Chapter 11 uses this example:
http://www.tei-c.org/release/doc/tei-p5-doc/en/html/PH.html#Note77_return
from Géométrie pratique by Bovelles, and credits it to this source:
http://ancilla.unice.fr/Illustr.html
The source is an ancient page which keeps disappearing from the web, breaking our link. We need to discover whether this particular page image has been published by the original copyright holders in some other location where it's more stable, or failing that, either link to the last Wayback machine version of the page (from Feb 2013):
https://web.archive.org/web/20130211170317/http://ancilla.unice.fr/Illustr.html
or remove the link altogether.
There are plenty of other places to find this particular page if we want to cite one of them. For example, the BNF has a copy at
http://gallica.bnf.fr/ark:/12148/bpt6k572723/f99.image
There are also other editions of course:
http://gallica.bnf.fr/ark:/12148/bpt6k107722q/f156.image
http://www.e-rara.ch/zut/content/pageview/2702737
Is it sufficient just to change the URL, or do we need to change the note text as well?
Yes, clearly the note text would need changing if we want to reference the BNF's copy.
I think you would have got permission from Marie-Luce last time around -- do we need to get extra permission, or would her content have been the source of the BNF copy?
If we use the BNF copy, we would presumably have to re-make the image, since it's not exactly the same one. My understanding is that the BNF has a policy of making its digital images available online : citing the ARK handle for it is all they ask you to do.
Alternatively, the easiest/least disruptive action would be to drop the link from the footnote concerned. Or substitute the Wayback link (the full one: https://web.archive.org/web/20051027034634/http://ancilla.unice.fr/images/Bouelles-49r.gif) if we want to be scrupulous about where we got the image from.