Books four and five of a landmark seven-volume work of medieval scientific study of optics. Ibn al-Haytham was perhaps the greatest mathematician and physicist of the medieval Arabic/Islamic world. The most famous book in which he applied his scientific method is his Optics , through which he dealt with both the mathematics of rays of light and the physical aspects of the eye in seven comprehensive books. His rethinking of the entire science of optics set the scene for the whole of the subsequent development of the subject, influencing figures such as William of Ockham, Kepler, Descartes, and Christaan Huygens. The immense work of editing, translating into English, and commenting on this work was undertaken by Abdelhamid I. Sabra. This English translation of Books IV-V was completed by Sabra just before his death in 2013 with an introduction and critical analysis. It has been extensively revised by Jan Hogendijk.