Wonderful record of an important moment in the evolution of drawing naturalistically: In the fifteenth century,  Albrecht Durer revolutionized drawing by adding precision and science to the process.  He invented this system for getting proportions right:  He set his eye to a point, so that his perception of the model did not change, and he set up a piece of class with a grid etched onto it that matched the grid on the paper.  This solved two huge problems with drawing that I see myself and others do all the time.  It reminds the artist not to keep their eye steady, and it shows what the true lines are.  Our perception tends to be faulty, and this system fixes those faults and facilitates realistic drawing.  I am not advocating that everyone make and use one of these, but i do offer the apparatus/concept in my classes as a way to show people how to perceive the truth of a subject.