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 great value of the slate for the beginner in freehand drawing is the ease with which the accuracy of a drawing may be tested. To obtain satisfactory results the models should be placed about a foot and a half in front of the spectator and the drawings made rather large. The drawing should be made freehand, in outline, and the greatest care taken to make it as accurate as possible before testing it because the object in making the drawing is to exercise the hand and eye. Drawing exercises should not be confounded with the preliminary exercises in tracing whose only object is to emphasize the fact that forms appear different as the position of the eye changes.
In order to test a drawing place the slate at right angles to a line from the eye to the model according to the directions in section 13. Holding the slate at this angle and keeping one eye closed move it backward and forward until the lines of the drawing cover the lines of the model. Any difference in the general direction of the lines or proportions can be readily observed. Corrections should not be made by tracing, but errors should be carefully noted and the alterations made freehand from a re-study of the models. If the drawing La too large to cover the lines of the model, errors may be discovered by testing the different angles of the drawing with those of the model. If all the angles coincide the drawing must be correct.
In making the tests the slate should be held firmly with both hands, and it cannot be emphasized too strongly that the test is of no value unless the state is at right angles to the direction in which the model en. When groups of models or other complicated subjects are being tested only the directions of important lines and proportions of leading masses can be compared. It must be clearly nnderstood that it takes some practice and much care to the drawing of a simple form, and that the slate is not to be used as a means of tracing. The student will soon discover that it is impossible to trace any form or group having much detail or multiplication of parts owing to the impossibility of holding the slate and the eye for long in the same position at the same time.
Do not expect too much of the slate. Even the first exercises in tracing simple forms will show the student that unless he has acquired some facility in making lines freehand he cannot trace lines. Indeed it has often been observed that no one can trace who cannot draw. Another difficulty in using the slate at first is the resistance which the pencil encounters on the glass. It calls for a different pressure and touch from that used with a pencil on paper, so that the beginner is often discouraged unnecessarily and becomes impatient with the slate, partly because he expects too much from it and partly because he has not learned how to use it. Do not try to make perfect lines on the slate. Be satisfied at first to indicate the general direction of lines. Understand also that the slate is only to be used in beginning to draw. The student should as soon as possible emancipate himself from the use of the tetsts and depend upon the eye alone for judging the relations of proportions and lines. From the beginning a drawing should be corrected by the eye as far as possible before applying any tests.