I think the mirror was for capturing sunlight and directing it to properly light the subject being photographed. the wooden part is either part of a stand for it or for making it so the "subject" doesn't squint - chances are good this was still when they used to clamp you into a chair to take your portrait.... maybe not that early, but still, it took a lot of light to make an image with those cameras, even with all the advances made by then - a whopping 20 or 30, maybe 40 years of 'modern' photography at the time or somewhere in there. Check out some photo history books to get some ideas of how mirrors were used for lighting. People used all kinds of contraptions for it but most of them redirected the sun.