15. THE FIXED STARS AND NEBULÆ. Passing now from our solar system, which appears to be subject to the action of the same forces as those we experience on our globe, there remains an innumerable host of fixed stars, nebulas, and nebulous clusters of stars. To these the attention of astronomers has been more earnestly directed since telescopes have been so much enlarged. Photography also has enabled a vast amount of work to be covered in a comparatively short period, and the spectroscope has given them the means, not only of studying the chemistry of the heavens, but also of detecting any motion in the line of sight from less than a mile a second and upwards in any star, however distant, provided it be bright enough. [Illustration: SIR WILLIAM HERSCHEL, F.R.S.—1738-1822. Painted by Lemuel F. Abbott; National Portrait Gallery, Room XX.] In the field of telescopic discovery beyond our solar system there is no one who has enlarged our knowledge so much as Sir William Herschel, to whom we owe the greatest discovery in dynamical astronomy among the stars—viz., that the law of gravitation extends to the most distant stars, and that many of them describe elliptic orbits about each other. W. Herschel was born at Hanover in 1738, came to England in 1758 as a trained musician, and died in 1822. He studied science when he could, and hired a telescope, until he learnt to make his own specula and telescopes. He made 430 parabolic specula in twenty-one years. He discovered 2,500 nebulæ and 806 double stars, counted the stars in 3,400 guage-fields, and compared the principal stars photometrically. Some of the things for which he is best known were results of those accidents that happen only to the indefatigable enthusiast. Such was the discovery of Uranus, which led to funds being provided for constructing his 40-feet telescope, after which, in 1786, he settled at Slough. In the same way, while trying to detect the annual parallax of the stars, he failed in that quest, but discovered binary systems of stars revolving in ellipses round each other; just as Bradley’s attack on stellar parallax failed, but led to the discovery of aberration, nutation, and the true velocity of light. _Parallax_.—The absence of stellar parallax was the great objection to any theory of the earth’s motion prior to Kepler’s time. It is true that Kepler’s theory itself could have been geometrically expressed equally well with the earth or any other point fixed. But in Kepler’s case the obviously implied physical theory of the planetary motions, even before Newton explained the simplicity of conception involved, made astronomers quite ready to waive the claim for a rigid proof of the earth’s motion by measurement of an annual parallax of stars, which they had insisted on in respect of Copernicus’s revival of the idea of the earth’s orbital motion.