I had assumed in the first book that the reader would have at least a passing knowledge of optics, or that he would wish to instruct himself in it by consulting some book on the matter, such as the Dioptrics of Descartes that I had indicated to him; for if authors were to pause to explain even the most general principles of the sciences related to the subject they treat, and to assume nothing is known to the readers, besides boring and offending even the learned they would be able to treat nothing in depth without producing enormous works.