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to search for any possible anomalies in the radial velocities given by these lines in a large number of stars. Single, as well as binary stars should be included in such an investigation, and preference might well be given to stars in low galactic latitudes.
Several of the binaries of this type furnish strong evidence of a calcium cloud, at rest in space, interposed between our Sun and the binary. An alternative explanation postulates a sheath of calcium around the binary system as a whole. It would seem that a decision between these theories must await the accumulation of additional evidence.
The concentration of seven of these in a relatively small area in Orion is suggestive, as is also the fact that all but three are located in or fairly close to the Milky Way. There are certain other peculiarities of position of these binaries which should be noted. The diffuse nebulosity I 348 is only 7' south of o Persei; £ Persei is y2° south of the great diffuse nebulosity 1499; the Orion stars are located in a region full of faint, extended nebulosity; vGeminorum is on the edge of a star cloud; pScorpii is in a remarkable region characterized by enormous diffused nebulosity and vacant or absorption lanes; a Aquilae is in or close to a fairly dark rift in the Milkv Way; Boss 6142 is fairly close to a similar rift (absorption lanes').
6. There have been a number of attempts to discover evidences of local absorption or occulting effect over larger areas: perhaps the best known of these is Turner's hypothesis of an "obscuring spiral." It would seem that the evidence is still insufficient to decide on the validity of such hypotheses. There is, however, one outstanding fact of sidereal arrangement for which no adequate explanation has as yet been found, the peculiar grouping of the spiral nebulae about the galactic poles, and the entire absence of these bodies in the Milky Way structure. If intragalactic and relatively close to us, their apparent abhorrence of the galactic plane is inexplicable. If these are individual galaxies enormously remote from our own. '•island universes." their grouping is equally difficult of explanation. The theory has been tentatively put forward of the existence of a great band of occulting matter in the plane of our galaxy and presumably for the most part outside its periphery, which serves to cut off from our view the distant spirals lying near the projection of our galactic plane in space. Aside from the fact that it offers a possible explanation of the grouping of the spiral nebulae, the theory is supported by no positive evidence at present, except such as may be drawn from the "coal sacks" and similar "stellar deserts." On the "island universe" theory our own galaxy, could it be viewed from a sufficiently great distance, would appear as a spiral (compare the work of Easton on our galaxy as a spiral). The results-of this paper, in that the occurrence of bands of occulting matter is shown to be a relatively common characteristic of the spiral nebulae, may be regarded as offering evidence in support of the validity of the hypothesis, that a similar band of occulting material at the periphery of our galaxy, obliterates the spirals in or near the projection of our galactic plane.
Campbell and Moore have suggested that a layer of absorptive material in the outer portions of certain planetary nebulae would explain the peculiar doubled lines which the3* have observed (Publ. A. S. P.. 29. 134-35, 1917).
It would appear that but little discussion or argument is needed as to the nature of the effect observed in the thirty-two nearly edgewise spirals showing dark lanes. Narrow, vacant regions extending through the entire diameter of these objects and dividing them into two parallel nebulae, would be mechanically inconceivable. Bands of occulting material form the only acceptable hypothesis.
It is my belief that the existence of this phenomenon in so large a proportion of the edgewise spirals is in itself the strongest argument that the same general cause is operative in the effects seen in the considerably elongated spirals, namely, the lanes visible on one side of the major axis and evanescent on the other, and other manifestations of asymmetry parallel to the major axis of such elliptical spirals. There appears to be no valid reason for separating the edgewise spirals, as a class, from spirals in general; these classes merge one into the other by imperceptible gradations.
Vacant lanes should show, in general, most clearly where they cross the major axis of the projected ellipse, as they would suffer no foreshortening at these points. The foreshortening caused by the inclination of the nebular plane to the line of sight should make them least distinct, in general, where they cross the minor axis. A familiar example of this effect is the Cassini division in Haturn's rings, always much more easily seen on the major axis of the projected ellipse. A similar effect is seen in the well separated whorls of many large spirals (cf. 3031 (76)). Very narrow lanes, relatively near the nucleus, as in 4526 (17) and 4429 (38), are presumably of a width comparable with the thickness of the nebular material where they occur; these, if vacant lanes, should suffer an additional effect of obliteration on the minor axis, as may be illustrated by the schematic diagram, figure 82. Here as drawn, the rays A and B to the observer will find no completely open path through open lanes near the minor axis of the projected ellipse. The observed effects are, on the contrary, most prominent near the minor axis of the projected ellipse, just where they should be weakest on the theory of open lanes.
It may be argued that a projecting, roughly spherical, central swelling cuts off and obliterates the supposedly open lanes on what we may term the farther end of the minor axis. But a moment's consideration will show that this argument is scarcely tenable. Such a protuberant central swelling, granting a reasonable amount of symmetry of form to the nebula, must, if it extends far enough above the nucleus to occult part of the lane at the farther end of the minor axis, extend also far enough below the nucleus to show through the lanes on the nearer end of the minor axis, were the appearance due to vacant lanes. Compare figure 83, and such a conclusive example as 4594 (6) or 4565 (4).
"' . S*\ -"N.*
Fig. 82 ^
In some of the cases illustrated it would seem that the streaks or rings of occulting matter must lie in a plane making a small angle with the plane of the nebula; compare 5866 (5), 3623 (23), 3169 (39), 4826 (54), and 2903 (57). A similar assumption might perhaps be made to explain the frequent cases where the nebular matter on one side of the major axis is fainter, though in that case it is difficult to see why we should not meet more frequently cases where such differences of brightness should exist parallel to the minor, rather than the major axis.
