sense excepting when the objects presented to it by means of light, reach it by the line a b; as, for instance, by the line b c. For although the lines m n and f g may be seen by the pupil they are not perfectly taken in, because they do not coincide with the line a b. And the proof is this: If the eye, shown above, wants to count the letters placed in front, the eye will be obliged to turn from letter to letter, because it cannot discern them unless they lie in the line a b; as, for instance, in the line a c. All visible objects reach the eye by the lines of a pyramid, and the point of the pyramid is the apex and centre of it, in the centre of the pupil, as figured above.
[Footnote: 51. In this problem the eye is conceived of as fixed and immovable; this is plain from line 11.]
Experimental proof of the existence of the pyramid of sight (52-55).
52.
Perspective is a rational demonstration, confirmed by experience, that all objects transmit their image to the eye by a pyramid of lines.
By a pyramid of lines I understand those lines which start from the edges of the surface of bodies, and converging from a distance, meet in a single point; and this point, in the present instance, I will show to be situated in the eye which is the universal judge of all objects. By a point I mean that which cannot be divided into parts; therefore this point, which is situated in the eye, being indivisible, no body is seen by the eye, that is not larger