Wishful thinking?

Our house has a lot of south-facing windows (the people who built it weren't dummies -- 1850 predates central heating...) Anyway, as I have mentioned before, some of those windows have prisms on the sash, to throw rainbows into our lives. And one of those rainbows seems to be inching higher...

(Horizontal prism, so the light refracts _up_, and thus doubles any angle change.)