In this chapter, you will have to move the book in different ways to see the second image. For the first few, you must hold the book at eye level and tilt it away from you. In many cases, we have placed an arrow to show you where you should look.
In the first four figures, lines that appear to be slightly bowed can be seen as perfectly straight and parallel when held at eye level and tilted away. The backgrounds on which they have been printed tend to draw your eye either inward or outward.