From Fraunhofer’s time, at the hands of Merz his immediate successor, Cauchoix in France, and Tully in England, the achromatic refractor steadily won its way. Reflecting telescopes, despite the sensational work of Lord Rosse on his 6-foot mirror of 53 feet focus (unequalled in aperture until the 6-foot of the Dominion Observatory seventy years later), and the even more successful instrument of Mr. Lassell (4 feet aperture, 39 feet focus), were passing out of use, for the reason already noted, that repolishing meant refiguring and the user had to be at once astronomer and superlatively skilled optician.
These large specula, too, were extremely prone to serious flexure and could hardly have been used at all except for the equilibrating levers devised by Thomas Grubb about 1834, and used effectively on the Rosse instrument. These are in effect a group of upwardly pressing counterbalanced planes distributing among them the downward component of the mirror’s weight so as to keep the figure true in any position of the tube.
Such was the situation in the 50’s of the last century, when the reflector was quite unexpectedly pushed to the front as a practical instrument by almost simultaneous activity in Germany and France. The starting point in each was Liebig’s simple chemical method of silvering glass, which quickly and easily lays on a thin reflecting film capable of a beautiful polish.
The honor of technical priority in its application to silvering telescope specula worked in glass belongs to Dr. Karl August Steinheil (1801-1870) who produced about the beginning of 1856 an instrument of 4-inch aperture reported to have given with a power of 100 a wonderfully good image. The publication was merely from a news item in the “Allgemeine Zeitung” of Augsburg, March 24, 1856, so it is little wonder that the invention passed for a time unnoticed.
Early the next year, Feb. 16, 1857, working quite independently, exactly the same thing was brought before the French Academy of Sciences by another distinguished physicist, Jean Bernard Léon Foucault, immortal for his proof of the earth’s rotation by the pendulum experiment, his measurement of the velocity of light, and the discovery of the electrical eddy currents that bear his name.
Fig. 24.—Dr. Karl August Steinheil.
Fig. 25.—Jean Bernard Léon Foucault.
The Inventors of the Silver-on-Glass Reflector.
To Foucault, chiefly, the world owes the development of the modern silver-on-glass reflector, for not being a professional optician he had no hesitation in making public his admirable methods of working and testing, the latter now universally employed. It is worth noting that his method of figuring was, physically, exactly what Jesse Ramsden (1735-1800) had pointed out in 1779, (Phil. Tr. 1779, 427) geometrically. One of Foucault’s very early instruments mounted equatorially by Sécrétan is shown in Fig. 26.