dark background. The Milky Way appears as a zone of faint light passing round the heavens nearly in a great circle. But besides this great zone, there are isolated spots, distinguishable some of them by the naked eye, which appear like very small luminous clouds. Now these isolated cloudy objects are denominated nebulæ, and the question arises, What is their constitution? Are they composed, as they appear to be, of nebulous matter like comets? The telescope, when turned to the Milky Way, which presents a cloud-like appearance, at once resolves it into distinct stars. The unaided eye is not sensitive enough to separate the individual stars; so they appear to blend in one uniform surface of light. But the telescope enables the eye to effect the separation, and to reveal its true character. Are we to conclude, that the nebulse are of the same character, and that they are congeries of stars, so closely set, that they present a cloudy appearance without distinct isolation? Thus, among the innumerable nebulas which the telescope reveals, there are some which are at once resolved into stars by moderate powers. Nebulae, according to their resolvability, may be divided into three grades. The first consists of those in which the stars are distinctly separated. The second consists of those presenting a granular appearance, which merely indicates the resolvability under a sufficient power. But besides these, there is a third class in which no indication of resolvability could for a long
