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About the same time also Mersenne sent to Descartes, as if they came from a friend in England, another set of objections which Hobbes had to offer on various points in the scientific treatises, especially the Dioptrics, appended by Descartes to his Discourse on Method in 1637; to which Descartes replied without suspecting the common authorship of the two sets.

That part of optics which treats of the refraction of light; Ð commonly called dioptrics.

Catoptrics, which treats of the reflection of light, and Dioptrics, which treats of its refraction.

The book will contain four essays, all in French, with the general title of ‘Project of a Universal science, capable of raising our nature to its highest perfection; also Dioptrics, Meteors and Geometry, wherein the most curious matters which the author could select as a proof of the universal science which he proposes are explained in such a way that even the unlearned may understand them.’

Besides the last two parts of the Principles of Philosophy, the physical writings of Descartes include the Dioptrics and Meteors, as well as passages in the letters.