This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The use of tests may easily be perverted and become mischievous. Since the object of all drawing is to train the hand and eye, it follows naturally that the more the student relies upon tests the less will he depend upon his perceptions to set him right, and the less education will he be giving to his perceptions. There is no greater mistake for a student than to use the measuring test before making a drawing. Spend any amount of time in calculating relative proportions by the eye, but put these down and correct them by the eye, not once but many times before resorting to tests. All the real education in drawing takes place before the tests are made. Let the student remember that the tests may help him to make an accurate drawing, but they will never make him an accurate draftsman in the true sense. Nothing but training the eye to see and the hand to execute what the eye sees, will do that. When the student has reached the end of his knowledge, has corrected by the eye as far as he can, then by applying tests he is enabled to see how far his perceptions have been incorrect. That is the only educational value of the test. Merely to make an accurate drawing with as little mental effort as possible, relying upon test measurements, requires considerable practice and skill in making the tests, but gives very little practice or training in drawing.