I bought a NeoLucida, a modern version of an old optical drawing aid. The ad can be seen on YouTube, or you can find out more at Big i Design. I also bought David Hockney's book Secret Knowledge about optical drawing aids as used in the Renaissance and later.
It's an interesting device, but I started wondering if I could build my own. So I bought a 50% beamsplitter pentaprism from Edmund Optics and built a mounting bracket that fits on a standard microphone stand.
I prefer to look forward at the subject and see the reflected image of the paper, rather than down at the paper as in the Neo Lucida. It's harder to maintain the right eye position, but easier to get the balance between the two images right because I can shine a light on the paper rather than the subject.
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