> Then you can't see it through this device either. The technical trick is to align an edge of a mirror which reflects part of the real image with the canvas a

> Then you can't see it through this device either. The technical trick is to align an edge of a mirror which reflects part of the real image with the canvas and add paint until there is no visible difference. I believe this trick can get around the shade corrections made by the brain, just like tracing over camera obscura gets around the proportion & angle corrections.