From my Notebooks: A (The?) Geniza Fragment on Optics

I. Introduction

Jews as a rule displayed little interest in the science of optics; this generalization applies to medieval communities in both east and west. Here follows a brief survey of the extant texts. The Euclidean Optics and Catoptrics were translated into Hebrew. Neither of these translations has been edited or compared in any way with the Arabic from which it derives.¹ As far as original Hebrew writings are concerned, we possess hardly anything beyond brief entries in some encyclopedias, such as Abraham bar Ḥiyya’s Migdal ha-Tevunah, and crisp exposés in works on the classification of the sciences, such as Shem Ṭov Ibn Falaquera’s Rešit Ḥoḵmah.²

Levi ben Gerson once again proves to be somewhat of an exception. He discusses meteorological optics in his supercommentary to Averroes.³ Moreover, he was the first person to use a pinhole camera in order to observe the apparent solar diameter at apogee and perigee, and, from these measurements, to determine the eccentricity of the sun.⁴

Nothing original was written in Judeo-Arabic, but a copy of Aḥmad ibn ʿĪsa’s Kitāb al-Manāẓir wa-l-marāya al-muḥriqa was transcribed into Hebrew characters.⁵ While that singular manuscript does not seem to have had any influence upon Jewish thinkers, it is nonetheless of great value to scholarship, since it preserves sections not extant in the Arabic-letter manuscripts. No texts on optics have been identified in any of the Geniza collections.⁶

However, the interest of the fragment we publish here is by no means limited to its status as the unique (to date) Geniza fragment on optics. Despite the difficulties in deciphering some portions of the preserved text, we can say with some confidence that its teachings are unusual for the Arabic tradition. In particular, it assigns to coloration (talwīn) a key role in activating vision; this is in line with what Ptolemy taught in his book on Optics.⁷ The fragment also speaks about artificially inducing a sense of depth on a flat surface by manipulating the lightness or darkness of the colors employed.⁸ The author, who, as we understand him, presents the fruits of his own research, may endorse some form of extramission (though not the well-known atomistic version).⁹

The fragment is found in the collection of the Alliance Israélite Universelle, where it is catalogued as IX A 22, and is published here with the kind permission of that institution. These Cairo Geniza fragments are the property of the Consistoire Israélite de Paris, brought [Begin Page 348] to Paris and given to it by the Baron de Rothschild in 1901 and ever since under the custodia of the Alliance Israélite Universelle. Side one has nineteen lines; side two, twenty. There are usually seven or eight words in each line. It is written in semi-cursive “eastern” hand which I date to the second half of the eleventh century.¹⁰

It is not possible to say much more about the place of this fragment in the history of medieval optics.¹¹ Painted in broad strokes, the history of optics is not unlike that of the other sciences. By the early ninth century, fundamental Hellenistic texts by or attributed to Euclid and Ptolemy had been translated into Arabic. Al-Kindī and his school prepared a new synthesis, based mainly upon the Hellenistic heritage, but by no means slavishly following any single text; Euclid in particular was singled out for criticism.¹² Later generations of Islamic scientists carried on work in the tradition; in the field of optics, a real breakthrough was achieved by Ibn al-Haytham in his monumental Kitāb al-Manāẓir.¹³

As far as the status of the texts is concerned, however, optics is different. Ptolemy’s text has not survived in either Greek or Arabic; all that we have is a Latin translation, prepared from an Arabic version. To make matters worse, book one of his Optics, in which he presumably displayed his theories of vision, light, and color, has...