Definite electrical changes can be detected in the eyes of many animals in response to illumination of the eyes. In the vertebrate eye these changes take the form of characteristic electrical variations known successively as the a-, b-, c-, and d-waves. Similar waves can be elicited from the compound eyes of a number of invertebrates. The simplest method of studying these electrical changes in invertebrates is to lead off by silver-silver chloride electrodes from fluid-filled chambers built around each eye. One eye is illuminated while the second eye is kept in darkness. The electrical potentials are amplified by means of a vacuum tube amplifier and are then recorded by means of a cathode ray oscillograph and moving film camera
