HELIOSTAT, an instrument containing a mirror driven by clockwork so as to reflect sunlight in a fixed direction; e.g., on to the slit of a fixed spectroscope. It is of more complicated de sign than the coelostat (q.v.). It has the advantage over the coelostat that the instrument fed by it need not be moved from day to day as the declination of the sun changes; the disadvan tage is that only one point of the image remains fixed, • the re mainder rotating around it, so that unlike the coelostat it cannot be used for photographing an extended region.