selves would not be recognized in the way intended. For instance, the lines in the upper left hand corner of Fig. 6, by
themselves, would not be understood as representing rain. But place an open umbrella in connection with the lines, and the eye interprets them as portraying rain. For practice, draw plain lines on the blackboard and add figures as here suggested.
Draw on the blackboard the lines A in Fig. 7.
The pupil should copy these as an exercise. The scallops, by themselves, represent nothing in nature. Invert them and place a sketch of a sailboat and waves are suggested.

The lines inside the frame, by themselves, would hardly be recognized as representing glass, but draw objects beyond, partly hidden, and the meaning becomes clear.