Abstract
In Chapters 6 through 8 of the Analyse, l’Hôpital studies various kinds of envelopes: curves that are tangent to all the members of some family of lines or curves. In Chapter 7, l’Hôpital studies caustics by refraction, or dicaustics. This problem derives from optics and is essentially the study of envelopes made by light rays refracted through a lens. L’Hôpital considers a variety of shapes of lenses and sources of light rays.