This illustration from the Encyclopédie of 1751 explains in great detail how a camera obscura works and how it can be constructed. It thus attempts to establish a closer connection between theoretical and practical knowledge which was by no means self-evident before the 18th century.
Camera obscura, Drawing, 1751, unknown artist, in: Encyclopédie, ou dictionnaire raisonné des sciences, des arts et des métiers, Planches, Neuchatel 1751, vol. III, p. 143 [??], scan: Jean-Jacques MILAN; source: Wikimedia Commons, http://commons.wikimedia.org/wiki/File:Camera_obscura.jpg, public domain.