Since Biblical references are often lost on me, I have occasion to look them up online*, which has the benefit of not just referring me to various Bible translations, but also to artistic interpretations. This is Belshazzar's Feast, by some Dutch guy with an R who was great with golden colors and light.
And not only do I now know what "the writing on the wall" refers to, but I also have an incling why almost all graffiti is illegible to me: it's Hebrew.
* Today's search was occasioned** by Wodehouse's Right Ho, Jeeves, in the context of which we get, "Oh, that, yes. I've oftn wodered how that gag was worked. With mirrors, I expect."
** And I'd never use "occasioned" if it weren't for Wodehouse. Highly recommendable, by the way (reading Wodehouse, that is; though using "occasioned" is, too).