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.
One of the chief difficulties in learning to draw is, as before stated, in learning to see correctly, because the appearance of objects so often contradicts what we know to be true of them. More than one beginner has drawn a handle on a mug because he knew it was there, regardless of the fact that the mug was turned in such a way that the handle was not visible. The changes which take place in the appearance of forms through changes in the position from which they are seen, are governed by the principles of perspective. Although students of this course are supposed to be familiar with the science of perspective, it is accessary to restate certain general principles of perspective with which the freehand draughtsman must be so familiar that he can apply them almost unconsciously as he draws. The most important of these are demonstrated in the following paragraphs, and their application should be so thoroughly understood that they become a part of the student's mental equipment. In theory the draughtsman draws what he sees, but practically he is guided by his knowledge as to how he sees. The principles can be most clearly demonstrated through the study of certain typical geometric forms which are purposely stripped of all intellectual or sentimental interest, so that nothing shall divert the attention from the principles involved in their representation. The student will readily recognize the great variety of subjects to which the principles apply and the importance of working out the exercises and mastering them for the sake of the knowledge they impart. These principles can be explained very clearly by the use of the glass slate, which is a part of the required outfit for this course. All drawings should be made from the models in outline and in freehand on the glass, using the Cross pencil. The drawing should be tested and corrected according to the instructions for testing.