A Treatise on Painting/Chapter 126
Chap. CXXVI.—How to draw accurately any particular Spot.
Take a glass as large as your paper, fasten it well between your eye and the object you mean to draw, and fixing your head in a frame (in such a manner as not to be able to move it) at the distance of two feet from the glass; shut one eye, and draw with a pencil accurately upon the glass all that you see through it. After that, trace upon paper what you have drawn on the glass, which tracing you may paint at pleasure, observing the aerial perspective.