Paper, 'Problema alhasein' [alhasein problem] by Christian Huygens at Paris On the ibn al-Haytham / alhasein / alhazen problem. This paper was enclosed in a letter to Henry Oldenburg dated at Paris [France] on 26 June 1669. The problem attributed to Alhazen (the Latinized name of Ibn al-Haytham (965-1045) is to find a point of reflection on the surface of a spherical mirror in relation to two points, the eye and the visible object. The diagram and text are printed using a method that Huygens claimed he had invented, making the acid bite all the way through the plate and inking over it, as with a stencil. Christopher Wren had come up with a similar copying method earlier in 1662. Subject: Geometry Published in Philosophical Transactions as 'Excerpta ex epistolis non-nullis, ultrò citróque ab illustrissimis viris, Slusio & Hugenio, ad editorem scriptis, de famigerato albazeni problemate circa punctum reflexionis in speculis cavis aut convexis; & primò quidem ex prima Hugenii'. 26 June 1669