to increase and diminish, indeed, but it is always present. Here, then, is this great stellar system, having a four-day revolution of its own, and yet swinging rapidly through space in our direction. Nor is this all. One of the component stars must be nearly or quite dark; else its presence would infallibly be detected by our instruments. And now we come to the most astonishing thing of all. How comes it that the average rate of approach of the "four-day system," as a whole, changed between 1896 and 1899? In 1896 only this velocity of the whole system was determined, the four-day period remaining undiscovered until the more numerous observations of 1899. But even without considering the four-day period, the changing velocity of the entire system offers one of those problems that exact science can treat only by the help of the imagination. There must be some other great centre of attraction, some cosmic giant, holding the visible double pole-star under its control. Thus, that which we see, and call the pole-star, is in reality threading its path about the third and greatest member of the system, itself situated in space, we know not where. [Illustration: Spiral Nebula in Constellation Leo. Photographed by Keeler, February 24, 1900. Exposure, three hours, fifty minutes.] NEBULÆ Scattered about here and there among the stars are certain patches of faint luminosity called by astronomers Nebulæ. These "little clouds" of filmy light are among the most fascinating of all the kaleidoscopic phenomena of the heavens; for it needs but a glance at one of them to give the impression that here before us is the stuff of which worlds are made. All our knowledge of Nature leads us to expect in her finished work the result of a series of gradual processes of development. Highly organized phenomena such as those existing in our solar system did not spring into perfection in an instant. Influential forces, easy to imagine, but difficult to define, must have directed the slow, sure transformation of elemental matter into sun and planets, things and men. Therefore a study of those forces and of their probable action upon nebular material has always exerted a strong attraction upon the acutest thinkers among men of exact science. Our knowledge of the nebulæ is of two kinds--that which has been ascertained from observation as to their appearance, size, distribution, and distance; and that which is based upon hypotheses and theoretical reasoning about the condensation of stellar systems out of nebular masses. It so happens that our observational material has received a very important addition quite recently through the application of photography to the delineation of nebulæ, and this we shall describe farther on.