Page:Popular Science Monthly Volume 2.djvu/676

656 effects—how far-reaching and grand the design!" How deeply they impress us with the wisdom, power, and glory of the Creator and Governor of the universe!

We now come to consider the physical reality of the fundamental assumption of the Nebular Hypothesis. Have nebulous masses a real existence in the universe? Is the Star-dust—the World-stuff—a physical reality, or a mere figment of the brain of the theorist? If the actual existence of self-luminous nebulous matter—the chaotic elements of future worlds and suns—can be established—the fundamental assumption of Laplace loses the character of a pure hypothesis: his conception becomes a physical theory, which, in proportion as it is verified by phenomena, approaches the domain of fact—a vera causa.

It was shown that the highly-diffused and attenuated matter constituting comets, as well as that constituting the zodiacal light—while affording some suggestive analogies to nebulous masses—do not furnish examples in all respects identical with the supposed nebula of Laplace. We are, therefore, compelled to fall back on Sir William Herschel's opinion, that there are numerous nebulæ which really consist—not of clusters of stars, but of a diffused, self-luminous, vaporiform matter. Such bodies are, beyond all question, self-luminous, but the question is, Are they clusters of stars or true nebulæ? In other terms, are they optically or physically nebulous?

For a long time, this question was keenly discussed, and opinions fluctuated in regard to the tenability of the fundamental assumption of the nebular hypothesis. It is well known that, since 1846, the tendency of telescopic observations, as revealed by the magnificent instruments of Lord Rosse, and corroborated by the splendid achromatic of Harvard University, has been to break down Sir William Herschel's distinction between stellar clusters and true nebulæ. After the sword-handle of Orion was broken into glittering fragments, shining with separate and distinct lustre, Sir John Herschel himself was disposed to abandon the opinion of his illustrious father.

But the development of a new and wonderful branch of physical science has recently furnished the most satisfactory proofs of the reality of such bodies. We allude to the application of Spectrum Analysis to the study of the celestial bodies. The well-matured speculations of Sir William Herschel, and the mathematical theory of Laplace, have been vindicated from the doubt under which they have been laboring, and the early nebulous condition of the cosmical matter has been demonstrated. The accomplished Sir John Herschel has been permitted to witness the complete verification of the previsions of his illustrious father; to see the link connecting the past with the present in the cosmogony of the universe—which seemed to have been almost ruptured by the extension of telescopic vision—restored and strengthened by this new branch of physical investigation.