“Fields of color. A special feature of this book is the use of color to depict fields that in themselves are unpicturable. Just as the color blue permeates the sky, so you will be asked to picture space as permeated with colors, with different colors representing different fields. Color is an appropriate tool for this job because color is something that does not exist in itself; it exists only as a property of something else. By using colors to represent physical fields, we remind ourselves that fields are a property of space, not a separate substance in space.”