Markings on Mars as seen with Small and Large Telescopes

Abstract

THAT large telescopes are liable to less accurate definition of planetary markings than small ones can easily be verified in the following manner, and tested for any given occasion. The spurious disc and rings made of a star by a telescope is a real image, due to the interference of the light-waves—as real an image, although produced by a different cause, as that of a planet's disc with its markings. If atmosphere and glass be perfect, the image consists of a round disc, encircled by concentric and continuous rings of light. The only difference in the image with different apertures is that the larger the aperture the smaller the disc, and the closer and thinner the rings. If this image assume any other form, it is either because the objective is poor, which is commoner with objectives than is supposed, as Hartmann's tests have shown¹, or because the seeing is defective. In proportion as the seeing is bad the rings of the image begin to waver, then break up into fragments, a sort of mosaic, and finally end in an indiscriminate assemblance of points. In certain kinds of bad seeing the parts may seem quite steady, but that the mosaic exists is proof positive of poor seeing.