I got vaguely fascinated by the documentary Tim's Vermeer a few years back, where Tim Jennison makes the argument that Vermeer used something not unlike a couple of these to project his scenes onto a canvas so he could effectively paint-by-numbers. Whether that's true or not, I found the idea fascinating, I just don't have a good enough understanding of optics to understand how the system of lenses and mirrors works to be able to know how to replicate it.