available for modern comparisons by C. S. Peirce's reduction of them in 1876 (Annals of Harvard Coll. Observatory, ix. 56). They were completed so as to embrace nearly all Flamsteed's stars, in two manuscript catalogues made known in 1883, together with a journal giving the dates of all the observations. Such as referred to variable stars thus acquired significance (Pickering, ib. xiv. 345; Proceedings of Amer. Acad. xix. 269; Observatory, vii. 256, &c.). Herschel discovered and assigned a period of sixty days to the variations of α Herculis (Phil. Trans. lxxxvi. 452). He ascribed stellar light-fluctuations to the display, through axial rotation, of unequally luminous hemispheres. His comparison in 1798 of the prismatic light of six bright stars was a venture upon new ground of unsuspected fertility (ib. civ. 264).
His theory of the constitution of the sun as a dark, cool body, surrounded by a shell of lucid clouds floating in a transparent atmosphere (ib. lxxxv. 46), held its ground until past the middle of last century. He surmised the periodicity of sun-spots, and attempted to substantiate his idea of a corresponding weather cycle by showing that the price of wheat rose as spots grew scarce (ib. xci. 310). His telescopic scrutiny of the solar surface was all but exhaustive. Among his few illusory observations were those of supposed volcanic outbursts on the moon in 1783 and 1787 (ib. lxxvii. 229) and of four additional Uranian satellites. He, however, established the retrograde revolutions of the pair genuinely seen. His results relative to Saturn, published in six memoirs between 1790 and 1808, included the first determination of its rotation and polar compression, with many observations of great interest on the rings. From recurrent changes of brightness in the fifth satellite (Japetus), he inferred the identity of its periods of rotation and revolution (ib. lxxxii. 14), and found the same law to prevail in the Jovian system. The ‘trade wind’ explanation of Jupiter's belts was suggested by him in 1781 (ib. lxxi. 118); he investigated in 1781 and 1784 the rotation of Mars, and adverted to the analogy between that planet and the earth, demonstrating the general permanence of its markings, and from their seasonal changes the glacial nature of its polar spots (ib. lxxiv. 233). A pungent repudiation in 1793 of Schröter's claim to the discovery of mountains in Venus formed a rare exception to the cordiality of his relations with his contemporaries. His proposal to designate the minor planets as ‘asteroids’ drew upon him a gratuitous attack, probably from Brougham, in the first number of the ‘Edinburgh Review.’ Herschel made important physical observations on the comets of 1807 and 1811, concluding them to be in part self-luminous and of nebular origin.
His discovery of the ‘infra-red’ solar rays renders him illustrious as a physicist. No one before him had suspected the unequal distribution of heat in the spectrum; and he pursued the subject with marvellous sagacity in four papers communicated to the Royal Society in 1800, dealing with the laws of reflection, refraction, and transmission of radiant heat. He traced the ‘heat’ and ‘light curves’ of the solar spectrum with maxima in the infra-red and yellow respectively, and conjectured that ‘the chemical properties of the prismatic colours might be as different as those which relate to light and heat’ (ib. xc. 270).
Herschel's achievements opened a new era in astronomical optics. The importance of large telescopic apertures, as giving proportionate power of ‘space penetration,’ was first by him insisted upon and exemplified, and his specula were as remarkable for perfect figure as for great size. When he began to observe, it was almost unheard of that a star should be seen without ‘rays’ or ‘tails.’ Henry Cavendish happening to sit next Herschel at dinner, slowly addressed him with, ‘Is it true, Dr. Herschel, that you see the stars round?’ ‘Round as a button,’ exclaimed the doctor, when the conversation dropped, till at the close of dinner, Cavendish repeated interrogatively, ‘Round as a button?’ ‘Round as a button,’ briskly rejoined the doctor, and no more was said. Herschel's extraordinary natural qualifications as an observer were diligently cultivated. ‘Seeing,’ he wrote to Dr. Watson in 1782, ‘is in some respects an art which must be learnt,’ and he compared its practice to that required for playing ‘one of Handel's fugues upon the organ.’ He presents a rarely happy combination of the speculative and experimental faculties, his thoughts transcending, yet eagerly seeking the control of visible facts. ‘As a practical astronomer,’ Professor Holden says, ‘he remains without an equal. In profound philosophy he has few superiors. … His is one of the few names which belong to the whole world.’