Portable heliostats (solar illuminators)

Summary

This paper examines the various types of small, portable heliostat which maintain a beam of sunlight in a constant direction. Used mainly in the second half of the nineteenth century, these instruments were intended for optical demonstrations and the illumination of microscopes and similar apparatus, rather than for astronomical applications. Three basic patterns are distinguished: polar heliostats (single- or two-mirror), biaxial single-mirror heliostats, and coelostats. The theory and geometry of each type is explained, followed by a consideration in chronological order of the corresponding working instruments. Heliostats by Fahrenheit, Meyerstein, 's Gravesande, Gambey, Silbermann, Foucault, Stoney and others are illustrated, and extensive references given.