Page:Popular Science Monthly Volume 5.djvu/271



t is well occasionally to cast our eyes back over a series of labors, in order that we may rightly judge of our progress.

Rough comparisons alone will not tell us: it is easy to contrast Galileo's piece of lead-tubing, having a lens in each end of it, with the gigantic telescopes of to-day; but we hardly learn much from such a comparison. The extremes of the series are too far removed; to get a just idea of the terms between, the comparisons must be made at shorter intervals.

If we can select some celestial object, which has been telescopically studied for a long period, we may, by contrasting the results obtained at different times, gain some accurate notion of the progress made, and of the way in which it was made.

This latter idea is of some importance when we consider that the day of startling discoveries is over. When Galileo found the four moons of Jupiter, the whole world of men of learning was astounded; and still more wonderful was his discovery of the ring of Saturn—that is to say, of the existence of a ring of Saturn—a portion of which, showing on each side of the planet, gave to Saturn the "tri-corporate" aspect which Galileo describes in a letter to his friend Kepler

Even the discovery of Uranus by Sir William Herschel, in 1789, was by no means an extraordinary event, although it was received with enthusiasm in all Europe, for Herschel's examination of the heavens was done with the aid of an instrument which could not fail to show Uranus with a sensible disk, if the planet should enter the field of his telescope.

The discovery of the first asteroid, Ceres, on the 1st day of January, 1801, by Piazzi, was received with some surprise, but these small bodies have lately become quite common (there being now 135