Neptune, to be filled with gas, luminous or not. It would not occupy more than a fortieth part of the space in the heavens occupied by the great nebula in Orion, and it is doubtful if our best telescopes reveal the whole of that nebula's extent in any direction. It is within such vast spaces that Herschel imagined this world-making process to be going on. Man's imagination quails in his attempt to grasp the space required for such a workshop, the tools employed, or the time taken to condense "nebulous matter" into dazzling suns or dark companions.
We are so much accustomed to feast our eyes on drawings of a few magnificent and singularly shaped nebulæ, that thought is apt to overlook the vast numbers of them scattered over the heavens in all stages of size or progress. Herschel did not fall into this mistake. His object was higher than to satisfy curiosity or to excite wonder. He had the feeling that there was a process going on, of which he believed he could trace not a few of the stages. The smallest and the least wonderful of the nebulæ might thus prove to be as important in tracing out this progress, as the most awe-inspiring. Nor did he look upon all of them as resolvable into stars or masses of shining matter, more or less rare. He believed that some of them were not luminous, but dark; but he made no attempt to explain, as may be at least attempted to-day, how a vast mass of invisible gas may become lighted up, and send its brightness off on a journey of ten or twenty or fifty years, to publish to us the changes that, in process of ages, had taken place in its nature. It was the discovery of world-making he was aiming at in these long and laborious, but not wearisome researches. Others
