Where did this rainbow come from???
I've just scanned the attached photo. There's nothing especially remarkable about how it was created; I braced the camera against the column, pointing at the capital, set the exposure (1/2 second or so at f/2, I believe), and went click. Bessa-R with a 50/2 Nikkor, Provia 100F; the roll was developed commercially and the other frames show nothing unusual.
The rainbow, I am pretty certain, was not visible at the time of exposure! Can anyone suggest what happened here? (Yes, it's on the slide, not just a scan artifact.)