Treasury of Optics

which depend on light and color, except through the reception of the forms of colors, and the transmission of them to the opposite parts: and it has been declared original: "[19 n]" that since the sight is opposed to the viewed object, the forms of light and color, which are in the viewed object, will be transmitted to the sight, and will arrive at the surface of the sensing organ: therefore, the sight does not sense the light and color of the viewed object, except from the form coming from the light and color, which are in the viewed object.

23. Vision does not occur through rays emitted from the sight. § 3.

It is permitted for us to say that a transparent body receives something from the sight, and returns it to the viewed object, and through the continuation of this viewed object between the sight and the viewed object, sensation occurs. And this is the opinion of those who posit rays exiting from the sight. Let it be posited, therefore, that it happens thus, that rays exit from the sight, and pass through the transparent body, arriving at the viewed object, and through these rays, sensation occurs. And when sensation happens in this way, I ask whether anything is returned to the sight through these rays, or not: If, indeed, sensation occurs through rays, and they do not return anything to the sight, the sight will not sense the viewed object, and it does not sense, except through the mediation of rays: therefore, these rays, which sense the viewed object, return something to the sight, through which the sight senses the viewed object. And since the rays return to the sight something through which it senses the viewed object, the sight will not sense the light and color, which are in the viewed object, unless from something coming from the light and color, which are in the viewed object, to the sight, and the rays return those things. Therefore, according to all arrangements, vision will not occur, except through the arrival of some viewed thing from the viewed object, whether rays have exited or not. And it has already been declared original: "[22 n]" that vision is not completed, except through the transparency of the medium body between the sight and the viewed object, and it is not completed when the medium between them is a non-transparent body. And it is manifest that a transparent body is distinguished from a non-transparent one in nothing, except according to the aforesaid manner. And when it is so, as we have said, and it has been declared that the forms of light and color, which are in the viewed object, arrive at the sight when they are opposite to the sight. That thing, therefore, which comes from the viewed object to the sight, through which the sight comprehends the light and colors which are in the viewed object according to every disposition, is nothing other than this form, whether rays exit or not. And it has already been declared original: "[14.18 n]" that the forms of light and color are always generated in the air, and in all transparent bodies, and are always extended in the air and in transparent bodies to the opposite parts, whether the eye is present or not. The exit of rays, therefore, is superfluous and idle. Therefore, the sight does not sense the light and color of the viewed object, except from the form coming from the light and color, which are in the viewed object. And it has been declared original: "[19 n]" that the form of any point of the viewed object, opposite to the sight, arrives at the sight according to many diverse directions, and that the sight cannot apprehend the form of the viewed object according to its ordering on the surface of the viewed object, except when the reception of forms is from the directions of straight lines, which are perpendicular to the surface of the sight, and to the surface of the sensing member, and that straight lines will not be perpendicular to these surfaces, except when the center of these surfaces is one point. And when all this is, as has been said: it is necessary that the center of the crystalline surface original: "glacialis" — the crystalline lens and the center of the surface of the sight be one point. Therefore, the sight can comprehend nothing from the forms of viewed things, except from the directions of straight lines, whose extremities concur at this center alone. And this is what we promised before to declare in this chapter in the preceding discourse original: "[12 n]" concerning the form of the sight: namely, that the center of the crystalline lens and the center of the surface of the sight are the same point. And since this has been declared, it remains now to consider the opinion of those who posit rays exiting from the sight, and to declare what in it is false, and what is true. Let us say, therefore, if vision occurs from something exiting from the sight to the viewed object: this thing is either a body, or not a body: If it is a body, when we look at the sky, and see the stars which are in it, it would be necessary that at that hour a body exit from our sight, and fill that which is between the sky and the earth, and that nothing be diminished from the sight: and this is false. Therefore, vision is not through a body exiting from the sight to the viewed object. And if that which exits from the sight is not a body, that thing will not sense the viewed object: for sensation does not exist, except in bodies. Therefore, nothing exits from the sight to the viewed object, sensing that object. And it is manifest that vision is through the sight: and when this is so, and the sight does not comprehend the viewed object, except when it exits from it to the viewed object, and that which exits does not sense that object. Therefore, that which exits from the sight to the viewed object does not return to the sight anything by which the sight comprehends the viewed object. And this is that which is not sensible, but a matter of opinion, and should not be thought of except through reason. However, those who posit rays exiting from the sight opine this, because they found this: that the sight comprehends the viewed object, and there is space between them, and it is a great thing to men that sensation does not occur, except through contact. Wherefore they opined that vision does not occur, except through something exiting from the sight to the viewed object, such that that which exits senses the object in its place, or takes something from the viewed object, and returns it to the sight, and then the sight senses it. And because a body sensing the viewed object cannot exit from the sight, and nothing senses the viewed object except a body: it remained to opine only that that which exits from the sight to the viewed object receives from the sight something, and returns it to the sight. And because it has been declared original: "[14.18 n]" that the air and transparent bodies receive the form of the viewed object, and return it to the sight, and to every body opposite the viewed object: then that which is opined, that it returns something of the viewed object to the sight, is nothing but the air and transparent bodies between the sight and the viewed object. And since the air and transparent bodies return to the sight something of the viewed object, in any time they return it, and according to all dispositions, when the sight is opposite to the viewed object, without the need for any thing exiting from the sight. Therefore, the reason which led those who posit rays to say that there are rays is superfluous: since that which led them to say that rays exist is their opinion: because vision cannot be completed, except through something extended between the sight and the viewed object, which returns to the sight something of the viewed object. And since the air and transparent bodies do this without the need for anything exiting from the sight, and are moreover extended between the sight and the viewed object without need: then for positing another thing returning something to the sight of the viewed object, there is no opinion.