Early Technology: Johnstone Stoney Heliostat circa 1869

Heliostats made by top makers such as Silbermann, Duboscq and Fuess from the mid nineteenth century and earlier, (see sold examples on this web site), were made by top scientific instrument manufacturers and were very expensive. George Johnstone Stoney in Ireland set himself a project to design a small portable heliostat carrying a larger mirror with greater accuracy than the Silbermann which could also be made for a fraction of the cost. He was brilliantly successful on both counts. The first commercial examples (c.1869) of his heliostat were made by Spencer & Son of Dublin and retailed for 5 guineas. Complete surviving examples are extremely rare. Spencer used a spring-driven domestic clock to power the polar axis via a linen belt connecting 2:1 wooden pulleys. The geometrical principle of Stoney’s heliostat is identical with the Gambey design. (See diagrammatic illustration on this web site by looking up the Stoney heliostat by Harvey & Peak). The declination arm is simplified and the mirror arm is supported upon a rotatable annulus moving in azimuth around a fixed horizontal table. This also acts as a sundial to aid the initial setting-up. The altitude of the exit beam is set by the screw bearing on the underside of the mirror arm which gives a rock solid support so that a large mirror can be used. Constructed for the latitude of 50º, this instrument can be set for places within a few degrees of this parallel by using the horizontal screws visible above the clock dial and offsetting on the degree scale visible at the rear and then restoring the spherical level to centre with the base adjusting screws. Base 10.5” (27 cm) x 6.25” (16 cm)

Other examples of the Stoney heliostat with a direct clockwork drive were made later (1890s) by Yeates & Son of Dublin, Harvey & Peak (see example on this website) and Watson & Sons in London – possibly others too. They are far more common than the original examples by Spencer not just because they were made later but being much more robust instruments many examples survive. They also lack something of the charm of the very rare early originals made by Spencer & Son.

Johnstone Stoney Heliostat circa 1869

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