His conjectures developed in 1811 into a formal theory. The cosmical fluid was met with in all stages of condensation. Nebulous tracts of almost evanescent lustre were connected in an unbroken series with slightly “burred” objects, wanting only a few last touches to make them finished stars. The extremes, as he said, had been, by his “critical examination of the nebulous system,” “connected by such nearly allied intermediate steps, as will make it highly probable that every succeeding state of the nebulous matter is the result of the action of gravitation upon it while in a foregoing one.”
In 1814 he traced the progress towards maturity of binary systems. Originating in double nebulæ incompletely dissevered—Siamese-twin objects, of which he had collected 139 examples—they next appeared as nebulously-connected stars, finally as a pair materially isolated, and linked together by the sole tie of gravitation. Scattered clusters represented, in his scheme of celestial progress, a state antecedent to that of globular clusters. “The still remaining irregularity of their arrangement,” he said, “additionally proves that the action of the clustering power has not been exerted long enough to produce a more artificial construction.” He made, too, the important admission that clusters apparently “in, or very near the Milky Way,” were truly part and parcel of that complex agglomeration.
But what of his “fifteen hundred universes,” which had now logically ceased to exist? The stellar and nebular “principles” had virtually coalesced; both were included in the galactic system. The question of “island universes” was accordingly left in abeyance; although Herschel certainly believed in 1818 that among the multitude of “ambiguous objects”—we should call them irresolvable nebulæ—many exterior firmaments were included. Yet what he had ascertained about the distribution of nebulæ should alone have sufficed to shatter this remnant of a conviction.
The fact became clear to him during the progress of his “sweeps” that nebulæ, to some extent, replace stars. He found them to occur in “parcels,” more or less embedded with stars, “beds” and “parcels” together being surrounded by blank spaces. This arrangement grew so familiar to him that he used to notify his assistant, when stars thinned out in the zone he was traversing, “to prepare for nebulæ.” A wider relationship, brought within view by the large scale of his labours, was defined by his fortunate habit of charting, for convenience of identification, each newly-discovered batch of nebulæ.
“A very remarkable circumstance,” he wrote in 1784, “attending the nebulæ and clusters of stars, is that they are arranged into strata, which seem to run on to a great length; and some of them I have already been able to pursue, so as to guess pretty well at their form and direction. It is probable enough that they may surround the whole apparent sphere of the heavens, not unlike the Milky Way.”
In the following year he spoke no longer of a zone, but of two vast groupings of nebulæ about the opposite poles of the Milky Way. That is to say, where stars are scarcest nebulæ are most abundant. The correspondence did not escape him; but he did not recognise its architectonic meaning. He had traced out the main plan of the stellar world; he had discovered, not merely thousands of nebulæ, but the nebular system; he had shown that stars and nebulæ were intimately associated; he had even made it clear that nebular distribution was governed by the lines of galactic structure. It only remained to draw the obvious inference that these related parts made up one whole—that no more than a single universe is laid open to human contemplation. This was done by Whewell thirty years after his death.
CHAPTER IV.
HERSCHEL’S SPECIAL INVESTIGATIONS.
Double stars were, when Herschel began to pay attention to them, regarded as mere chance productions. No suspicion was entertained that a real, physical bond united their components. Only the Jesuit astronomer, Christian Mayer, maintained that bright stars were often attended by faint ones; and since his observations were not such as to inspire much confidence, his assertions counted for very little. “In my opinion,” Herschel wrote in 1782, “it is much too soon to form any theories of small stars revolving round large ones.” He, indeed, probably even then, suspected that close equal stars formed genuine couples; but he waited, if so, for evidence of the connection. The chief subject of his experiments on parallax was Epsilon Boötis, an exquisitely tinted, unequal pair. But he soon became aware that either stellar parallax was elusively small, or that he was on the wrong track for detecting it. And, since his favourite stars have proved to be a binary combination, it was, of course, drawing water in a sieve to make one the test of perspective shifting in the other.
The number of Herschel’s double stars alone showed them to be integral parts of an express design. Such a crop of casualties was out of all reasonable question. And it was actually pointed out in 1784 by John Michell, a man of extraordinary sagacity, that the odds in favour of their physical union were truly “beyond arithmetic.”