noun. a hemispherical tool utilized to plot the limits of the visual region. Someone is seated within the hemisphere and is requested to concentrate on a central area of light, the visual region is then invaded with minor flashes of light, which the involved party is asked to identify. The brightness and size of the light flashes can be differentiated to evaluate visual sensitivity at multiple points within the region.
PERIMETER: "The perimeter is missing from the laboratory- has anyone seen it?"