Hi Meg,
The Biblical symbolism is fairly obvious as illustrations of Old and New Testament texts. It compares well with contemporary artwork in illustrated manuscripts. To get a grasp of the non-biblical images like zodiacs, astrolabes, sirens and geometrical stuff you need to look at the texts of Natural Philosophy that the scholars of the period were reading and writing. So look to Boethius, Macrobius, Capella, Plato (Timaeus), Euclid and all the sources that fed into the study of the quadrivium.
The best introduction I've found to the natural philosophy of the period is Peter Ellard's A Sacred Cosmos. See also Edouard Jeauneau's Rethinking the School of Chartres and his Lectio Philosophorum.
I think you also need to consider that rhetorical devices used by students of the trivium were applied also to the visual and touchable forms of expression found in the sculpture and glass of the cathedrals. I think there is no device more important than what they called the integument. The best source I've found on that is Winthrop Wetherbee's Platonism
and Poetry in the Twelfth Century: The Literary Influence of the School of Chartres.
I've posted some draft papers (with lots of pictures) touching on many of these issues as they relate to Chartres. Many primary and secondary sources are listed.
As for sunlight entering certain windows, my hands-down favorite is what happens on the floor of Vézelay Abbey around noon on the summer solstice. See image at:
http://www.bacimu.be/users/alistair
Cheers,
Richard J Legault
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