“A knowledge of the construction of the heavens,” Herschel wrote in 1811, “has always been the ultimate object of my observations.” The “Construction of the Heavens”! A phrase of profound and novel import, for the invention of which he was ridiculed by Brougham in the Edinburgh Review; yet expressing, as it had never been expressed before, the essential idea of sidereal astronomy. Speculation there had been as to the manner in which the stars were grouped together; but the touchstone of reality had yet to be applied to them. This unattempted, and all but impossible enterprise Herschel deliberately undertook. It presented itself spontaneously to his mind as worth the expenditure of a life’s labour; and he spared nothing in the disbursement. The hope of its accomplishment inspired his early exertions, carried him through innumerable difficulties, lent him audacity, fortified him in perseverance. For this,
“He left behind the painted buoy
That tosses at the harbour’s mouth,”
and burst his way into an unnavigated ocean.
Herschel has had very few equals in his strength of controlled imagination. He held the balance, even to a nicety, between the real and the ideal. Meditation served in him to prescribe and guide experience; experience to ripen the fruit of meditation.
“We ought,” he wrote in 1785, “to avoid two opposite extremes. If we indulge a fanciful imagination, and build worlds of our own, we must not wonder at our going wide from the path of truth and nature. On the other hand, if we add observation to observation without attempting to draw, not only certain conclusions, but also conjectural views from them, we offend against the very end for which only observations ought to be made.”
This was consistently his method. If thought outran sight, he laboured earnestly that it should be overtaken by it: while sight, in turn, often took the initiative, and suggested thought. He was much more than a simple explorer. “Even at the telescope,” Professor Holden says, “his object was not discovery merely, but to know the inner constitution of the heavens.” He divined, at the same time that he observed.
The antique conception of the heavens as a hollow sphere upon which the celestial bodies are seen projected, survived then, and survives now, as a convenient fiction for practical purposes. But in the eighteenth century the fiction assumed to the great majority a sort of quasi-reality. Herschel made an exception in being vividly impressed with the depth of space. How to sound that depth was the first problem that he attacked. As a preliminary to further operations, he sought to fix a unit of sidereal measurement. The distances from the earth to the stars were then altogether unknown. All that had been ascertained was that they must be very great. Instrumental refinements had not, in fact, been carried far enough to make the inquiry profitable. Herschel did not underrate its difficulty. He recognised that, in pursuing it, one hundredth of a second of arc “became a quantity to be considered.” Justly arguing, however, that previous experiments on stellar parallax had been unsatisfactory and indecisive, he determined to try again.
He chose the “double star method,” invented by Galileo, but never, so far, effectually put to trial. The principle of it is perfectly simple, depending upon the perspective shifting to a spectator in motion, of objects at different distances from him. In order to apprehend it, one need only walk up and down before a lamp placed in the middle of a room, watching its apparent change of position relative to another lamp at the end of the same room. Just in the same way, a star observed from opposite sides of the earth’s orbit is sometimes found to alter its situation very slightly by comparison with another star close to it in the sky, but indefinitely remote from it in space. Half the small oscillation thus executed is called that star’s “annual parallax.” It represents the minute angle under which the radius of the terrestrial orbit would appear at the star’s actual distance. So vast, however, is the scale of the universe, that this tell-tale swing to and fro is, for the most part, imperceptible even with modern appliances, and was entirely inaccessible to Herschel’s observations. Yet they did not remain barren of results.
“As soon as I was fully convinced,” he wrote in 1781, “that in the investigation of parallax the method of double stars would have many advantages above any other, it became necessary to look out for proper stars. This introduced a new series of observations. I resolved to examine every star in the heavens with the utmost attention that I might fix my observations upon those that would best answer my end. The subject promises so rich a harvest that I cannot help inviting every lover of astronomy to join with me in observations that must inevitably lead to new discoveries. I took some pains to find out what double stars had been recorded by astronomers; but my situation permitted me not to consult extensive libraries, nor, indeed, was it very material; for as I intended to view the heavens myself, Nature, that great volume, appeared to me to contain the best catalogue.”