Pablo Garcia and Golan Levin at CMU reconstructed the Camera Lucida, which was a common artists' tool up until the last century, and describe the history of it here: http://neolucida.com/history/lucida-history/

Basically, it's a tiny prism on a stick that projects what is in front of you to your drawing surface.

(disclaimer: I have no financial connection to these guys, but I did buy one of their Neolucidas, and it is a lot of fun)