Treasury of Optics

The original text explains the mechanics of vision by distinguishing between light rays that arrive perpendicularly versus those that are refracted.

...are understood to be refracted according to the mode affirmed by the diversity of transparency, which exists between the transparency of the cornea and that of the air. They arrive at different locations, and at different points among the number of points that are on the surfaces of the objects opposite to the eye at one time; and none of these lines encounter the point which is at the extremity of the perpendicular. And the forms of the points that are at the extremities of all those lines of the surface of the viewed objects extend according to the straightness of those lines, and they arrive at the surface of the eye, and they are refracted to the same point of the surface of the glacialis crystalline lens, except for the form of the point which is at the extremity of the perpendicular; for it extends according to the straightness of the perpendicular, and passes through to that point of the glacialis.

If, therefore, the glacialis senses from one point all the forms coming to it from all directions, it will sense from every point forms admixed from many different forms, and from many colors of the viewed objects opposite to the eye at that time; and thus nothing will be distinguished by it from the points that are on the surfaces of the viewed objects, nor will the forms of the points coming to that point be ordered. But if the glacialis senses from one point that which comes to it from one direction only, the points that are on the surfaces of the viewed objects will be distinguished by it. And no point, whose forms arrive at the glacialis along refracted lines, is more worthy than another among the refracted forms, nor is any refracted direction more worthy than another; and the forms refracted to one point of the glacialis at one time are many and indeterminate.

And the point whose form comes according to the straightness of the perpendicular to one point of the glacialis is one point only, and no other form comes with it according to the straightness of the perpendicular; for all refracted forms are not refracted except along declining lines. And when the center of the surface of the eye is the same as the center of the surface of the glacialis per 12 n, the line that is perpendicular to the surface of the eye is perpendicular to the surface of the glacialis. The form, therefore, that comes along the perpendicular is distinguished from other forms by two dispositions: the first of which is that it extends from the surface of the viewed object to the point of the glacialis along a straight line, and the rest arrive along refracted lines; the second, however, is that this perpendicular erected upon the surface of the eye is also perpendicular to the surface of the glacialis; and the remaining lines, along which the remaining refracted forms come, are declining upon the surface of the eye. And the operation of light coming along the perpendiculars is stronger than the operation of light coming along inclined lines. It is, therefore, more fitting that the glacialis should not sense from any point except the form coming to that point along the straightness of the perpendicular only, and should not sense from that point that which comes to it according to refracted directions.

And again, when the center of the surface of the eye and the center of the surface of the glacialis are the same point, all perpendiculars raised upon the surface of the glacialis and the surface of the eye will concur at the common center, and will be diameters in the surfaces of the coats of the eye, perpendicular to those coats of the eye; and every perpendicular will be one that meets the surface of the cornea at one point, and meets the surface of the glacialis at one point; and there does not emerge to that point of the cornea except one perpendicular, nor does there emerge to that point of the glacialis except one perpendicular only. The form, therefore, that emerges from any point of the surface of the viewed object along the perpendicular which extends from it to the surface of the eye, meets the surface of the eye at one point, upon which no other form of those not coming along the perpendiculars meets it.

19. Vision occurs through a pyramid, whose vertex is in the eye, and whose base is on the viewed object. 18. 21. 22 p 3.

AND again it has already been determined 14. 18 n that from any point of any body that is colored and illuminated with any light, light and color emerge along any straight line which can be extended from that point; therefore, between any point of the eye, and any point opposite to any surface, and any point of that surface, there is an imaginable straight line, and between that point and that surface there is an imaginable pyramid, whose vertex is that point, and whose base is that surface; and that pyramid contains all the straight lines understood to be between that point and all the points of that surface.

When, therefore, the form of light and color have emerged from any point of the surface of the colored and illuminated body along any straight line which can be extended from that point to any point opposite to the illuminated and colored body, the form of light and color that are on the surface of that body will extend from any point of the surface of that body to that point opposite to it along the straight line extended between that body and that point. The form, therefore, of light and color of any body colored and illuminated with any light extends from its surface to any point opposite to that surface according to the direction of the pyramid that is formed between that point and that surface; and the form will be ordered in that pyramid along those lines concurring to that point which is the vertex of the pyramid, just as is the ordering in the parts of the color which is on the surface of that body.

When, therefore, the eye is opposite to any viewed object, there will be formed between the point that is the center of the eye and the surface of that viewed object an imaginable pyramid, whose vertex will be the center of the eye and whose base will be the surface of that viewed object; and when the air between that viewed object and the eye is continuous, and there is no dense body between the viewed object and the eye, and that viewed object is illuminated with any light, the form of light and color that are on the surface of that viewed object will extend to the eye according to the direction of that pyramid, and the form of any point of the surface of that viewed object will extend along the straightness of the line that is between that point and the vertex of that pyramid, which is the center of the eye.