Museo Galileo
Virtual Museum

In the oldest instruments, a rule (Latin regula) denoted a kind of index for positioning certain parts of the instrument. The rule often consisted of a short rod, occasionally graduated. Other models were later developed with various independent characteristics (such as the slide rule) according to the type of divisions etched on the surfaces. In the orthographic planisphere of Juan de Rojas (16th C.), the rule constitutes the diameter carrying the orthographic projection of the meridians; on it slides the cursor with the orthographic projection of the parallels.