A heliostat is an instrument designed to capture a reflection of the Sun (or other celestial body) over a period of time, by rotating a clockwork mirror to match the Sun's apparent path through the sky. Prior to the advent of high-powered electric lights, this would be the most efficient way of providing sustained and adequate illumination for microscopy, or study of the sun itself and the properties of its beams. Their construction and refinement flourished in the second half of the nineteenth century, but by the start of the twentieth, and the advent of the electric light, they had become more or less obsolete. The coelostat (in which the moving mirror is arranged to produce a non-rotating image of the celestial body under examination) was invented by E.F. August in 1839; the heliostat was developed by O. von Littrow (1863) after August's design, and by Prazmowski (1877), who gave no credit to any previous inventor. His 1877 model was illustrated in the Nachet trade catalogue of 1910.