faint stars in the far distant outposts of the Galaxy, but we have no reason to suspect that their characteristic motions are unique. We can see no escape from the condition that all the stars are in motion. Under these circumstances we are unable to explain how within a great volume of FIG. 15. Irregular Nebula N.G.C., 5146, photographed by Curtis with the Crossley Reflector of the Lick Observatory. (The circular halos around the brighter stars are unavoidable defects and are not real.) space that is rich in stars there can be a smaller, but still enormous volume of space, nearly free of stars. Their random motions should distribute them more uniformly than we observe to be the case. Let us illustrate by the celebrated black holes, the so-called "coal sacks," in the constellation. of Sagittarius (Fig. 17). In a region where the stars are especially plentiful are two small areas all but empty of visible stars. An enlargement of a photograph by Barnard, the preeminent student of this subject, shows the effect still better. How can such great "holes" through the stellar system be surrounded by a plenitude of stars moving more or less at random so as to give a high density of star distribution right up to the sharply defined edges of the holes, and yet leave the holes empty of stars? With the help of all astronomical experience we can not explain the phenomena by the absence of stars. I think we must assume, with Barnard and others, that the stars are actually there, and that they are invisible because invisible materials between us and the stars are absorbing or occulting the light which the stars are trying to send us. Much of the interesting structure in the Milky Way is probably due in a like manner to obstruction by materials lying between us and the great clouds of stars (see Fig. 18). It is characteristic of the galactic structure that where luminous nebulæ seem to reduce the numbers of faint stars visible, the reduction in numbers of stars extends also far out beyond the limits of visible nebulosity (see Figs. 14, 15 and 16), and we can scarcely resist the conclusion that invisible extensions of the luminous nebular fabric exist as obstructing agents. There are many other lines of evidence in support of the hypothesis that invisible. matter exists in abundance within the stellar system: 1. Newcomb and Kelvin, working independently and on the assumption that the motions of the stars are generated by gravitational attractions originating within the stellar system, were unable to account for the high observed velocities of stars, except on the hypothesis that the visible stars contain only a small fraction of the matter existing in the system; the greater part of the attracting material in the stellar sys FIG. 16. The North America Nebula, photographed by Barnard with the 10-inch Bruce Telescope of the Yerkes Observatory. tem being non-luminous, or at least invisible. 2. There must be an enormous amount of comet material distributed throughout space. The astronomers discover only a negligibly small proportion of the comets which pass near the center of our solar system. May not comet materials exist also in abundance in the systems of the other stars? 3. Students of meteors have established that the separate little bodies which collide with the earth's atmosphere and are respon sible for the so-called shooting stars are stupendous in numbers. It has been estimated that as many as twenty or thirty millions of such bodies collide with the earth every twenty-four hours. How incomparably and inconceivably greater must be the total number in our solar system. The power of these countless particles to obstruct the passage of light is not negligible. There may be as many such particles on the average around the other stars in our stellar system. 4. The so-called new stars, otherwise known as temporary stars, afford interesting evidence on this point. These are stars which suddenly flash out at points where previously no stars were known to exist; or, in a few cases, where a faint existing star has in a few days become immensely brighter. Twenty-nine such stars have been observed in the past three centuries, nine FIG. 17. The Dark Holes, or "Coal Sacks," in Sagittarius, photographed by Barnard with the 10-inch Bruce Telescope of the Yerkes Observatory. teen of them since 1886 when the photographic dry plate was applied systematically to the mapping of the heavens, and fifteen of the nineteen are to the credit of the Harvard observers. This is an average of one new star in less than two years in the last three decades; and, as some of the fainter new stars undoubtedly come and go unseen, it is evident that they are by no means rare objects. Now all of the temporary stars except five appeared in the Milky Way, and four of the five exceptions are worthy of note. Two of the five appeared in well-known nebula; another was located close to the edge of a spiral nebula, and quite probably in a faint outlying part of it; a fourth was observed to have a nebulous halo about it; and the fifth was but meagerly and imperfectly observed. Keep ing the story as short as possible, a temporary star is seemingly best explained on the theory that a dark or relatively dark star traveling rapidly through space has met with resistance, such as a great nebula or cloud of particles would