“Such telescopes as Herschel worked with,” Dr. Kitchiner wrote in 1815, “could only be made by the man who used them, and only be used by the man who made them.” The saying is strictly true. His skill in one branch promoted his success in the other. He was as much at home with his telescopes as the Bedouin are with their horses. Their peculiarities made part of his most intimate experience. From the graduated varieties of his specula he picked out the one best suited to the purpose in hand. It was his principle never to employ a larger instrument than was necessary, agility of movement being taken into account no less than capacity for collecting light. The time-element, indeed, always entered into his calculations; he worked like a man who has few to-morrows.
His sense of sight was exceedingly refined, and he took care to keep it so. In order to secure complete “tranquility of the retina,” he used to remain twenty minutes in the dark before attempting to observe faint objects; and his eye became so sensitive after some hours spent in “sweeping,” that the approach of a third-magnitude star obliged him to withdraw it from the telescope. A black hood thrown over his head while observing served to heighten this delicacy of vision. He despised no precaution. Details are “of consequence,” he wrote to Alexander Aubert, an amateur astronomer, “when we come to refinements, and want to screw an instrument up to the utmost pitch.”
This was said in reference to his application of what seemed extravagantly high magnifying powers. He laid great stress upon it in the earlier part of his career. The method, he said, was “an untrodden path,” in which “a variety of new phenomena may be expected.” With his seven-foot Newtonian he used magnifications up to nearly 6,000, proceeding, however, “all along experimentally”—a plan far too much neglected in “the art of seeing.” “We are told,” he proceeded, “that we gain nothing by magnifying too much. I grant it, but shall never believe I magnify too much till by experience I find that I can see better with a lower power.” The innovation was received with a mixture of wonder, incredulity, and admiration.
Herschel showed his customary judgment in this branch of astronomical practice. He established the distinctions still maintained, and laid down the lines still followed. It is true he went far beyond the point where modern observers find it advisable to stop. The highest power brought into use with the Lick refractor is 2,600; and Herschel’s instruments bore 5,800 (nominally 6,500) without injury to definition. But only at exceptional moments. His habitual sweeping power was 460; he “screwed-up” higher only for particular purposes, and under favourable conditions. Although his strong eye-pieces seem, for intelligible reasons, to have been laid aside on the adoption of the “front-view” form of construction, they had served him well in the division of close pairs, as well as for bringing faint stars into view—an effect correctly explained by him as due to the augmented darkness, under high powers, of the sky-ground. But the most important result of their employment was the discovery that the stars have no sensible dimensions. This became evident through the failure of attempts to magnify them; the higher the power applied, the smaller and more intense they appeared. Herschel accordingly pronounced stellar telescopic discs “spurious,” but made no attempt to explain their origin through diffraction.
He never possessed an instrument mounted equatoreally—that is, so as automatically to follow the stars. In its absence, his work, had it not been accomplished, would have seemed to modern ideas impossible. No clockwork movement kept the objects he was observing in the field of view. His hands were continually engaged in supplying the deficiency. How, under these circumstances, he contrived to measure hundreds of double stars, and secure the places of thousands of nebulæ, would be incomprehensible but for the quasi-omnipotence of enthusiasm.
The angle made with the meridian by the line joining two stars (their “position angle”) was never thought of as a quantity useful to be ascertained until Herschel, about 1779, invented his “revolving-wire micrometer.” This differed in no important respect from the modern “filar micrometer;” only spider-lines have been substituted for the original silk fibres. For measuring the distances of the wider classes of double stars, he devised in 1782 a “lamp-micrometer;” while those of the closest pairs were estimated in terms of the discs of the components. In compiling his second catalogue, however, he used the thread-micrometer for both purposes. It is true that “even in his matchless hands”—in Dr. Gill’s phrase—the results obtained were “crude;” but the fact remains that the whole system of micrometrical measurement came into existence through Herschel’s double-star determinations.
