ALHAZEN

...understood to be refracted according to the manner dictated by the difference in transparency diaphanitas|transparency or the quality of allowing light to pass through between the body of the cornea and the air, these rays arrive at different locations and at various points among the many points on the surfaces of visible objects facing the eye at one time. None of these refracted lines meet the point located at the end of the perpendicular line. The forms In medieval optics, a "form" refers to the likeness or physical information of color and light emanating from an object. of the points at the extremities of all these lines on the surfaces of visible objects extend along the straight paths of those lines, reach the surface of the eye, and are refracted toward the same point on the surface of the crystalline lens Original: glacialis—the "icy" or crystalline lens, then considered the essential organ of sight., except for the form of the point located at the end of the perpendicular. This specific form extends along the straight path of the perpendicular and passes directly to that point on the crystalline lens.

Therefore, if the crystalline lens were to sense from a single point all the forms coming to it from every direction verticatio|a line of direction or orientation, it would sense at every point a mixture of many different forms and many colors of the objects facing the eye at that time. Thus, nothing would be distinguished by the lens from the points on the surfaces of visible objects, nor would the forms of the points coming to that spot be ordered. But if the crystalline lens senses from a single point only that which comes to it from a single direction, the points on the surfaces of visible objects will be clearly distinguished.

None of the points whose forms reach the crystalline lens along refracted lines are more "worthy" than any other refracted form, nor is any one refracted direction more significant than another; indeed, the forms refracted to a single point of the crystalline lens at one time are numerous and undefined. However, the point whose form arrives along the straight path of the perpendicular to a point on the crystalline lens is only one single point, and no other form arrives with it along that same perpendicular path, because all refracted forms are only bent along sloping lines.

And since the center of the surface of the eye is the same as the center of the surface of the crystalline lens by note 12, the line that is perpendicular to the surface of the eye is also perpendicular to the surface of the crystalline lens. Therefore, the form that arrives along the perpendicular is distinguished from other forms by two conditions: first, it extends from the surface of the visible object to the point on the crystalline lens in a perfectly straight line, while the others arrive along refracted lines; and second, the perpendicular itself, standing upright upon the surface of the eye, is also perpendicular to the surface of the crystalline lens. The remaining lines, along which the other refracted forms arrive, are slanted relative to the surface of the eye.

The action of light arriving along perpendicular lines is stronger than the action of light arriving along slanted lines. It is therefore more appropriate that the crystalline lens should not sense from any given point anything except the form arriving at that point along the straight path of the perpendicular alone, and should not sense from that point anything that arrives by way of refracted directions. Furthermore, since the center of the surface of the eye and the center of the surface of the crystalline lens are the same point, all perpendiculars raised above the surface of the lens and the surface of the eye will meet at this common center. They will be diameters within the surfaces of the layers Original: tunicarum—the "tunics" or membranes of the eye. of the eye, perpendicular to those layers. Each perpendicular will meet the surface of the cornea at one point and the surface of the crystalline lens at one point. No more than one perpendicular passes through that point of the cornea, and no more than one perpendicular passes through that point of the crystalline lens. Thus, the form that exits from any point on the surface of a visible object along a perpendicular—extending from it to the surface of the eye—meets the surface of the eye at a single point, where no other form (of those not arriving along perpendiculars) meets it.

19. Vision occurs by means of a pyramid, whose vertex is in the eye and whose base is in the visible object. 18. 21. 22 p 3.

Furthermore, it has already been determined 14. 18 n that from every point of any colored body, illuminated by any light, light and color go out along every straight line that can be extended from that point. Therefore, between any point of the eye and any point facing a surface, and every point of that surface, an imaginary straight line exists. Between that point [of the eye] and that surface, there is an imaginary pyramid whose vertex is that point and whose base is that surface. That pyramid contains all the understood straight lines that exist between that point and all the points of that surface.

Since the form of light and color exits from every point of the surface of an illuminated, colored body along every straight line that can be extended from that point to any point facing that body: the form of the light and color on the surface of that body will extend from every point of its surface to the point facing it along the straight line stretched between the body and that point. Therefore, the form of light and color of any colored body illuminated by any light extends from its surface to any point facing that surface according to the orientation of a pyramid, which is formed between that point and that surface. The form will be ordered within that pyramid by those lines converging at the point which is the vertex of the pyramid, just as the parts of the color are ordered on the surface of the body itself.

When the eye is placed opposite any visible object, an imaginary pyramid will be formed between the point that is the center of the eye and the surface of that visible object; its vertex will be the center of the eye and its base will be the surface of that object. When the air between the object and the eye is continuous, and no dense body stands between them, and the object is illuminated by any light: the form of the light and color on the surface of the object will extend to the eye according to the orientation of that pyramid. The form of every point of the surface of that visible object will extend along the straightness of the line that exists between that point and the vertex of the pyramid, which is the center of the eye.