This book is primarily intended for medical students as a text-book on the subject of Geometrical Optics for their preliminary scientific examinations, though it practically contains all the Optics required by an ophthalmic surgeon. It is hoped that it will also prove of service to students of physics, as some knowledge of the subject is indispensable if the laboratory experiments are to be understood.It requires prolonged and deep study to form any vivid conception of the now accepted theory of light, and in all elementary books the form in which the undulatory theory is presented is so crude that it is both untrue and useless. The subject of Physical Optics therefore has been avoided entirely; indeed, I am convinced that no thorough elementary knowledge of that intricate subject can be obtained in the short space of time allotted to the student for studying Optics.As an introduction to mathematical analysis the subject of Geometrical Optics has no equal, for it insists on the importance of paying due attention to the meaning of algebraic signs, and it is also an easy introduction to several somewhat difficult mathematical conceptions. For instance, the vectorial significance of the line Ba being considered as equal to - AB, or AB taken in the reverse direction, opens up a new vista to the student of Euclid and elementary geometrical methods: equally novel is the conception of a virtual image. At the same time every student can verify for himself the results of his calculations so simply by experiment that it will convince him of the reality of the analytical methods employed.