Lenses and Waves

114 CHAPTER 4
methodological, epistemological and conceptual features of perspectivist
optics are perceptible throughout seventeenth-century optics. 24
4.1.1 ALHACEN ON THE CAUSE OF REFRACTION
In the eleventh century, the Islamic scholar Ibn al-Haytham composed a
voluminous work on optics, Kitb al-Manzir. The Optics of Alhacen, as they
are commonly referred to in the West, was intended to bring together
mathematicians’ and physicists’ accounts of light and vision by giving a
systematic treatment of optics that met the demands of both. 25 This required
the combination of the Aristotelian doctrine of forms received by the eye
and Ptolemy’s ray-wise analysis of the perception of shape and position.
According to Alhacen both these notions were partly true but incomplete. 26
The synthesis he had in mind - ‘tarkb’ - consisted of a theory in which the
forms of light and color issue from every point of the object and extend
rectilinearly in all directions. 27 It met the demands of the Aristotelian doctrine
by considering light rays as the direction in which light extended and those
of Ptolemy by appointing rays as the ultimate tool of analysis. As contrasted
to his mathematical precursors, Alhacen regarded a ray as a purely
mathematical entity: “Thus the radial lines are imaginary lines that determine
the direction in which the eye is affected by the form.” 28 In a later work on
optics, the Discourse on light, Alhacen expounded his conception of light and
reflected on the character of the science of optics by discussing the
distinction between mathematical and physical aspects of light. In his view,
each provided answers to different kinds of questions: in physics one
investigates the essence of light; in mathematics the radiation or spatial
behavior of light. Physical theory classified the various kinds of bodies:
luminous, shining, transparent, opaque. Mathematical theory described the
perception of things by means of rays, rectilinear and inflected. 29 In the Optics
Alhacen adopted the Aristotelian concept of forms without further
philosophical inquiry. His exposition on the nature of light in book 1 served
as a physical foundation for the mathematical and experimental investigation
of light and vision that constituted the heart of the Optics.
Alhacen provided the basis for the flourishing of the study of light and
vision in thirteenth-century Europe given shape to by Robert Grosseteste,
Roger Bacon, John Pecham and Witelo. 30 Alhacen’s work reached the west in
24
This theme is amplified by, among others, Schuster, Descartes, 332-334: Smith, Descartes’s Theory of Light
and Refraction, 4-12.
25
Alhacen, Optics I, 3-6 (book 1). The content and scope of Alhacen’s optics is discussed in Sabra’s
introduction to his translation of its first three books: Alhacen, Optics II, xix-lxiii. See further Lindberg,
Theories, 85-86.
26
Alhacen, Optics I, 81 (book 1, section 61).
27
Alhacen’s account for the subsequent one-to-one correspondence between the points of the object and
the image in the eye is discussed in section 2.2.1 above.
28
Alhacen, Optics I, 82 (book 1, section 62)
29
Alhacen, Optics I, li (Sabra’s introduction). See also Sabra, “Physical and mathematical”, 7-9.
30
Lindberg, Theories, 120-121 and 109-116.