Humans are primarily visual creatures. Not all animals depend on their eyes, as we do, for most of the information received about their surroundings (the characteristics of human vision are discussed in Chapter 2). This bias in everyday life extends to how we pursue more technical goals as well. Scientific instruments commonly produce images to communicate their results to the operator, rather than generating audible tones or emitting a smell. Space missions to other planets and equally arduous explorations of the ocean depths always include cameras as major components, and the success of those missions is often judged by the quality of the images returned. This suggests a few of the ways in which humans have extended the range of natural vision. Optical devices such as microscopes and telescopes allow us to see things that are vastly smaller or larger than we could otherwise. Beyond the visible portion of the electromagnetic spectrum (a narrow range of wavelengths between about 400 and 700 nanometers) there are sensors capable of detecting infrared and ultraviolet light, X-rays, and radio waves, and perhaps soon even gravity waves. Figure 1.1 shows an example, an image presenting radio telescope data in the form of an image in which color represents the Doppler shift in the radio signal. Such devices and presentations are used to further extend imaging capability.