Geometrical optics is both the object of abstract study and a body of knowledge necessary for design and engineering. The subject of geometric optics is small, since so much can be derived from a single principle, that of Fermat. and large since the consequences are infinite and far from obvious. Geometric optics is deceptive in that much that seems simple is loaded with content and implications, as might be suggested by the fact that some of the most basic results required the likes of Newton and Gauss to discover them. Most of what appears complicated seems so because of obscuration with mathematical terminology and excessive abstraction. Since it is so old. geometric optics tends to be taken for granted and treated too casually by those who consider it to be "understood." One consequence is that what has been long known can be lost if it is not recirculated by successive generations of textbook authors, who are pressed to fit newer material in a fairly constant number of pages.