  Now, this account based on “rays” also seems to underlie al-Kindi’s most ambitious work on the physical sciences: a lengthy treatise entitled On Rays (de Radiis, for which see D’Alverny and Hudry 1974) and preserved only in Latin. There is some question as to its authenticity, but it seems plausible that On Rays represents al-Kindi’s attempt to explain all physical interaction – from heating and cooling, to vision, to astral influence, to magical incantations – in terms of a fundamentally geometrical mechanism. (For connections to the optical works, see Travaglia 1999.)