I thought of a photographic diaphragm, like some people use film to make zone plates, but I wonder if that was a likely way of doing things in 1911. Film was in use, certainly, but I wonder if it had yet become more common for large format photography than dry plates. I've had filmholders dating from that era and earlier, so it was certainly possible.
In the article on soft focus lenses (in french) http://www.galerie-photo.com/soft-fo...rait-flou.html
There's a picture of a fruitbasket shot with tthe imagon. The highlights reproduce more or less the shape of the whole diaphragm with the sieve structure.