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present entertained respecting the Via Lactea. The elder Herschel, whose nobly speculative views of nature were accompanied by practical common sense, and a wonderful power of patient observation, applied to the heavens his now celebrated method of gauging. He assumed as a first principle, to be modified by the results of observation, that there is a tolerable uniformity in the distribution of stars through space. Directing his twenty-feet reflector successively towards different parts of the heavens, he counted the number of stars which were visible at any single view. The field of view of this reflector was 15' in diameter, so that the portion of the sky included in any one view was less than one-fourth of that covered by the moon. He found the number of stars visible in different parts of the heavens, in a field of view of this size, to be very variable. Sometimes there were but two or three stars in the field; indeed, on one occasion he counted only three stars in four fields. In other parts of the heavens the whole field was crowded with stars. În the richer parts of the galaxy as many as 400 or 500 stars would be visible at once, and on one occasion he saw as many as 588. He calculated that in one quarter of an hour 116,000 stars traversed the field of his telescope, when the richest part of the galaxy was under observation. Now, on the assumption above-named, the number of stars visible when the telescope was pointed in any given direction was a criterion of the depth of the bed of stars in that direction. Thus, by combining a large number of observations, a conception-rough, indeed, but instructivemight be formed of the figure of that stratum of stars within which our sun is situated.

One section of the galactic nebula, as determined from Herschel's observations, is given in Figure 3. The projections extending to the left correspond to those portions of the

Fig. 3.

particular great circle considered, which cross the double part of the Milky Way; the opposite projection represents the portion crossed by the single stream; while the comparative flattening of the central part indicates the gradual diminution of star-density in directions removed from the galactic zone. It is, of course, to be understood that Herschel was far from

supposing that such a figure correctly represented the figure of the section. He looked upon it as affording but a rough indication of the true figure. He had indeed noticed, so early as 1785, that there is a tendency in the Milky Way to cluster around definite regions of the heavens; and he saw that the fact of such clustering was sufficient to account for many irregularities of the figure, quite irrespectively of the absolute extent of the Milky Way in space. If we are looking from a height at the lights of a large town, we may fairly assume that a row of many lights very closely ranged, lies at a greater distance from us than another row containing lights more widely dispersed, if we have reason to suppose that throughout all the streets of the town the lights are separated by distances approximately equal. But if we have reason to suspect that there are some streets lighted more fully than others, the inference would be no longer valid. And, again, Herschel suspected that there are stars so large, as to bear a sort of sway among other stars by superior attractive influence. Here, then, was another element of difficulty, since it becomes clear (1) that the brilliancy of a star is no positive evidence of proximity; and (2) that there may be (besides the obvious clusterings already considered) laws of systematic distribution, which might largely modify the evidence afforded by stargauging. For instance, returning to the illustration given above, if we have reason to suspect that there are many lights of superior brilliancy, in some parts of a town, and that further there are in some streets laws of arrangement among the lights, or that there are irregularities of surface-contour, which produce here and there a greater or less foreshortening than would result on a level ground, we should have to make allowance for these points in attempting to form an estimate of the distances at which the different parts of the town are removed from us.

Still, the results obtained by Sir W. Herschel have very properly been accepted as affording general evidence of high value.

Sir J. Herschel, during his residence at the Cape of Good Hope, carried out an extensive series of observations of the southern heavens. Applying his father's method of gauging, with a telescope of equal power, he obtained a result agreeing, in a most remarkable manner, with those obtained by Sir William Herschel. It appeared, however, that the southern hemisphere is somewhat richer in stars than the northern, a result which has been accepted as indicating that our system is probably somewhat nearer the southern than the northern part of the galactic nebula.

Combining the results obtained by the two Herschels, we should assign to the stratum of stars a figure somewhat resem

bling that of the solid cloven disc exhibited in Figs. 4 and 5. The latter figure, being a side view, gives the figure of the

Fig. 4.

Fig. 5.

imaginary section which appears so often in works on astronomy. In Figs. 3 and 5 the small circle near the centre indicates the probable extent (on the theory in question) of the sphere within which stars down to the fourth magnitude may be supposed to be included.

The main difficulties in attempting to form an estimate of the real configuration of the galactic nebula are those which have been already mentioned. Have we evidence confirming or disproving (1) the tendency to clustering suggested by the elder Herschel, (2) the possible variability among star-magnitudes, and (3) of influences exerted by large stars in guiding or swaying others? It appears to me that there are indications of a very obvious and important character, which have been either altogether unnoticed, or much less noticed than they deserve. In considering these indications, I would refer the reader chiefly to Figs. 1 and 2; but some portions of the evidence cannot be thoroughly understood without reference to star-maps, in which (at a single view, if possible) the course of the Milky Way is exhibited in a manner which enables us at once to determine its relations to the constellations not included in these figures. I write with my black star-maps before me, my object being to consider the special evidence afforded by stars of the leading magnitudes. It may seem (and, indeed, on the assumption of any approach to uniformity in the true magnitudes, or distribution of stars, it must necessarily be) a very imperfect method to refer to star-maps including only the first five magnitudes, still more to consider the first four magnitudes which are alone represented in Fig. 1. It is

obvious that if our sun is placed within such a stratum as is exhibited in Figs. 3, 4, and 5, no evidence whatever as to the structure of that stratum can be afforded by considering the comparatively few stars included within the small central circle. But it is this very fact on which I wish to dwell. If any connection does appear between the configuration of our galaxy, and the arrangement of stars which are assumed to be much nearer to us than the Milky Way, it will be obvious that we must somewhat modify the views illustrated by these figures.

