[Illustration:  Diagram IV.

Showing three principles of construction used in observing Fig.  X,
masses; Fig.  Y, curves; Fig.  Z, position of points]

In measuring comparative distances the needle should always be held at arm’s length and the eye kept in one position during the operation; and, whether held vertically or horizontally, always kept in a vertical plane, that is, either straight up and down, or across at right angles to the line of your vision.  If these things are not carefully observed, your comparisons will not be true.  The method employed is to run the thumb-nail up the needle until the distance from the point so reached to the top exactly corresponds with the distance on the object you wish to measure.  Having this carefully noted on your needle, without moving the position of your eye, you can move your outstretched arm and compare it with other distances on the object. #It is never advisable to compare other than vertical and horizontal measurements.# In our diagram the points were drawn at random and do not come in any obvious mathematical relationship, and this is the usual circumstance in nature.  But point C will be found to be a little above the half, and point D a little less than a third of the way up the vertical line.  How much above the half and less than the third will have to be observed by eye and a corresponding amount allowed in setting out your drawing.  In the horizontal distances, B will be found to be one-fourth the distance from X to the height of C on the right of our vertical line, and C a little more than this distance to the left, while the distance on the right of D is a little less than one-fifth of the whole height.  The height of B is so near the top as to be best judged by eye, and its distance to the right is the same as B. These measurements are never to be taken as absolutely accurate, but are a great help to beginners in training the eye, and are at times useful in every artist’s work.

[Illustration:  Plate XVII.

Demonstration drawing made before the students of the goldsmiths College
school of art

Illustrating how different directions of lines can help expression of form.]

It is useful if one can establish a unit of measurement, some conspicuous distance that does not vary in the object (if a living model a great many distances will be constantly varying), and with which all distances can be compared.