Although lacking the optical resources which now enable us to recognize the structure of the Milky-way, some of the ancient philosophers had succeeded tolerably well in their speculations regarding its nature. It was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy was nothing else but a vast and confused assemblage of stars, whose faint light was the true cause of its milky appearance.
Before the invention of the telescope, no well-founded theory in regard to the structure of the Milky-way could, of course, be attempted. Although Kepler entertained different ideas in regard to the structure of this great belt from those now generally admitted, yet in them may be found the starting point of the modern conception of the structure of the Galaxy and of the visible universe. In the view of this great mind, the Milky-way, with all its stars, formed a vast system, the centre of which, and of the universe, was occupied by our Sun. Kepler reasoned that the place of the Sun must be near the centre of the galactic belt, from the fact this last object appears very nearly as a great circle of the celestial sphere, and that its luminous intensity is about the same in all its parts.
Half a century later, another attempt to explain the Milky-way was made by Wright, of Durham, who rejected the idea of an accidental and confused distribution of the stars as inconsistent with the appearance of the Galaxy, and regarded them as arranged along a fundamental plane corresponding to that of the Milky-way. These ideas which were subsequently developed and enlarged by Kant, and then by Lambert, constitute what is now known as Kant's theory. According to this theory, the stars composing the Galaxy are conceived as being uniformly arranged between two flat planes of considerable extension, but which are comparatively near together, the Sun occupying a place not very far from the centre of this immense starry stratum. As we view this system crosswise through its thinnest parts, the stars composing it appear scattered and comparatively few in number, but when we view it lengthwiser through its most extended parts, they appear condensed and extremely numerous, thus giving the impression of a luminous belt encircling the heavens. In the conception of Kant, each star was a sun, forming the centre of a planetary system. These systems are not independent, but are kept together by the bonds of universal gravitation. The Galaxy itself is one of these great systems, its principal plane being the equivalent of the zodiac in our planetary system, while a preponderant body, which might be Sirius, is the equivalent of our Sun, and keeps the galactic system together. In the universe there are other galaxies, but as they are too distant to be resolved into stars, they appear as elliptical nebulæ. Such are, in brief, the grand speculations of Kant and Lambert on the Milky-way, and the structure of the universe.
Kant's theory rested more on conjectures than on observed facts, and needed therefore the sanction of direct observations to be established on a firm basis. With this view, Sir William Herschel investigated the subject, by a long and laborious series of observations. His plan, which was that of "star-gauging," consisted in counting all the stars visible in his twenty-foot telescope, comprised in a wide belt cutting the Galaxy at right angles, and extending from one of its sides to the opposite one, thus embracing 180° of the celestial sphere. In this belt he executed 3,400 telescopic star-gaugings of a quarter of a degree each, from which he obtained 683 mean gaugings giving the stellar density of the corresponding regions.
The general result derived from this immense labor was that the stars are fewest in regions the most distant from the galactic belt; while from these regions, which correspond to the pole of the Galaxy, they gradually increase in number in approaching the Milky-way. The star density was found to be extremely variable, and while some of the telescopic gaugings detected either no star at all, or only one or two, other gaugings gave 500 stars and even more. The average number of stars in a field of view of his telescope, obtained for the six zones, each of 15°, into which Herschel divided up the portion of his observing belt, extending from the Galaxy to its pole, is as follows: In the first zone, commencing at 90° from the galactic belt and extending towards it, 4 stars per telescopic field were found; 5 in the second; 8 in the third; 14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated in the Galaxy itself. Very nearly similar results were afterwards found by Sir John Herschel, for corresponding regions in the southern hemisphere.
From these studies, Herschel concluded that the stellar system is of the general form supposed by the Kantian theory, and that its diameter must be five times as extended in the direction of the galactic plane, as it is in a direction perpendicular to it. To explain the great branch sent out by the Galaxy in Cygnus, he supposed a great cleft dividing the system edgewise, about half way from its circumference to its centre. From suppositions founded on the apparent magnitude and arrangement of stars, he estimated that it would take light about 7,000 years to reach us from the extremities of the Galaxy, and therefore 14,000 years to travel across the system, from one border to the opposite one.
But Herschel's theory concerning the Milky-way rested on the erroneous assumption that the stars are uniformly distributed in space, and also that his telescopes penetrated through the entire depth of the Galaxy. Further study showed him that his telescope of twenty feet, and even his great forty-foot telescope, which was estimated to penetrate to a distance 2,300 times that of stars of the first magnitude, failed to resolve some parts of the Galaxy into stars. Meanwhile, the structure of the Milky-way being better known, the irregular condensation of its stars became apparent, while the mutual relation existing between binary and multiple systems of stars, as also between the stars which form clusters, was recognized, as showing evidence of closer association between certain groups of stars than between the stars in general. Herschel's system, which rested on the assumption of the uniform distribution of the stars in space, and on the supposition that the telescopes used for his gauges penetrated through the greater depths of the Galaxy, being thus found to contradict the facts, was gradually abandoned by its author, who adopted another method of estimating the relative distances of the stars observed in his gaugings.
This method, founded on photometric principles, consisted in judging the penetrating power of his telescope by the brightness of the stars, and not, as formerly, by the number which they brought into view. He then studied by this new method the structure of the Milky-way and the probable distance of the clustering masses of which it is formed, concluding that the portion of the Galaxy traversing the constellation Orion is the nearest to us. This last result seems indicated by the fact that this portion of the Milky-way is the faintest and the most uniform of all the galactic belt.
More recently Otto Struve investigated the same subject, and arrived at very nearly similar conclusions, which may be briefly stated as follows: The galactic system is composed of a countless number of stars, spreading out on all sides along a very extended plane. These stars, which are very unevenly distributed, show a decided tendency to cluster together into individual groups of different sizes and forms, separated by comparatively vacant spaces. This layer where the stars congregate in such vast numbers may be conceived as a very irregular flat disk, sending many branches in various directions, and having a diameter eight or ten times its thickness. The size of this starry disk cannot be determined, since it is unfathomable in some directions, even when examined with the largest telescopes. The Sun, with its attending planets, is involved in this immense congregation of suns, of which it forms but a small particle, occupying a position at some distance from the principal plane of the Galaxy. According to Struve, this distance is approximately equal to 208,000 times the radius of the Earth's orbit. The Milky-way is mainly composed of star-clusters, two-thirds, perhaps, of the whole number visible in the heavens being involved in this great belt. In conclusion, our Sun is only one of the individual stars which constitute the galactic system, and each of these stars itself is a sun similar to our Sun. These individual suns are not independent, but are associated in groups varying in number from a few to several thousands, the Galaxy itself being nothing but an immense aggregation of such clusters, whose whole number of individual suns probably ranges between thirty and fifty millions. In this vast system our globe is so insignificant that it cannot even be regarded as one of its members. According to Dr. Gould, there are reasons to believe that our Sun is a member of a small, flattened, bifid cluster, composed of more than 400 stars, ranging between the first and seventh magnitude, its position in this small system being eccentric, but not very far from the galactic plane.
The study of the Milky-way, of which Plate XIII. is only a part, was undertaken to answer a friendly appeal made by Mr. A. Marth, in the Monthly Notices of the Royal Astronomical Society, in 1872. I take pleasure in offering him my thanks for the suggestion, and for the facility afforded me in this study by his "List of Co-ordinates of Stars within and near the Milky-way," which was published with it.