Theory of the Callendar automatic pyrheliometer. The indications of the Callendar pyrheliometer differ from the calculated intensities of radiation upon a horizontal surface on account of (1) greater sensitiveness for low intensity than for high; (3) selective absorption of short-wave radiation by platinum compensating grids; (3) internal reflection of light from glass cover to grids; (4) selective absorption by cover glass of ultra-violet and infra-red radiation from sun, and total absorption of radiation from grids; (5) grid surfaces not geometrical planes; (6) lag of registration behind radiation.