In his catalogue of Arabic manuscripts at the Université St. Joseph, Beirut, L. Cheiko describes MS 223,20 as follows: Traité d'Aristanés (?) sur la construction des deux moyennes proportionelles par la méthode de la géométrie fixe. The purpose of this note is to point out that the tract mentioned is actually an Arabic translation of a letter concerning the construction of two mean proportionals between two given straight lines, purporting to be by Eratosthenes, and of which several copies are extant.