FIG. 10. Planetary Nebulæ.

Herschel and Proctor had established, they cluster more densely in the neighborhood of the pole of the Galaxy. In the southern hemisphere they show the same tendency, but not so strongly marked. Two or three hundred of the brighter nebulæ have long been known to exist in and near the Milky Way, but faint nebulæ, such as those scores of thousands which Keeler showed are still awaiting discovery, are practically nonexistent in the galactic region.

Let us examine more carefully the distribution of the nebula with reference to the

Galaxy, and with reference to the physical conditions which seem to exist within them. Out of approximately fifteen thousand nebulæ thus far discovered, fewer than 150 are planetaries. They exist in a variety of

It is not intended to convey the impression that the nebular distribution is merely a function of the galactic latitude; the observed nebulæ are more numerous in certain galactic longitudes than in others.

1 inch 8'.5.

sizes, from a few that are only two or three seconds of are up to others a quarter of a degree in diameter. The difference in size is due, at least in large part, as Curtis has recently made clear, to a difference in the distances of the bodies from us. A considerable number of them appear to be more and more condensed as we approach their centers, but the ring-form planetaries are the prevailing type (see Fig. 10). These rings of nebulosity are apparently not true rings, existing chiefly in two dimensions, but ellipsoidal shells of matter seen as rings in projection on the background of the sky. If they were true rings, we should see some of them as extremely elongated ellipses, and others ought to be long and slender as a result of seeing them edgewise. Those forms are wholly unknown. Now all of the planetaries give spectra consisting chiefly of isolated bright lines (see Fig. 32); that is, they are gaseous in constitution, and are

Planetary nebulæ.

Right Ascen. Oh to 12". Irregular bright-line nebulæ.

shining by their own light. A very large proportion of them are in or adjacent to the structure of the Milky Way, or in the Magellanic Clouds (see Fig. 11). There are a few exceptions, but the exceptions almost certainly find their explanation in the relative nearness of these few to us, so that, being a little to one side of the central plane of the stellar system, they are seen in projection at some distance to one side or the other of the galactic structure.

The large irregular nebula whose spectra are known to consist of bright lines are charted as open circles. Such nebulæ at a considerable distance from the Galaxy are very scarce indeed.

The motions of approach and recession of the bright-line nebulæ have been observed with spectrographs at the Lick and D. O. Mills observatories. The large and formless bright-line nebulæ, such as the Trifid and Orion nebulæ, are almost at rest amongst the stars; their individual veloc

ities are small, and the mean velocity of the group, with reference to our stellar system, is zero. These nebulæ, considered as a system, are not moving through the stellar system. They are in it and a part of it. Many of the planetary nebulæ (see Figs. 1 and 10) have high velocities, as individuals, but when we consider them collectively their motion with reference to the stellar system is in effect zero. They too are of our system.

There are in or not very far from the Milky Way many irregular nebulæ, of a great variety of sizes, whose types of spectra are for the most part unknown. They are intrinsically faint, and their investigation is a most promising problem of the immediate future. Two of these nebulæ, according to Slipher, have spectra identical with the brilliant stars which seem to be immersed in them; that is, continuous spectra, except as the absorption lines of helium and hydrogen are present in both

ing regions. The bright stars in the Pleiades, those really belonging to the cluster, are numerous, but within the cluster as we see it, and in a considerable area of the adjacent sky, the faint stars are markedly scarcer than in the areas farther away. Barnard has found that the sky in the region around the Pleiades group is possessed of much nebulosi'y. It is a nat

ural question, are the faint stars scarce because the nebulosity there existing has not yet condensed into stellar forms? This may be true in part, but we shall find much more probable the view that the faint stars are deficient in numbers because the nebular materials, at a certain distance away from us, are obstructing the light of the faint stars that are farther away from us than

the nebula is. A similar deficiency of faint stars exists within the great nebula of Orion, and likewise in the adjacent areas, where Mr. William H. Pickering has found a very large part of the constellation of Orion to be covered with faint nebulosity. We shall give illustrations of several regions (see Figs. 14, 15, 16) where this condition -the presence of nebulosity and the scarcity of faint stars-is so marked as to be at once apparent. There are so many regions in and near the Galaxy where these relationships exist that we can not doubt their significance. The faint stars are relatively scarce chiefly because the nebular materials cut off the light of the more distant stars.

This explanaticn is reached by several lines. of evidence, but we take time to present only one.

It is established by modern astronomy that the individual stars are in rapid motion. The speeds of the naked-eye stars average about sixteen miles per second. The distant fainter stars, so far as they have been observed, are also traveling rapidly. There is a tendency to favor certain directions of motion, and the stars in certain small groups are keeping company through space; but a large share of stellar motion is at random. There are stars traveling in all directions. We have not the direct evidence as to the motions of the