Horopter

Horopter, surface in the field of view, all points of which give images in paired identical points of the retina and therefore do not give double images. Rays from points lying outside the H. always give double images, as they fall on disparate places of the retina. A person usually does not notice these double images for the following reasons: a) images from points lying outside the H. are not in the fovea centralis, but in other places of the retina, where visual acuity is less; b) these images are not sharp, since the eye is usually accommodated to the plane of the object being viewed; c) if two images fall on identical points of the retina, they are not seen simultaneously, but alternately (struggle of visual fields). The doctrine of the H. is complex and difficult to explain. When fixing a nearby point F (see fig.), when the eyes are symmetrically located relative to it and converged, the H. has the shape of a circle. When looking into the distance, when the visual axes are parallel, the H. represents a plane perpendicular to the axes and located at an infinitely great distance from the observer.

The large circle, partly dotted, is the horopter; F - the fixed point; A and B - points on the horopter; f - fovea centralis; a, a', b and b' - images of points A and B on the retina in identical points.

The concept "horopter" was introduced into science by Johann Müller (J. Müller).