Astronomy and perspective were linked indirectly through instruments, such as the planisphere and astrolabe, which involved planisphere projection. Etymologically, perspective was linked with optics (perspectiva), and with the verb seeing through (perspicio). In the early seventeenth century, the telescope, which literally involved seeing through, became known as a perspective tube and, in at least one case, Hycynianus, a work on the telescope was included in a bibliography on perspective.