Diagram, 'Solutions to certain geometrical problems' by John Collins Diagram for solving two problems posited in the mathematical writings on the calculation of conic sections by Apollonius of Perga. Shows triangles and lines with angles. John Collins used geometrical methods to solve these problems. Collins also refers to the seventeenth lecture of Dr Isaac Barrow's Dioptricks, i.e. Barrow's Lectiones opticae (1669). See also a longer explanation by Collins in his letter [CLP/24/38]. The solutions begin: 'In a rectangle angle there is knowne x = difference of its sides, and n = difference of the segment made by the perpendicular drawn from the right angle on its base and the said perpendicular'. Subject: Mathematics 17th century