**Here**, **too**, **he** **made** **the** **acquaintance** **of** **Claude** **Mydorge**, **one** **of** **the** **foremost** **mathematicians** **of** **France**, **and** **renewed** **an** **early** **intimacy** **with** **Malin** **Mersenne** (**q**.**v**.), **now** **Father** **Mersenne**, **of** **the** **order** **of** **Minim** **friars**.

**By** **this** **time** **he** **had** **ceased** **to** **devote** **himself** **to** **pure** **mathematics**, **and** **in** **company** **with** **his** **friends** **Mersenne** **and** **Mydorge** **was** **deeply** **interested** **in** **the** **theory** **of** **the** **refraction** **of** **light**, **and** **in** **the** **practical** **work** **of** **grinding** **glasses** **of** **the** **best** **shape** **suitable** **for** **optical** **instruments**.

**It** **was** **about** 1648 **that** **Descartes** **lost** **his** **friends** **Mersenne** **and** **Mydorge** **by** **death**.

**Claude** **Mydorge** (1585**-**1647), **a** **French** **geometer** **and** **friend** **of** **Descartes**, **published** **a** **work** **De** **sectionibus** **conicis** **in** **which** **he** **greatly** **simplified** **the** **cumbrous** **proofs** **of** **Apollonius**, **whose** **method** **of** **treatment** **he** **followed**.