Fortunately, however, other means than telescopic observation for the settlement of this question are available. In 1846, it was discovered by the author of this book that the spectrum of an ignited solid is continuous, that is, has neither dark nor bright fixed lines. Fraunhofer had previously made known that the spectrum of ignited gases is discontinuous. Here, then, is the means of determining whether the light emitted by a given nebula comes from an incandescent gas, or from a congeries of ignited solids, stars, or suns. If its spectrum be discontinuous, it is a true nebula or gas; if continuous, a congeries of stars. In 1864, Mr. Huggins made this examination in the case of a nebula in the constellation Draco. It proved to be gaseous. Subsequent observations have shown that of sixty nebulæ examined, nineteen give discontinuous or gaseous spectra; the remainder continuous ones. It may, therefore, be admitted that physical evidence has at length been obtained, demonstrating the existence of vast masses of matter in a gaseous condition, and at a temperature of incandescence. The hypothesis of Laplace has thus a firm basis. [Sidenote: Opposition to the nebular hypothesis.] Notwithstanding the great authority of the astronomers who introduced it, the nebular hypothesis has encountered much adverse criticism; not so much, however, from its obvious scientific defects, such as its inability to deal with the cases of Uranus and Neptune, as from moral and extraneous considerations. There is a line in Aristophanes which points out precisely the difficulty: Ho Zeus ouk ôn, all' ant' autou Dinos nuni basileuôn. A reluctance to acknowledge the presidency of law in the existing constitution and movements of the solar system has been yielded only to be succeeded by a reluctance to acknowledge the presidency of law in its genesis. And yet whoever will reflect on the subject will be drawn to the conclusion that the principle involved was really settled by Newton in his "Principia"--that is to say, when it became geometrically certain that Kepler's laws originate in a mathematical necessity. As matters now stand, the nebular hypothesis may be regarded as the first superficial, and therefore imperfect, glimpse of a series of the grandest problems soon to present themselves for solution--the mathematical distribution of matter and force in space, and the variations of that distribution in time.