The heliostat is used to provide a strong and steady beam of sun light, for either astronomical study or illumination, using a mirror driven in two axes by clockwork to track the sun's transit. Although the idea was propsed by the Italian mathematician Giovanni Borelli (1608-79) in a manuscript dating from circa 1665, it is not believed that this design was constructed. The earliest heliostat known to have been constructed was made in about 1716 by Gabriel Farenheit (1686-1736), the thermometer maker who gave his name to the unit of temperature. Following this first model came others, using a variety of techniques, including a model by Gambey, described and illustrated by Mills (pp. 389-393, figures 19 and 20). The present example is, however, based on the design of 1843 by Johann Theobald Silbermann (1806-65), first published in Compte Rendu Hebdomadaire Des Sciences De L'Académie Des Sciences 17 (pp. 1319-24). An example manufactured by Duboscq-Soleil of Paris, held by the Whipple Museum, is illustrated by Mills (p. 396, figure 23, "circa 1850"), who comments that, "the Silbermann is the heliostat most likely to be encountered today - although still rare enough" (p. 396), perhaps because, "the Silbermann ... heliostats were produced commercially by well known firms of instrument makers, and were beautiful examples of their skill and art. As such they were necessarily expensive" (p. 399). Brenni describes and illustrates another example by Duboscq (pp. 35 and 52, item 3), noting that, "solida e relativamente compatta essa fu molto diffusa durante la seconda metà del seculo scorso" (p. 52). The Silbermann heliostat was not sufficiently precise to be used for astronomical purposes (with the exception of certain spectroscopical applications), but was normally used as a source of illumination by microscopists, particularly in the fields of petrology and mineralogy, where dense, light-absorbing material was under examination. The rapid developement of electric light sources meant that the heliostat had become obsolescent by the end of the 19th century.