Like photometers, microphones, compasses, and smoke detectors, sense organs have the task of detecting physical energy or the chemical nature of substances; and, like man-made instruments, the quality of their performance depends on their sensitivity to weak signals and their ability to provide accurate information over a wide range of intensity of stimulation. They also have the problem of internal noise: distinguishing genuine but weak signals from random false activity within the organ. This program looks at the extreme sensitivity of the rods in the human eye; examines the limits of sensitivity of the human ear; looks at some examples of extraordinary sensual sensitivity in animals; and analyzes how the human senses analyze and discriminate between signals over wide operating ranges and then transmit their messages along the rather low-quality cables constituted by the nerve fibers connecting them to the brain. (60 minutes)