What Is the History of Medieval Optics Really About?1
Smith, A. Mark, Proceedings of the American Philosophical Society
SINCE ITS PUBLICATION IN 1975, David Lindberg's Theories of Vision from Al-Kindi to Kepler2 has become the canonical source for our understanding of medieval optics and its place in the development of modern optics. Lindberg's ulterior purpose in writing this book was to show that, contrary to prevailing opinion, Johan Kepler's account of sight, which is based on the casting of point-by-point images through the lens at the front of the eye onto the retina at the back, represented a continuation of, rather than a break with, the medieval optical tradition, whose foundations were laid by the Arab Ibn al-Haytham, or Alhacen as he came to be known in the Latin West.3 To be sure, Lindberg concluded, "Kepler attacked the problem of vision with greater skill than had theretofore been applied to it, but he did so without departing from the basic aims and criteria of visual theory established by Alhazen in the eleventh century."4 So, even if we credit Kepler with opening the way toward the development of modern optics, neither the opening itself nor what came out of it during the seventeenth century marked a fundamental departure from the past. The transition from medieval to modern optics was evolutionary, not revolutionary.
Lindberg's case rests on the fact, which he demonstrated in meticulous fashion, that Alhacen and Kepler based their accounts of vision on similar theoretical and methodological grounds. Both, that is, used the same basic model of punctiform light-radiation, and both relied upon the same ray-analytic procedure in tracing out the logical implications of that model. That the two reached markedly different conclusions is beside the point. The path that led to them was the same; Kepler simply followed it more rigorously. It would therefore seem reasonable to conclude with Lindberg that, in sharing the same analytic principles, Alhacen and Kepler operated within the same conceptual framework.
But this conclusion is grounded in a problematic assumption: namely, that, like Kepler and his seventeenth-century successors, Alhacen and his Latin medieval followers were primarily concerned with the physics of light and issues more or less directly relating to it.5 As we shall see in due course, this assumption applies to the first group, but not to the second. For, unlike Kepler and his seventeenth-century successors, Alhacen and his medieval Latin followers were far more concerned with making sense of sight than with understanding light. Thus, while there is an undeniable link between Alhacen's and Kepler's accounts at the procedural level, the two are worlds apart at the conceptual level.
Before examining Alhacen's account of vision in detail, let us briefly set the background. The primary source for that account is the Kitab al-Manazir, or "Book of Optics." Written in the 1030s, this work comprises seven books, the first three of which are devoted to a close analysis of visual perception, the second three to an equally close analysis of reflection, and the last to a study of refraction.6 Alhacen seems to have intended the Kitab al-Manazir as a critical response to Ptolemy's Optics, which predated it by nearly nine centuries. In particular, Alhacen challenged the central supposition of Ptolemaic optics that the eye emits visual flux, which passes outward from its centerpoint through the pupil in a radial bundle that forms a cone, as illustrated in figure 1. The rays within this cone are to be thought of by analogy to fingers that reach out to external objects and put us into visual touch with them. The information garnered from this visual contact is conveyed back through the cone of flux in the form of color-impressions. These color-impressions are subject to visual scrutiny after reaching the anterior surface of the eye, and from them we eventually get an internal "picture" of the objects they represent.7 But why posit such visual radiation, Alhacen demanded, when it is perfectly sufficient to assume that objects are seen by means solely of what they radiate to the eye? …