Letter, from Renatus Slusius to Henry Oldenburg, dated at Luttich Discussing some problems with Barrow's 'Lectiones opticae'; concerning Collins's work on geometrical curves; shows that Grienberger's discovery of the foci of conic sections can be deduced from Slusius's work; concerning translations of books and other literary notices; encloses a letter to Wallis (see EL/S1/64) Mathematical figures in text 25 July 1670