Hi KMR, I bet you're right about the mirror. Looking at the shot again (thanks MB for including it in your comment), I suddenly flashed back to the huge mirror Julia had hanging over her lab in 1840. (Whenever I watch those episodes now, I get totally distracted wondering just how Julia got that thing up there. A mirror that size would have been extremely heavy. Did Julia have levitation powers she never told anybody about??)
I remember reading decades ago in KLS's very first book about how they did a lot of shots in mirrors on the set, so I'm sure you're correct about this.
Thanks!
G.