The limit of visual resolution

Abstract

The diffraction image of a long line source of light at the focus of a lens with a circular aperture has been calculated numerically. These figures are used to find what intensity drop between two bright lines is at the threshold of visibility, by making observations on the separation of two line sources at which a dark line is visible between their images. After means had been taken to avoid a well-known contrast effect which produces a false dark line, it was found that, on the average, a 3 per cent drop of intensity is required for threshold resolution.