Galaxy (Gr. , milk; the milky zone), the via lactea, or milky way, an irregular band of light visible in the heavens on a clear night. The following is an abridgment of Sir John Herschel's account of this phenomenon. In the northern heavens the milky way is for the most part faint. From Cepheus over Cassiopeia, Perseus, Auriga, Arc, to Monoceros, it forms a single stream, except where in Perseus it throws out a branch which can be traced as far as e Persei, and probably to the Pleiades and Hyades. Beyond Monoceros, southward, it becomes broader, brighter, and more complicated, opening out in Argo into a fanlike expansion 20° wide. Here the continuity of the stream is interrupted, a broad black rift extending right across it in this part of its course. Beyond the rift is another ianlike expansion, whose widest part, like that of the other, abuts upon the rift. As the milky way narrows down toward the neck of this expansion, it becomes brighter, and its outline is in places singularly well marked. In Crux it expands again, but in the very heart of the expansion is a large black space perfectly clear of lucid stars and of milky light.

This is the Southern Coalsack. Passing on toward Scorpio, we find the milky way dividing close by a Centauri into two branches, of which one only can be traced as a distinct branch for any distance. This stream passes northward over Sagittarius, where it exhibits a remarkably rich condensation, over Aquila, where are several such condensations, and thence, rapidly diminishing in brightness, to Cygnus. The other branch, as soon as it enters Scorpio, exhibits a multitude of complicated divisions, subdivisions, and detached portions. Near Antares it throws out a great projection toward Libra -that is, nearly at right angles to that of the main stream. Another subdivision, passing toward Serpens, seems to seek the main stream, but cannot be traced quite up to it, coming to an end a few degrees to the north of the star u Sagittarii. Returning to the other stream near Cygnus, we find it proceeding to Cassiopeia, throwing out a projection from Cepheus toward the north pole, while from Cygnus a branch extends southward, very rich in Cygnus, but thence rapidly fading in brightness, until it comes to an end on the equator.

In most maps this branch is carried southward beyond the equator to meet the branch which terminates near u Sagittarii; but the two branches do not meet in reality. -The ancients held a variety of opinions concerning the milky way. Aristotle regarded it as constituted of the same substance as comets. Theophrastus looked upon it as the band along which the celestial hemispheres had been knit together, so carelessly that the fiery heavens beyond could be discerned. But Democritus formed the just opinion that the milky way consists of a multitude of stars. It was not until the invention of the telescope that its real nature could be demonstrated. Galileo, even with the small telescopic power at his disposal, was able to resolve the galaxy in many places into discrete stars. The labors of the Herschels, father and son, furnish the means of forming definite ideas respecting its constitution. In the first instance, Sir W. Herschel. regarding the milky way as of the same constitution as the star groups in our neighborhood, applied to it his famous method of star gauging.

Where he counted most stars in the field of view of his telescope, he judged that the extension of the sidereal system was greatest, and thus he was led to the theory which has been called the cloven grindstone theory, according to which the sidereal svs-tern is grreatly extended in the direction of the milky way, and so forms a flat stratum, divided into two laminae opposite the part of the milky way which appears double. Herschel advanced this view in 1785; but the progress of his labors compelled him to abandon the theory that the milky way is constituted like the star regions in our neighborhood. Thus in 1802 he writes: The stars we consider as insulated are also surrounded by a magnificent collection of innumerable stars, called the milky way, which must occasion a very powerful balance of opposite attractions, to hold the intermediate stars in a state of rest. For though our sun, and all the stars we see, may truly be said to be in the plane of the milky way yet I am now convinced, by a long in-spection and continued examination of it, that the milky way itself consists of stars very differently scattered from those immediately around us. On a very slight examination it will appear that this immense starry aggregation is by no means uniform.

The stars of which it is composed are very unequally scattered, and show evident marks of clustering together into many separate allotments." In 1811 he abandoned even more definitely the principle on which his system of star gauging had been based ".By continuing my sweeps of the heavens," he says, my opinion of the arrangement of the stars and their magnitudes, and of some other particulars, has undergone a gradual change. . . . An equal scattering of the stars may be admitted in certain calculations; but when we examine the milky way, or the closely compacted clusters of stars of which my catalogues have recorded so many instances, this supposed equality of scattering must be given up." In 1817 Herschel adopted a new method of estimating the profundity of certain of the richer parts of the milky way.

