THE BREWSTER STEREOSCOPE AND MODIFICATIONS
THE FAMILIAR forms of the Brewster stereoscope consist of a pair of plus lenses decentered outward and mounted to permit convenient viewing of a pair of stereoscopic pictures through the lenses. These pictures have usually been set near the focal plane of the lenses to increase the illusion of distance by encouraging relaxation of accommodation and convergence.Accommodation is relaxed and the lines of sight are parallel when two conditions are satisfied: when the targets are in the focal plane of the lenses, and when the centers of the targets for the two eyes and the optical centers of the viewing lenses are separated by the same distance. It is apparently the latter condition which accounts for the usual decentering of the lenses, for it permits the use of larger pictures.When the stereoscope is used as a training device, it is customary to move