afford. While passing through the cloud the star is in effect bombarded at high velocity by the resisting materials. The surface strata become heated, and the luminosity of the star increases rapidly. The new star of February, 1901, in Perseus afforded interesting testimony. Wolf at Heidelberg photographed in August an irregular nebulous object near the star. Ritchey's photograph of September showed extensive areas of nebulosity in all directions from the star. In October Perrine and Ritchey discovered that the nebular structure had apparently moved outward from the star (see Fig. 19). Going back to a March photograph, taken for a different purpose, Perrine found recorded upon it an irregular ring of nebulosity closely surrounding the star which was not visible on later photographs. The region seemed to be full of nebulosity not visible to us under normal conditions. The rushing of the dark star into and through this resisting medium made the star the brightest one in the northern sky for several days. The great wave of light going out from the star when at this maximum brightness traveled far enough in five weeks to fall upon non-luminous materials and made a ring of nebulosity visible by reflection. Continuing its progress, with a speed of 186,000 miles per second, the wave of light illuminated the material which Wolf photographed far away from the star in August, the material which Ritchey photographed still farther away in September, and the still more distant materials which Perrine and Ritchey photographed in October, November and in later months. We FIG. 18. The Milky Way in Ophiuchus showing apparent absorption or obstruction effects; photographed by Barnard with the 10-inch Bruce telescope of the Yerkes Observatory. were able to see this material only as the very strong wave of light which left the star at maximum brightness made the material luminous in passing. We can scarcely doubt, in view of all these facts, that there is a stupendous amount of obstructing material scattered throughout our stellar system. This may eventually be condensed into stars; but the point I wish to emphasize is that the material is there, and is evidently obstructing the passage of light. The efficiency of the obstructing material is no doubt the greater in the long dimensions of the system; in the direction of the Milky Way. Let us now consider the nebulæ which are not in or near the Galaxy. The overwhelming majority of the stars are in the galactic zone of the sky. If we call this zone 30° wide, which makes it a shade over one fourth the entire sky, we may say that this fourth-area contains certainly three fourths of all the stars known to exist in our stellar system. The remaining three fourths of the sky do not show more than one fourth of all the stars. Of about 15,000 nebulæ recorded probably not over 300,5 certainly 5 It is not practicable to state the number more definitely. To illustrate the difficulty, we mention the fact that several small adjoining nebulæ re FIG. 19. Apparent movement of nebulous matter about Nova Persei. Photographed by Perrine with the Crossley Reflector of the Lick Observatory. [The motion is best shown by the bright mass above and to the right of the center, in comparison with the surrounding stars.] not over 400,5 are found in the galactic zone. That is, not more than one fortieth or one fiftieth of the known nebulæ are in that one fourth of the sky which contains the Milky Way; and these possible 400 galactic nebulæ include nearly all of the planetary nebulæ, nearly all of the large gaseous nebulæ, nearly all of the regions where large absorbing or obstructing nebulæ are seen to be effective; in other words, as I have endeavored to make clear, nearly all of the nebulæ that are really within our stellar system. The other three quarters of the sky contain, on the contrary, nearly 15,000 recorded nebulæ. The nebulæ to the south of the galaxy have not been so well observed as those to the north, and we shall here consider the northern galactic hemisphere alone. The one quarter of the northern hemisphere immediately around the pole of the Galaxy contains three fourths of all the recorded nebulæ in the corded with a short exposure may be seen to be but parts of one great nebula when the exposure is longer. whole hemisphere, and the quarter of the hemisphere adjoining the central line of the Galaxy contains about one fiftieth of all the recorded nebulæ in the hemisphere. The density of nebular distribution in the Galaxy is only one fortieth that in the quarter-area farthest away from the Galaxy. A still more interesting fact concerning nebular distribution is this: thousands of spiral nebula are known to exist, but not a single spiral nebula has been found within the galactic structure. Some spirals have been found in regions adjoining the Galaxy, but they are relatively few. The spirals in particular abhor the Milky Way. As we said above, the very avoidance of the Milky Way seems at first sight to show that they are arranged with reference to it; that they hold some relation to it. Is this relationship real, or only apparent? Are the spiral nebulæ in or attached to our system, or are they outside of our system, at tremendous distances from us? This question is a live one in the astronomy of to-day. The old hypothesis that the unresolved nebulæ are |