I think that I'm going to go with Gaines and Nakis on this one, it looks to me like light coming from the window. Judging from the shadows the light is coming not only from the window to the side but perhaps one lower down from the stairwell as well-or maybe a lightbulb there or simply from the flash of the camera, I noticed that the bust shadow is taller than the bust itself- indicating a light source below it. Though I don't think any manipulation of the pic went on here it actually would have been possible even in 1929, you can get many neat special effects in a darkroom, possibly even more convincing than those done on a computer. But like I said, I don't think that's what happened here.