When Kenyon College built a new science building in 1926, a massive stone ledge was placed outside the south-facing window of the physics lecture hall. This was to hold the Heliostat that directed the sun's rays to the Gaertner wavelength spectrometer. The students, one at a time, could then look through the eyepiece of the spectrometer and see the dark Fraunhofer lines crossing the otherwise continuous spectrum of the sun.
The heliostat has to take the light from the sun as it appears to track across the sky, and redirect it in a constant direction. The light is reflected from a front-surface mirror that reproduces the motion of the sun, except at twice the rate. The factor of two comes from the fact that as the mirror moves through a certain angle, the reflected ray turns through twice that angle. (The angle of reflection, measured from the normal to the surface of the mirror, is equal to the angle of incidence.)
Although he did not invent the heliostat, it is first mentioned
prominently in an 1742 textbook by the Dutch physicist, William Jacob
He coined the word from the Greek words for Sun and Stationary.
| At the right is an example of the heliostat designed in
1843 by the Frenchman J. T. Silbermann in 1843. The system is adjusted to
make the rotating shaft attached to the clockwork motor on the right-hand
side parallel to the rotation axis of the earth. Gears give a second, perpendicular
motion to the mirror to direct the sunlight in the desired direction.
This example, at Allegheny College in Meadville, Pennsylvania, was made by the Parisian firm of Jules Duboscq, and is listed at 900 francs ($180.00) in the 1885 Duboscq catalogue
| Here are two more Silbermann-type heliostats made by Duboscq.
At the left is an instrument in the Garland Collection of Classical Physics Apparatus at Vanderbilt University. Like most of the Vanderbilt apparatus, it was probably bought in 1875.
At the right is a heliostat at Washington and Lee University in Lexington, Virginia.
The heliostat at the right below is at Denison University in Granville, Ohio, and was made by Franz Schmidt and Haensch of Berlin, probably about 1900. At the left is a heliostat on display at Transylvania University. It is unmarked, and was bought in either London or Paris in 1857 for $42.00.
| Finally, there are hand-powered heliostats, in which the
mirror is moved slowly with a drive rod.
The heliostat at the left is by the Societe Genovoise, and is in the collection of the United States Military Academy at West Point. The drive shaft projects at the right side of the base.
At the right is the heliostat built by Prof. Joseph Naylor of DePauw University in Greencastle, Indiana. He nicknamed this heliostat "Joshua".
This large heliostat, made by the firm of E.S. Ritchie and Sons of Boston, stands about 65 cm high, and is remarkably heavy, probably due to the massive driving spring.
The 1881 Ritchie catalogue describes a much smaller instrument, so this must be after that date.
It is in the Greenslade Collection.