On Alberti's "sign": vision and composition in quattrocento painting - Leon Battista Alberti's commentary 'On Painting'

Pelacani draws on but modifies a definition of the visible developed by Alhazen. Euclid defines the visible as a lighted, colored, opaque body seen under a visual angle. However, as Sabra explains, Alhazen treats vision in terms of "punctiform elements" rather than angles.(78) Since Aristotle establishes that immediate corporeal vision comprehends only color and light (De anima, 418a26 - 419a24), Alhazen describes visible bodies as aggregates of illuminated points radiating colored light in all directions.(79) This analysis leads him to define the visible as a lighted, colored opaque body of moderate size placed before the eye at moderate distance, and this definition is adopted by Pecham and Bacon.(80)

Alhazen's punctiform analysis poses a problem. If every point radiates light and color in all directions, rays from different points can reach the eye at the same place. How then does visual sense sort out which ray corresponds to which point? For students of optics the physiology of the eye provides the answer to this question(81) [ILLUSTRATION FOR FIGURE 6 OMITTED]. Alhazen holds that the sensitive part of the eye is "a small, delicate, white and humid" lentil-shaped globe mounted within the uveal sphere at the end of the optic nerve directly opposite the pupil. This small globe is filled with two gelatinous liquids of differing density, which are separated by a very thin, frail membrane (indicated in Fig. 6 by a dashed arc): at the front is the "crystalline humor," which resembles ice in its transparency; behind it is the denser but clearer "vitreous humor." Sight occurs when rays pass through the crystalline and vitreous humors to the optic nerve, which transmits sensations to the brain. Rays entering the cornea obliquely are refracted to the periphery as they pass from one humor to another; this refraction weakens them so that they make little if any impression on the optic nerve. Rays perpendicular to the anterior surface of the crystalline humor (which is concentric with the cornea) pass unrefracted to the vitreous humor, which refracts them toward the back of the eye where the optic nerve is most sensitive. Although they do not actually converge in the eye, these perpendicular rays form a "visual cone" or "round pyramid" whose apex is at the center of the eye and whose base is at the object seen. The clearest impression is made by the incident ray at the center of this pyramid, because it passes unrefracted through the crystalline and vitreous humors to the center of the optic nerve. This sorting out of rays into a visual pyramid preserves the "structure" and "order" of visual form.(82)

Nonetheless, perception by a single pyramid is imperfect. Although the centric ray gives a clear perception of apparent color and indicates the direction of the gaze, the other rays, being refracted, give only an uncertain sense of apparent size, shape, and extent. Only by sweeping the centric ray over the surfaces of perceived objects does the viewer obtain a secure knowledge of visual form. This secure knowledge is then certified by judgment - a faculty that is like reason but that belongs to inner sense, not to intellect.