The World's Greatest Books [Volumes 1-15, 17-20]

I.—The Wonders of the Milky Way

There is no science which draws more largely than does astronomy on that intellectual liberality which is ready to adopt whatever is demonstrated or concede whatever is rendered highly probable, however new and uncommon the points of view may be in which objects the most familiar may thereby become placed. Almost all its conclusions stand in open and striking contradiction with those of superficial and vulgar observation, and with what appears to everyone the most positive evidence of his senses.

There is hardly anything which sets in a stronger light the inherent power of truth over the mind of man, when opposed by no motives of interest or passion, than the perfect readiness with which all its conclusions are assented to as soon as their evidence is clearly apprehended, and the tenacious hold they acquire over our belief when once admitted.

If the comparison of the apparent magnitude of the stars with their number leads to no immediately obvious conclusion, it is otherwise when we view them in connection with their local distribution over the heavens. If indeed we confine ourselves to the three or four brightest classes, we shall find them distributed with a considerable approach to impartiality over the sphere; a marked preference, however, being observable, especially in the southern hemisphere, to a zone or belt passing through epsilon Orionis and alpha Crucis. But if we take in the whole amount visible to the naked eye we shall perceive a great increase of numbers as we approach the borders of the Milky Way. And when we come to telescopic magnitudes we find them crowded beyond imagination along the extent of that circle and of the branches which it sends off from it; so that, in fact, its whole light is composed of nothing but stars of every magnitude from such as are visible to the naked eye down to the smallest points of light perceptible with the best telescopes.

These phenomena agree with the supposition that the stars of our firmament, instead of being scattered indifferently in all directions through space, form a stratum of which the thickness is small in comparison with its length and breadth; and in which the earth occupies a place somewhere about the middle of its thickness and near the point where it subdivides into two principal laminæ inclined at a small angle to each other. For it is certain that to an eye so situated the apparent density of the stars, supposing them pretty equally scattered through the space they occupy, would be least in the direction of the visual ray perpendicular to the lamina, and greatest in that of its breadth; increasing rapidly in passing from one to the other direction, just as we see a slight haze in the atmosphere thickening into a decided fog-bank near the horizon by the rapid increase of the mere length of the visual ray.

Such is the view of the construction of the starry firmament taken by Sir William Herschel, whose powerful telescopes first effected a complete analysis of this wonderful zone, and demonstrated the fact of its entirely consisting of stars.

So crowded are they in some parts of it that by counting the stars in a single field of his telescope he was led to conclude that 50,000 had passed under his review in a zone two degrees in breadth during a single hour's observation. The immense distances at which the remoter regions must be situated will sufficiently account for the vast predominance of small magnitudes which are observed in it.

The process of gauging the heavens was devised by Sir William Herschel for this purpose. It consisted simply in counting the stars of all magnitudes which occur in single fields of view, of fifteen minutes in diameter, visible through a reflecting telescope of 18 inches aperture, and 20 feet focal length, with a magnifying power of 180 degrees, the points of observation being very numerous and taken indiscriminately in every part of the surface of the sphere visible in our latitudes.