Astronomical Images : Rete and mater plates marked with red ink, cut out and mounted in line with Blagrave's instructions

John Blagrave

Astronomical Images

<p style='text-align: justify;'>This image comes from John Blagrave's <i>The Mathematical Jewel </i>(1585). In this work, Blagrave drew on a long tradition of guides to the construction and use of astrolabes, referencing works by Gemma Frisius and Johann Stoeffler amongst others. The new instrument that Blagrave presented as the mathematical jewel was an astrolabe of his own devising which had the benefit that it could be used anywhere in the World without the need to substitute different plates according to the latitude. At some point a reader of this copy of Blagrave's book cut out the two full-page images at the front of the work, mounted them, trimmed away numerous tiny sections and assembled the two sheets so as to make a working version of the mathematical jewel. The reader has also filled in several numbers on the scale around the edge of the mater, according to the written instructions that Blagrave gave in the body of the work. Although a reader has taken pains to assemble the pages as we see here, it is interesting that they should then mount the paper instrument on the inside cover of the book. This arrangement would have meant that the instrument was always to hand while reading the book, although it would have made it impossible to use for many of the observational techniques that Blagrave recommended. Furthermore, there is no evidence that this reader attempted to reconstruct the back of the instrument, which was much simpler and intended for use in basic surveying, rather than in astronomical observation. This all hints at the variety of interests that Blagrave's book could support. In spite of the fact that this paper instrument could not have been used in the sort of observational work that Blagrave at times described, constructing the instrument and using it while reading the book could have helped a reader to understand some of the astronomical and instrumental principles presented in the text.</p>