Ars Magna Lucis et Umbrae contains all there was to know about light and optics in the middle of the seventeenth century. It begins with a survey of the properties of light (according to Kircher) and continues with the design, building and use of astronomical, and sometimes astrological, instruments. One section deals with the design of various types of sundials (in some cases "moondials") for different purposes, some highly advanced and some designed for use in different parts of the world. Much of the book is devoted to astronomy in practice and in theory, and it also supplies the beginner astronomer with the necessary knowledge of mathemathics. Another subject in the book is how to grind and polish various kinds of lenses and mirrors.

The last of the ten books of Ars Magna Lucis et Umbrae treats the "natural magic of mirrors and lenses", i. e. optical toys and other devices, such as "magical" parabolic mirrors that could project pictures of angels, saints and even of Christ in suitable places, an everchanging picture, a stroboscope or "star-wheel", a giant focusing mirror and a "high-power" parabolic floodlight. In a separate treatise, Cryptologia Nova, he describes how to build and to use projecting devices.

There are also some really weird designs in the book, e.g. a tiny replica of the Memnon sculptures that sings when air, heated by the sun in a specially designed vessel, expands through an organ pipe, hidden in the throat of the sculpture, and "Flying Dragons", i.e. illuminated box-kites decorated with "prodigous images".