This is the title page of Christopher Clavius' *Euclidis Posteriores Libri IX* from 1654; the original edition appeared in 1607. In translation this page reads approximately as

*Beyond Euclid’s Nine Books.*

Included is Book XVI, *On the Comparison of Regular Solids of all Kinds*, all clearly demonstrated and illustrated with the most accurate commentary; now published in quarto and enriched with much material not contained in the first edition.

By the author Christopher Clavius

Bamburg, Society of Jesus

Frankfurt: Published by the heirs of Jonah Rocia in the year 1654

The publisher’s seal is particularly interesting showing the biblical scene of a whale vomiting Jonah onto the shore. The whale appears to be an imaginary creature conceived from the mind of the artist.

The *Posteriores* appears to be an extension of Clavius’ *Euclidis Elementorum *of 1574, in which he considered fifteen books of Euclid. Here we find a sixteenth book added by François de Foix-Candale (1512-1594). De Foix was a French mathematician and alchemist and a noted geometer who established a Chair of Geometry at the University of Bordeaux in 1591. The three books comprising an addendum to Euclid’s *Elements*, Books XIV-XVI, all consider the regular polygons and their properties.

Page 275 of *Posteriores* discusses two propositions of Book X:

Proposition 35: To find two straight lines in square which make the sum of the squares on them medial and the rectangle contained by the medial and moreover incommensurable with the sum [of] the squares on them.

Proposition 36: If two rational straight lines commensurable in square only are added together, then the whole is irrational; let it be called binominal.

### Acknowledgments

Professor Michael Twomey of Ithaca College, Ithaca, New York, assisted with the Latin translation.

*The images above are provided courtesy of Archives and Special Collections, Dickinson College, Carlisle, Pennsylvania.*