As the science of astronomy has advanced, the ideas men have formed respecting the extent of the universe have gradually become more and more enlarged. In far-off times, when astronomers were content to judge of the conformation of the universe by the appearances directly presented to their contemplation, the ideas formed respecting the celestial bodies were singularly homely. We read that Theophrastus looked upon the Milky Way as the fastening of the stellar hemispheres, which are 'so carelessly knitted together, that the fiery heavens beyond them can be seen through the spaces.' Anaximenes believed the heavens to be made of a kind of fine earthenware, and that the stars are the heads of nails driven through the domed vault formed of this material. And even Lucretius, whose views of nature were so noble, has referred without disapproval to the bizarre theory of Xenophanes that the stars are fiery clouds collected in the upper regions of air.
While the Ptolemaic system of astronomy was accepted there were no means of forming any trustworthy views respecting the extent of the stellar universe. If the earth were ever at rest we could never know how far the stars are from us; and therefore the old astronomers were free to invent whatever theories they pleased as to the scale on which the sidereal scheme is constructed. It was only when the earth was set free by Copernicus from the imaginary chains which had been conceived as holding it in the centre of the universe that it became possible to form any conception of the distances at which the stars lie from us. Indeed Tycho Brahe immediately pointed this out as an overwhelming objection against the new theory. 'Are we to suppose,' he argued, ' that the stars are placed at such enormous distances from us as to seem wholly unchanged in position while the earth sweeps round the sun in an orbit millions of miles in diameter ? If this amazing theory were true, the stars would be hundreds of millions of miles from us, a view which is utterly monstrous and incredible.'
But strange as this new view appeared, it gradually gained ground. It became presently so well established that when Cassini discovered that the earth travels in a much wider orbit than Tycho Brahe had supposed-so that the stars were at once thrown many hundreds of millions of miles farther from us - astronomers still held to the new order of things. 'With Briarean arms,' as Humboldt has described their labours, the fellow-workers of Cassini thrust farther and farther away the 'heaven of the fixed stars,' until the immensity of the universe grew so great beneath their labours, that new modes of expressing its dimensions had to be adopted. They were not satisfied with the obvious circumstance that the stars seem to remain unchanged in position as the earth sweeps round the sun. They tested this apparent fixity of position with instruments of greater and greater power, - yet always with the same result. They made observations ten, twenty, even fifty times more exact than Tycho Brahe's, and the fact that they still detected no change of position signified nothing less than the universe of the fixed stars is ten, twenty, even fifty times farther from us than Tycho Brake had imagined.
Thus when Sir W. Herschel began the noble series of researches amid the stellar depths which has rendered his name illustrious, the world of stars was already of inconceivably enormous extent. Yet so widely did he increase our appreciation of the vastness of the universe, that it has been thought no exaggeration to say of him, that 'he broke through the barriers of the heavens :' 'Caelorum perrupit claustra,' says his monument at Upton, and every student of astronomy who has carefully examined Herschel's labours understands the justice of the expression. For consider what Herschel did. When he began his survey of the heavens, astronomers had proved indeed that the nearest of the fixed stars lie at enormous distances from us, and some of the more advanced thinkers had begun to form noble speculations respecting the relations of the stars which lie beyond the sphere of those visible to us. But it was reserved for Sir W. Herschel to apply exact observations to the unseen star-systems. He literally gauged the celestial depths. With a telescope whose light-gathering power extended the range of vision to about eight hundred times its natural limit, he swept the whole of the northern heavens. He estimated the depth of the system of stars in every direction by a simple and natural process. For, like all great thinkers, he struck out modes of inquiry which, the moment they were presented to the world, seemed so obvious, that the wonder was how they could have remained so long undetected. He said that precisely as the quantity of water passed through by the sailor's lead-line marks the depth of the sea, so the number of stars which can be seen when a telescope of given power is turned towards any part of the heavens is a measure of the depth of the sidereal system in that direction. In individual cases, indeed, the law may not be true, just as the sailor's lead-line may light on the peak of some sunken rock, and so give no true measure of the general depth of the sea in the neighbourhood. But when the average of a great number of such 'star-gaugings' is taken, then we may feel tolerably certain that on applying the simple rule devised by Herschel we shall form no inaccurate estimates of our system's extent in any direction.
Thence arose his great theory of the stellar system. He showed that our sun is but one of an immense number of suns, distributed in a region of space resembling a cloven disc in figure. When we look along the thickness of the disc we see the enormous beds of stars, which lie round us in that direction as a cloud of milky light, which so comes to form a cloven ring round the heavens. But when we look out towards the sides of the disc, where the stars are less profusely scattered, we see between them the black background of the sky.