Twice in the course of this discussion we have called attention to the change of color invariably undergone by temporary stars in the later stages of their career. This was conspicuous with Nova Persei which glowed more and more redly as it faded, until the nebulous light began to overpower that of the stellar nucleus. Nothing could be more suggestive of the dying out of a great fire. Moreover, change of color from white to red is characteristic of all variable stars of long period, such as “Mira” in Cetus. It is also characteristic of stars believed to be in the later stages of evolution, and consequently approaching extinction, like Antares and Betelgeuse, and still more notably certain small stars which “gleam like rubies in the field of the telescope.” These last appear to be suns in the closing period of existence as self-luminous bodies. Between the white stars, such as Sirius and Rigel, and the red stars, such as Aldebaran and Alpha Herculis, there is a progressive series of colors from golden yellow through orange to deep red. The change is believed to be due to the increase of absorbing vapors in the stellar atmosphere as the body cools down. In the case of ordinary stars these changes no doubt occupy many millions of years, which represent the average duration of solar life; but the temporary stars run through similar changes in a few months: they resemble ephemeral insects—born in the morning and doomed to perish with the going down of the sun. VI Explosive and Whirling Nebulæ One of the most surprising triumphs of celestial photography was Professor Keeler’s discovery, in 1899, that the great majority of the nebulæ have a distinctly spiral form. This form, previously known in Lord Rosse’s great “Whirlpool Nebula,” had been supposed to be exceptional; now the photographs, far excelling telescopic views in the revelation of nebular forms, showed the spiral to be the typical shape. Indeed, it is a question whether all nebulæ are not to some extent spiral. The extreme importance of this discovery is shown in the effect that it has had upon hitherto prevailing views of solar and planetary evolution. For more than three-quarters of a century Laplace’s celebrated hypothesis of the manner of origin of the solar system from a rotating and contracting nebula surrounding the sun had guided speculation on that subject, and had been tentatively extended to cover the evolution of systems in general. The apparent forms of some of the nebulæ which the telescope had revealed were regarded, and by some are still regarded, as giving visual evidence in favor of this theory. There is a “ring nebula” in Lyra with a central star, and a “planetary nebula” in Gemini bearing no little resemblance to the planet Saturn with its rings, both of which appear to be practical realizations of Laplace’s idea, and the elliptical rings surrounding the central condensation of the Andromeda Nebula may be cited for the same kind of proof.