It will be evident that on the hypothesis of rings of occulting material that side of an elongated spiral which is fainter or shows absorption lanes, is presumably the closer to us. As the ratio of the major and minor axes of such elongated spirals furnishes an approximate value of the inclination of the nebular plane to the line of sight, this asymmetry, when obtaining, should serve to give the quadrant of the inclination as well.
It is difficult to resist the impression, on studying carefully the original negatives of many of the nearly round spirals which lie almost at right angles to our line of sight, that there are frequent evidences of whorls of dark matter between the bright whorls of nebular matter. No definite proof of this can be adduced, and it is possible that the effect mentioned is largely one of contrast, and subjective. The narrow dark lanes in 5194 (M. 51) are worth study in this connection, particularly that alongside the longest whorl which connects with the companion nebula 5195, and which is apparently connected with the lanes showii in the latter (74). Another very marked example is afforded by the beautiful nebula 5236. There are many other, nearly round nebulae where similar narrow, clearcut dark lanes are seen; in fact the majority of such nebulae show appearances of this sort. Among these the following may be specially noted (the number following each N.G.C. number indicates the plate in L. 0. Publications, vol. VIII):
253; 2 2403; 17 4303; 34 4736; 44
598; 3 2903; 20 4321; 35 5457; 49
628; 4 3627; 29 4501; 37 6946; 62
1068; 7 3726; 30 4725; 42
While the hypothesis of occulting matter in the outer portions of the spiral nebulae seems sufficiently established by the pictorial evidence adduced for the edgewise arid for many of the greatly elongated spirals, other hypotheses should be considered as well for that class of spirals which show differences of intensity on opposite sides of the major axis. Two theories, other than that which presupposes the presence of whorls of occulting matter, may be mentioned as meriting some consideration.
Phase effect.—Assuming that the spiral arms are composed of finely divided matter shining by reflected light originating in the nuclear portions, the result of a phase effect in spirals making but a small angle with the line of sight, would be a marked difference in intensity on opposite sides of the major axis, that side which was nearer the observer being the fainter. So many unknown factors would enter into any calculation of such a difference in intensity, for instance, size and reflective power of the small particles, diffraction effects, illumination by scattered light, occulting effect, etc., that it would appear futile to attempt to include any such assumptions. Considering a phase effect only, for an inclination of the nebular plane of 10° to the line of sight, we should expect a thin uniform sheet of particles shining by light from a central nucleus to show roughly the following differences in brightness:
Farther end of minor axis 1.00
Ends of major axis 0.56
Nearer end of minor axis 0.12
Polarisation effects.—These should be expected as a consequence of any reflection theory. Because of the exceedingly faint character of the nebulae, no effects of polarization have as yet been observed.1
Phase effect, as a possible cause for differences in brightness in various parts of the spiral nebulae, is manifestly inseparably bound up with the hypothesis that the spirals shine by reflection from a central nucleus. It is impossible to exercise any dogmatism in discussing a class of objects which offer so many unexplained pecularities as do the spiral nebulae, and with regard to which we have so little precise evidence. If the differences in brightness observed on opposite sides of the major axis of elongated spirals could with probability be attributed to a phase effect, it would be a strong argument in favor of the reflection theory, as against the wellknown "island universe" theory of the spiral nebulae. It may be an advantage, then, to combine such arguments as may be advanced against the phase effect as such, and the reflection theory.
i Reynolds, Preliminary Observations of Spiral Nebulae in Polarised Light, Monthly Notices, 72, 553, 1912. An extensive series of photographic tests has been made by Dr. W. K. Green at the Lick Observatory during the past year; no certain evidence of polarisation could be detected.
1. Phase effect can not be assumed to be universal; quite a number of the greatly elongated spirals show no marked difference in brightness on opposite sides of the major axis.
2. A phase effect can not explain the lanes so frequently found showing on one side, and faint or invisible on the other. In many cases the nebular whorls on one side of the major axis seem to be of equal intensity on both sides of this axis at equal distances from the nucleus, but the dark lanes are so much more prominent on one side that this side appears much the fainter.
3. The reflection hypothesis presupposes a central star or collection of stars of sufficient brightness to produce the observed effects of illumination in the outlying parts of the nebula. Many spirals show such bright centers; in fully as many others the central star or condensation is so faint in comparison with the brighter outlying whorls as to be absolutely inadequate as a source of illumination under the reflection hypothesis. A number of large spirals appear to have no true nucleus whatever; such an example is 2403; others like 2503, have nuclei, little if any brighter than the nebular matter in some of the whorls. The nucleus of the enormous elongated nebula 253 (L. 0. Publications, VIII, plate 2), if one exists at all, is very much fainter than the nebular condensations in the outer portions of the spiral.
4. On a reflection hypothesis, we should expect in general, a diminution in the brightness of the nebular material following the inverse square law. Reynolds has found2 that the Great Nebula in Andromeda satisfies this requirement. In most spirals such an effect is so bound up with the varying density of the nebular matter at different portions of the spiral that it would be difficult to establish the law; only the existence of polarisation effects, thus far never certainly detected, could decide the point. A study of the round or nearly round spirals of the Crossley collection leads me to the belief that in the majority of cases, a reflection hypothesis is absolutely untenable.3 Many nebulae show bona fide nebular matter (not apparently stellar condensations) in whorls at considerable distances from the nucleus, nearly as bright or brighter than the matter contiguous to the nucleus.
2 Monthly Notices, 74, 132, 1913.
3 It is scarcely necessary for me to emphasize at this point, that the discussion and argument is based solely upon the spiral nebulae. Diffuse nebulosity associated with bright stars, e.g., the Meropc nebula in the Pleiades, the variable nebulae 2261 and 6729, seem well explained as a reflection effect.