was deflected. We shall declare this in the discourse on refraction, and we will show the way by which one can test this arrangement; it will be apparent to the senses, and certainty will be established upon it.

18. Distinct vision occurs along straight lines perpendicular from the visible object to the surface of the eye. Thus, individual visible points occupy the same position on the surface of the eye as they do on the visible object. Original reference: 17 p 3.

Since this is the case, regarding the form of light and color that comes from any point of the visible object to the surface of the eye: when it reaches the surface of the eye, nothing will pass through the transparency of the tunics of sight|the various layers of the eye, such as the cornea and humors in a straight line except that which is on a straight line raised above the surface of the eye at right angles. Anything on any other line will be refracted and will not pass through straight, because the transparency of the eye's tunics is not the same as the transparency of the air touching the surface of the eye. And that which is refracted from these forms will also be refracted along slanting lines, not along the perpendicular lines extending from the point of refraction. Only one straight line goes out to a point on the surface of sight from one point on the surface of the visible object such that it is perpendicular to the surface of the eye; and infinite slanting lines go out from it to the surface of the eye. The form coming along the straight path of the perpendicular passes through the tunics of the eye in a straight perpendicular line; but all forms coming along slanting lines to that point are refracted at that point and pass into the tunics of the eye along slanting lines. None of them pass according to the extension of the lines along which they came, nor according to the straightness of the lines erected perpendicularly over that point. At any point on the surface of the eye, the forms of all points on the surfaces of all visible and illuminated objects opposite to it arrive at the same time, because there is a straight line between that point and every point opposite to it. From any of the points on the surfaces of illuminated visible objects, forms extend along every straight line that can be drawn from that point; but the form of only one point out of all the points opposite the eye reaches that point of the eye's surface at that time along a perpendicular raised above that point of the eye's surface. The forms of all the remaining points reach that point of the eye's surface along slanting lines. At every point on the surface of the eye, the forms of all visible points opposite it pass through at the same time; yet the form of only one point passes straight through the transparency of the eye’s tunics—specifically the point at the end of the perpendicular emerging from that point of the eye’s surface. The forms of all other points are refracted at that point of the eye's surface and pass through the transparency of the eye's tunics along lines slanted to the surface of the eye. And from every point on the surface of the crystalline lens original: glacialis, only one line emerges perpendicular to the surface of the eye; while from the same point, infinite lines emerge to the surface of the eye, and these are slanting upon it. Therefore, from a point on the surface of the crystalline lens from which a perpendicular emerges to the surface of the eye and passes through the opening of the uvea|the pupil or opening in the iris, infinite lines emerge which pass through the opening of the uvea and reach the surface of the eye, besides that perpendicular. The ends of all these lines emerging from some point of the surface of the crystalline lens, passing through the opening of the uvea and reaching the surface of the eye while slanting upon it, when they are...