Excerpt from Journal de Mathématiques Pures Et Appliquées, Vol. 2: Quatrième Série, Année 1886On sait qu'on appelle foyers les deux points où un rayon lumineux réfracté touche les surfaces caustiques ces points ne sont pas toujours réels. Les plans tangents des surfaces caustiques aux deux foyers sont les plans focaux.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.