He regarded the dimensions of the telescope necessary to effect the complete resolution of such regions as affording a measure of the distance to which the milky way extended outward into space. It is not too much to say, however, that this method was as imperfect as that of star gauging, since it involved an assumption equally opposed to existing analogies. In star gauging Herschel assumed that there was a general equality of scattering; he now assumed a general equality of stellar lustre. If we consider his application of this principle to the great cluster in the sword hand of Perseus, we shall see that it was unsound. For from the gauging powers necessary to effect incipient resolution on the one hand and perfect resolution on the other (the latter not attained, but only a lower limit indicated), he inferred that the nearest part of this cluster is at about the 12th order of distance, the furthest certainly beyond the 344th order. But the cluster occupies but a minute space; it is indeed double, and the moon's disk would nearly hide both clusters at once.

Is it credible, then, that we have here to deal with a long conical space having a minute vertical angle, and the sun placed exactly at the vertex, while the remotest portion of the space thus occupied with stars is at least twenty-seven times further away than the nearest ? Such a portion of space would have the shape of a long straight rod very delicate in its proportions. Apart from the antecedent improbability of such an arrangement, it is certain that a cluster of stars so shaped would have no dynamical stability. Moreover, the cluster in Perseus is not a solitary instance, since upward of thirty similar clusterings were counted by Herschel in the northern heavens alone, and Sir John Herschel observed many more in the southern portions of the milky way. These considerations seem to dispose of the principle on which Sir W. Herschel based this his latest method of star gauging. It seems demonstrated by the evidence that the stars seen in the clustering aggregations of the milky way are of many orders of real magnitude, and arranged at distances among which there is not even an approach to general uniformity.

Sir John Herschel's observations of the milky way in the southern heavens go far to confirm these conclusions, though he himself adopted a theory in some sense resembling that which his father advanced in 1785; only that instead of regarding the galaxy as shaped like a cloven disk, he held that it resembles in figure a flat ring (cloven, necessarily, to explain the double portion of the milky way). The elder Struve was among the first to point out that the arrangement of the brighter stars over the heavens does not accord with either the cloven disk or the cloven ring theory of the galaxy. He found that the stars down to the eighth magnitude, which according to either theory should show no marked gathering toward the milky way zone, are nevertheless aggregated in the most striking manner upon that region. Hence Struve inferred that there is an aggregation of stars toward the medial plane of the milky way; and he adopted (quite unnecessarily, as it appears to the present writer) the theory that the range of stars constituting the milky way stratum is illimitable in all directions lying within that stratum.

Struve's theory of an indefinite extension of the milky way in its own plane seems disposed of by the younger Herschel's observation that throughout by far the largest portion of the extent of the milky way in both hemispheres, the general blackness of the ground of the heavens on which its stars are projected, and the absence of that innumerable multitude and excessive crowding of the smallest visible magnitudes, too small to affect the eye singly, which the contrary supposition would appear to necessitate, must, we think, be considered unequivocal indications that its dimensions, in directions where these conditions obtain, are not only not infinite, but that the space-penetrating power of our telescopes suffices fairly to pierce through and beyond it." Moreover, Sir John Herschel disposed very completely of the reasoning on which Struve based the theory that light is gradually extinguished in its passage through space. We are not at liberty," he said, "to argue that at one part of the milky way's circumference our view is limited by this sort of cosmical veil which extinguishes the smaller magnitudes, cuts off the nebulous light of distant masses, and closes our view in impenetrable darkness; while at another we are compelled, by the clearest evidence our telescopes can afford, to believe that star-strewn vistas lie open, exhausting their powers and stretching beyond their utmost reach, as is proved by that very phenomenon which the existence of such a veil would render impossible, viz., infinite increase of number and diminution of magnitude, terminating in complete irresolvable nebulosity." Recent researches have led to the inference that the structure of the galaxy is not so simple as any of the theories advanced by the Herschels or Struve would imply.

The stars, even in one and the same portion of the galaxy, seem to present all those varieties of size and aggregation which have hitherto been ascribed to the effects of distance. It appears that often where the Herschels supposed that they were passing further and further, by means of their powerful telescopes, into the depths of space, they were in reality merely searching more and more scrutinizingly a particular region of our star system. The galaxy, according to these more modern views, would come to be regarded as an infinitely complicated spiral, with outlying branches extending beyond the range of the most powerful telescopes yet made. Moreover, it seems as if those mysterious objects the nebulae, instead of being distant galaxies as had been supposed (at least as respects the stellar nebulae), were in reality but portions of our own sidereal system. It is at least certain that the mysteries of the galaxy have not yet been fully solved, even if any noteworthy advance has been made toward their solution.