The two names have been linked on account of Kamāl-al-Dīn’s critical revision of Ebn al-Hayṯam’s Ketāb al-manāẓer, which represents a watershed in the scientifi;c understanding of light and vision. Kamāl-al-Dīn’s work, entitled Tanqīḥ al-manāẓer le-ḏawī al-abṣār wa’l-baṣāʾīr, was for long assumed to be a commentary (šarhá) on the Ketāb al-manāẓer. This impression was partly reinforced by the autobiographical information in the Tanqīhá, which is the main source of what little we know about him.
Kamāl-al-Dīn relates (Tanqīh, ed. Hyderabad, I, pp. 4-9) having come to Tabrīz (possibly sometime before 1290) to study under Qoṭb-al-Dīn Šīrāzī (634-710/1236-1311), one of the distinguished team of astronomer-philosophers from the Marāḡa observatory in Azerbaijan. Kamāl-al-Dīn’s concern with optics was already suffi;ciently established to question the statements of “leading philosophers” such as Naṣīr-al-Dīn Ṭūsī (Sabra, p. lxxi, n. 112), on the refraction of rays in water and on why stars appeared larger near the horizon than at higher altitudes. In response to Kamāl-al-Dīn’s dissatisfaction with his readings, his teacher recollected having seen during his youth “a book on optics in two large volumes attributed to Ebn al-Hayṯam” in a library in Fārs and subsequently obtained a copy for Kamāl-al-Dīn “from a distant land.” Fortuitous circumstances thus placed in Kamāl-al-Dīn’s hands a unique work which, in an extensive series of mathematical and experimental studies, had brought together for the fi;rst time the physics of light (dealing with rectilinear propagation, refl;ection, and refraction) and ocular anatomy to explain vision. Previous explanations based on visual rays, qualitative impressions, and indivisible forms were replaced by a new theory of an “optical” punctate image formed in the eye by light refl;ected from the surface of the object—a theory which marks the beginning of physiological optics (Russell, 1996). Qoṭb-al-Dīn Šīrāzī urged Kamāl-al-Dīn to write a commentary on Ebn al-Hayṯam as he himself was preparing one on the Qānūn of Avicenna (q.v.).
Kamāl-al-Dīn’s work may have intended initially to prepare a “summary” (eḵteṣār) of the Ketāb al-manāẓer with a commentary (šarhá), as further indicated by the stylistic use of “he said” for paraphrases of Ebn al-Hayṯam’s text and “I say” for his own statements. It clearly evolved into a critical revision (tanqīhá), not only of Ebn al-Hayṯam’s work but also of the study of “optics” itself. First of all, the Tanqīḥ goes beyond the seven books of the Ketāb al-manāẓer to include in a sequel (ḏayl) and three appendices (lawāḥeq) recensions of other treatises by Ebn al-Hayṯam—on the halo and the rainbow (Maqāla fī qaws qozaḥ wa’l-hāla); the burning sphere (al-Kora al-moḥreqa), shadows (Kayfīyat al-aẓlāl), the shape of the eclipse (Ṣūrat al-kosūf), and a discourse on light (Fi’l-żawʾ)—some of which were associated with astronomy and metereology. In so doing, Kamāl-al-Dīn redefi;ned the boundaries and provided a more comprehensive presentation of the science of optics (manāẓer). In the concluding section (ḵātema) of the Tanqīhá, he expanded Ebn al-Hayṯam’s research on refraction in book 7 of the Ketāb al-manāẓer (Sabra, II, pp. lxii-lxiii). At the same time, utilizing the result of Ebn al-Hayṯam’s investigations as well as experimental techniques in dark rooms or camera obscura (al-bayt al-moẓlem), he made innovative contributions in both physical and physiological optics which deviated from those of Ebn al-Hayṯam.
For example, a new explanation of the rainbow by Kamāl-al-Dīn accompanies Ebn al-Hayṯam’s Maqāla fī qaws qozaḥ wa’l-hāla in the sequel to the Tanqīḥ. Radically departing from previous theories, it correctly accounts for the shape of the arc, the presence of the primary and secondary bows, and describes the order of colors in both, showing their reversal in the secondary bow. Kamāl-al-Dīn’s explanation is distinguished by the originality of its experimental procedure where he substitutes a glass sphere fi;lled with water for an individual droplet, places it in a dark room with a single aperture, and investigates what happens as rays of light pass through his model. Inspired by Avicenna’s observation in the Šefāʾ that rainbows occur independently of the presence of dark clouds (traditionally assumed to serve as a concave mirror), Kamāl-al-Dīn ingeniously brings together in his experiment Avicenna’s emphasis on water droplets and Ebn al-Hayṯam’s studies of refraction in the Ketāb al-manāẓer and of parallel rays through transparent spheres in al-Kora al-moḥreqa. He correctly describes the lower primary bow as a result of two refractions (of rays entering and emerging from the waterdrop) with one internal refl;ection; and the secondary bow as a result of two internal refl;ections between the two refractions (for a detailed exposition, see Weidemann; Naẓīf; and esp. Rashed, 1973, pp. 213-18). His attempt to account for the colors, though unsuccessful, is of considerable interest in itself.
Fazlallah Khunji Isfahani