Now, taking Figs. 1 and 2, I think one can trace a connection between the stars there depicted, and that stream of nebulous light which the view we are examining teaches us to consider as at an indefinite distance beyond those stars. In the northern portion, perhaps, the connection is not very remarkable. We see that a large number of the brighter stars lie on or near the Milky Way, but it would require the examination of a somewhat wider zone than that here presented to exhibit this arrangement as positive evidence of aggregation. However, I think no one who has attentively examined the glories of Orion, the richly-jewelled Taurus, the singular festoon of stars in Perseus, and the closely-set stars of Cassiopeia, but must have felt that the association of splendour along this streak of the heavens is not wholly accidental. The stars here seem to form a system, and a system which one can hardly conceive to be wholly unconnected with the neighbouring stream of the Milky Way. But in the southern portion the arrangement is yet more remarkable and significant. From Scorpio, over the feet of the Centaur, over the keel of Argo, to Canis Major, there is a clustering of brilliant stars, which it seems wholly impossible not to connect with the background of nebulous light. It is noteworthy, also, that this stream of stars merges into the stream commencing with the group of Orion already noticed. Nor is this all. It is impossible not to be struck by the marked absence of stars in that region of the sky which lies in the upper right-hand corner of Fig. 2. One has the impression that the stars have been attracted towards the region of the stream indicated, so as to leave this space comparatively bare.

Now, this last circumstance would appear less remarkable if the paucity of stars here noticed were common also in parts of the heavens far removed from the Milky Way. But this is not the case. Beyond this very region, which we find so bare of stars, we come upon a region in which stars are clustered in considerable density, a region including Crater, Corvus, and Virgo, with the conspicuous stars Algores, Alkes, and Spica. But, what is very remarkable, while we can trace a connection between the stream of bright stars in Fig. 2, and the stream

of nebulous light in the background, it is obvious that the two streams are not absolutely coincident in direction. The stream lies (in the figure) above the Milky Way near Scorpio, crosses it in the neighbourhood of Crux, and passes below it along Canis Minor, Orion, and Taurus. Does the stream return to the Milky Way? It seems to me that there is clear evidence of a separation near Aldebaran, one branch curving through Auriga, Perseus, and Cassiopeia, the other proceeding (more nearly in the direction originally observed) through Aries (throwing out an outlier along the band of Pisces), over the square of Pegasus, and along the streams which the ancients compared to water from the urn of Aquarius (but which in our modern maps are divided between Aquarius and Grus). The stream-formation here is very marked, as is evident from the phenomenon having attracted the notice of astronomers so long ago. But modern travels have brought within our ken the continuation of the stream over Toucan, Hydrus, and Reticulum (the two latter names being doubtless suggested by the convolutions of the stream in this neighbourhood). Here the stream seems to end in a sort of double loop, and it is not a little remarkable that the Nubecula Major lies within one loop, the Nubecula Minor within the other. It is also noteworthy that from the foot of Orion there is another remarkable stream of stars, recognised by the ancients under the name of the River Eridanus, which proceeds in a sinuous course towards this same region of the Nubecula.

Having thus met with evidence-striking at least, if not decisive,―of a tendency to aggregation into streams, let us consider if, in other parts of the heavens, similar traces may not be observable. We traced a stream from Scorpio towards Orion, and so round in a spiral to the Nubeculæ. Let us now return to Scorpio, and trace the stream (if any appear) in the contrary direction. Now although over the northern hemisphere starstreams are not nearly so marked as over the southern, yet there appears a decided indication of stream-formation along Serpens and Corona over the group on the left hand of Bootes to the Great Bear. A branch of this stream, starting from Corona, traverses the body of Bootes, Berenice's Hair, the Sickle in Leo, the Beebive in Cancer, passing over Castor and Pollux in Gemini, towards Capella. A branch from the feet of Gemini passes over Canis Minor, along Hydra (so named doubtless from the obvious tendency to stream-formation along the length of this constellation), and so to the right claw of Scorpio. Four small stars of Hydrus are indicated in Fig. 2 between Scorpio and Centaurus near the upper edge of the figure.

One other remarkable congeries of stars is to be mentioned.

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