CHAPTER II.

THE EXTERNAL GALAXIES.-CLASSIFICATION ACCORDING TO THEIR FORMS.-THEIR DISTRIBUTION IN SPACE.

AMONG the regions where now we propose to adven

ture, Twilight reigns. They are the regions in which, faint as shadows, those vast galaxies are strewn of which our own glorious one is an illustration, and whose remoteness has dwarfed them into clouds of an hair's

breadth. The watchword for the

solicitude, as well as reverence.

Wanderer here, is

Let us ascertain

first, by what rights, and with what securities, we may feel ourselves environed amid abysses so dread?

I.

In researches thus evanescent and arduous, our reliance is, of course, wholly on the telescope: wherefore it behoves that, however briefly, we ascertain the source and measure of its power. Fortunately, the principle on which that power depends is most simple; nay, the

44

THE TELESCOPE.

smallest and rudest instrument involves a full expression of it. To produce the sense of seeing, it is necessary that a quantity of light enter the pupil of the eye. Now light itself-overlooking the consideration of its intensity-radiates from every luminous point throughout all space; so that if the simple reception of light were adequate to the production of vision, no material point from which rays stream, could, however small or remote, ever be invisible; but inasmuch as the rays from any object become thinner and fainter as we recede from it, a limit is soon reached, beyond which the quantity reaching the eye is insufficient to enable the object to be perceived. The quantity of light, however, which enters our visual organ, in any given case, depends on the size of the opening that receives the ray―viz., the pupil; so that if, by the processes and ingenuity of Art, we could virtually enlarge that pupil, our sense of sight might be rendered more penetrating precisely to that extent; and fortunately, one of the most widely recognised characteristics of light has enabled us in this way to overpass the boundaries of natural vision, and altogether change our relations with the scheme of external things. In the diagram below, the point S is represented sending