Speculum metal was not destined to stay; ten years later (1857) the genius of Léon Foucault introduced glass mirrors with a thin coating of silver deposited chemically, and these have now universally superseded the metallic ones.
The long supremacy of Germany in the matter of refractors was broken down ultimately by the famous English optician and engineer, Thomas Cooke, of York. His first considerable instrument, one of seven inches aperture, was finished in 1851; and in 1865, a year before his lamented death, he completed the first of our present giant refractors, one of twenty-five inches aperture, for Mr. Newall, of Gateshead. In consequence of the success of Cooke’s achievement other large refractors were soon undertaken.
Alvan Clarke, the famous optician of Cambridgeport, Massachusetts, at once commenced a twenty-six-inch for the Washington Observatory. The next was one of twenty-seven inches, made by Grubb for the Vienna Observatory. Object-glasses now grew inch by inch in size, depending on the increased dimensions of disks that could be satisfactorily cast. Gautier, of Paris, completed a twenty-nine-and-a-half-inch for the Nice Observatory, while Alvan Clarke made an objective of thirty inches for Pulkowa. In 1877 the latter successfully completed the mounting of an objective of thirty-six inches for the Lick Observatory, but this immense lens was only achieved after a great number of failures. Even this large object-glass was surpassed in size by the completion in 1892 of the forty-inch which he made for the Yerkes Observatory, and by that made by Gautier for the Paris Exhibition of 1900.
So much, then, for the largest refractors. In recent years, since the introduction of the silver on glass mirrors, with their stability of figure and brilliant surface, which can be easily renewed, reflectors of large apertures are again being produced. The first of these was one of thirty-six inches aperture made by Calver for Dr. Common, who demonstrated its fine qualities and his own skill by the beautiful photographs of the nebula of Orion he was enabled to secure with it. Dr. Common himself has since turned his attention to the making and silvering of large mirrors of this kind, and the largest he has actually completed and mounted equatorially is one with a diameter of five feet. Another of thirty-six inches aperture is in use at the Solar Physics Observatory at Kensington.
The progress of depositing silver on glass has led of late years to important developments in which plane mirrors are used. Foucault was the first to utilize such mirrors in his “siderostat,” in which such a mirror is made to move in front of a horizontal fixed telescope, which may be of any focal length, and no expensive dome or rising floor is required. The plane mirror of the siderostat in the Paris Exhibition telescope is six feet in diameter.
A variation of this instrument is the cœlostat more recently advocated by Lippmann. The Coudé equatorial mounting also depends upon the use of plane mirrors; with such a telescope the observer is at rest at a fixed eye-piece or camera in a room which may be kept at any temperature.
Now that in astronomical work eye observations are indispensably supplemented by the employment of photography, an important modification of the refracting telescope has become necessary; this was first suggested by Rutherfurd.
The ordinary achromatic object-glass consists, as a rule, of two lenses, one made of flint and the other of crown glass; but in this form the photographic rays are not brought to the same focus as the visual rays. This, however, can be achieved by employing three lenses instead of two, each of different kinds of glass. The most modern improvement in the telescope is due to Mr. Dennis Taylor, of Cooke & Sons, and to Dr. Schott and Professor Abbe, whose researches in the manufacture of old and new varieties of optical glass have rendered Mr. Taylor’s results feasible. By the Taylor lens outstanding color is abolished, all the rays being brought absolutely to the same focus; such lenses can therefore be used either for visual observations or for photography for spectroscopy.
SPECTROSCOPIC ASTRONOMY
The branch of physics which at the present day has assumed such mighty and far-reaching proportions in astronomical work is that dealing with spectrum analysis, which, although suggested as early as the time of Kepler, did not receive any impetus as regards its application to celestial bodies until the beginning of the present century at the hands of Wollaston and Fraunhofer. Then, however, it still lacked the chemical touch supplied afterwards by Kirchhoff and Bunsen. They showed us that the spectrum observed when the light from any heated body is passed through a prism is an index to the chemical composition of the light source; the constitution of a vapor when in a condition to absorb light can be determined by an extension of the same principle, first demonstrated by Stokes, Angström, and Balfour Stewart, when the century was about half completed.