which blots out the light of the more distant stars; and (2) the misty matter associated with the more condensed star-groups may obscure the light of external galaxies which therefore are better seen through the thinner places in our own stellar mass. Either of these causes would account for the stellar voids which Sir William Herschel describes as even a warning of the proximity of nebulæ; but it will be seen that there is no foundation for the inference which Mr. Herbert Spencer has built upon the supposed fact, namely, that none of the nebulæ can be external galaxies, because "thousands of nebulæ. . . agree in their visible positions with the thin places in our own Galaxy," and that they are necessarily most intimately linked with its structure. The connection, if established, will in no wise invalidate the wider generalization that external galaxies must appear to be most numerous in those regions where the mists or gaseous masses attendant on our Galaxy thin out and permit the light from the outside to penetrate the starry walls.
Mr. Roberts bears this testimony to the fact that the larger part of the nebulæ are situated beyond the confines of the Galaxy: There are "to be seen," he says, "stars apparently in a complete state of development, scattered over the surfaces of the most prominent of the nebulæ, but it will be observed that they do not conform with the trends of the spirals nor with the curves of the nebulous stars [or stellar condensations[1]?] involved in them. This fact I apprehend to be strong evidence that they are independent of the nebulæ—that they are not in any way involved in the nebulosity, but are seen by us either in front, or else in space beyond the nebulæ. If they were beyond them, their light would have to penetrate through the nebulosity, and we should therefore expect it to be duller in character and the margins of the stars to be surrounded by more or less dense nebulous rings; but these effects are not traceable in the photo-images, and we are consequently led to adopt the alternative inference that they are between us and the nebulas. If they were involved in the nebulosity, they would conform with the trends of the convolutions and appear like nebulous stars."[2]
The dark lanes in the Milky Way are sometimes called "rifts," a term which implies that the stars are distributed in a relatively thin sheet which can be rent asunder. Moreover, the word is not used in a merely metaphorical or descriptive sense, but in its full significance, as in the following quotation from "Worlds in the Making" by Svante Arrhenius (p. 173): "The presumption that these rifts represent the tracks of large celestial bodies which have cut their way through widely expanded nebular masses has been entertained for a long time." And
- ↑ Of the larger spiral nebulæ, Professor G. W. Ritchey says (Astrophys. J., Vol. 32, p. 32, July, 1910): "All of these contain great numbers of soft star-like condensations which I shall call nebulous stars." It appears not improbable that these represent irresolvable stellar groups.
- ↑ "Photographs of Stars, Star Clusters and Nebulæ," Vol. 2, pp. 23–24.