Their consequences have developed enormously within the last few years. Mr. Burnham’s discoveries of excessively close pairs have been so numerous as to leave no reasonable doubt that their indefinite multiplication is only a question of telescopic possibility. Then in 1889, another power came into play; the spectroscope took up the work of resolving stars. Or rather, the spectroscope in alliance with the photographic camera; for the spectral changes indicating the direction and velocity of motion in the line of sight can be systematically studied, as a rule, only when registered on sensitive plates. The upshot has been to bring within the cognisance of science the marvellous systems known as “spectroscopic binaries.” They are of great variety. Some consist of a bright, others of a bright and dark, pair. Those that revolve in a plane nearly coinciding with our line of vision undergo mutual occultations. A further detachment seem to escape eclipse, yet vary in light for some unexplained reason, while they revolve. Others, like Spica Virginis, revolve without varying. Their orbital periods are counted by hours or days. The study of the disturbances of these remarkable combinations promises to open a new era in astronomical theory. For they are most likely all multiple. Irregularities indicating the presence of attractive, although obscure bodies, have, in several cases, been already noticed.
The revolutions of spectroscopic binary stars can be studied to the greatest advantage when they involve light-change; and photometric methods have accordingly begun to play an important part in the sidereal department of gravitational science. And here again we meet with Herschel’s initiative. His method of sequences has been already explained; and he made the first attempt to lay down a definite scale of star-magnitudes. He failed, and it was hardly desirable that he should succeed. On his scale, the ratio of change from one grade to the next constantly diminished. In the modern system it remains always the same. A star of the second magnitude is by definition two and a-half (2·512) times less bright than one of the first; a star of the third magnitude is two and a-half times less bright than one of the second, the series descending without modification until beyond telescopic reach. This uniformity in the proportionate value of a magnitude is indispensable for securing a practicable standard of measurement. Herschel, however, took the great step of introducing a principle of order.
His estimates of stellar lustre were purely visual. And although various instruments, devised for the purpose, have since proved eminently useful, the ultimate appeal in all is to the eye. But there are many signs that, in the photometry of the future, not the eye but the camera will be consulted. Their appraisements differ markedly. Herschel’s incidental remark on the disturbance of light-valuation by colour touches a point of fundamental importance in photographic photometry. The chemical method gives to white stars a great advantage over yellow and red ones. They come out proportionately much brighter on the sensitive plate than they appear to the eye. And to these varieties of hue correspond spectral class-distinctions, the spectrum of an object being nothing but its colour written at full length. This systematic discrepancy between visual and photographic impressions of brightness, while introducing unwelcome complications in measures of magnitude, may serve to bring out important truths. The inference, for example, has been founded upon it that the Milky Way is composed almost exclusively of white, or “Sirian” stars; and there can be no question but that the arrangement of stars in space has some respect to their spectral types.
Herschel’s plan of inquiry into the laws of stellar distribution by “photometric enumeration,” or gauging by magnitudes, was a bequest to posterity which has been turned to account with very little acknowledgment of its source. Argelander’s review of the northern heavens (lately completed photographically by Dr. Gill to the southern pole) afforded, from 1862, materials for its application on a large scale; but the magnitudes assigned to his 324,000 stars do not possess the regularity needed to make deductions based on them perfectly trustworthy. Otherwise the distance from the earth of the actual aggregations in the Milky Way could have been ascertained in a rough way from the numerical representation of the various photometric classes. As it is, the presumption is strong that the galactic clouds are wholly independent of stars brighter than the ninth magnitude—that they only begin to gather at a depth in space whence light takes at least a thousand years to travel to our eyes. Confirmatory evidence, published in 1894, has been supplied by M. Easton’s research, based on the same principle, into the detailed relations of stars of various magnitudes to Milky Way structure. They are exhibited only by those of the ninth magnitude, or fainter; for with them sets in a significant crowding upon its condensed parts, attended by a scarcity over its comparative vacuities. Counts by magnitudes have, besides, made it clear that the stars, in portions of the sky removed from the Milky Way, thin out notably before the eleventh magnitude is reached; so that, outside the galactic zone, the stellar system is easily fathomed.