Levi ben Gerson's tranversale scale on his Jacob's staff, following a drawing of Bernard R. Goldstein
<a href="//commons.wikimedia.org/wiki/User:Kaidor" title="User:Kaidor">Kaidor</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>, following Bernard R. Goldstein <i>Levi ben Gerson's Contributions to astronomy</i>, in G. Freudenthal (ed.), <i>Studies on Gersonides. A Fourteenth Century Jewish Philosopher</i>, Leiden - New York,1992, <a href="//commons.wikimedia.org/wiki/Special:BookSources/9004096418" title="Special:BookSources/9004096418">ISBN 9004096418</a>, p 5 <a rel="nofollow" class="external text" href="https://books.google.fr/books?id=DXSUpHqPiMgC&amp;pg=PA5#v=onepage&amp;q&amp;f=false">link</a>.