Camera Lucida

I purchased a prism and have been trying to figure out how a camera lucida works. I've started to get the basic principle. The prism allows you to see a double image; a combination of your drawing surface and a space in front of you. This allows you to basically trace what you see. Lighting, as with all of these pre-20th century optical tools, is vital. Also, a steady and workable structure needs to be built. This one, constructed from metal bits, begins to work except that it is too upright, forcing you to draw only the right side of the image, and it is very difficult to align the prism, since it is held willy-nilly using an overstretched clothes pin.