Page:The American Cyclopædia (1879) Volume VII.djvu/571

GALAXY 559 black rift extending right across it in this part of its course. Beyond the rift is another fan- like 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 perfect- ly 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 dimin- ishing in brightness, to Cygnus. The other 'branch, as soon as it enters Scorpio, exhibits a multitude of complicated divisions, subdi- visions, and detached portions. Near Antares it throws put a great projection toward Libra that is, nearly at right angles to that of the main stream. Another subdivision, pass- ing 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. Eeturning to the oth- er 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 // Sagittarii; but the two branches do not meet in reality. The ancients held a variety of opinions con- cerning 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. Her- schel, regarding the milky way as of the same constitution as the star groups in our neigh- borhood, 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 sys- tem is greatly extended in the direction of the 343 VOL. vii. 36 milky way, and so forms a flat stratum, di- vided 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 magnifi- cent collection of innumerable stars, called the milky way, which must occasion a very pow- erful 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- rtion and continued examination of it, that 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 aggrega- tion is by no means uniform. The stars of which it is composed are very unequally scat- tered, 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 scatter- ing 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 dis- tance to which the milky way extended out- ward 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 analo- gies. 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 ne- cessary 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