Judging by their loose and irregular distribution, the easily resolvable clusters would appear, in general, to be the nearer to us. It is probable that the globular clusters do not possess, to a very great degree, the regular form which they ordinarily present to us. It seems rather more natural to infer that they are irregular, and composed of many wings and branches, such as are observed in the cluster in Hercules; but as these appendages would necessarily be much poorer in stars than the central portions, they would be likely to become invisible at a great distance, and therefore the object would appear more or less globular; the globular form being simply given by the close grouping of the stars in the central portion. It would seem, then, that in general, the most loosely scattered and irregular clusters are the nearest to us, while the smallest globular clusters and those resolvable with most difficulty are the most distant. In accordance with the theory that the clusters are composed of stars, the spectrum of these objects is in general continuous; although, in many cases, the red end of the spectrum is either very faint or altogether wanting. Many objects presenting in a very high degree the principal characteristics exhibited by the true star-clusters, namely, a circular or oval mass, whose luminous intensity is greatly condensed toward the centre, have not yielded, however, to the resolving power of the largest telescopes, although their continuous spectrum is in close agreement with their general resemblance to the star-clusters. Although such objects may remain irresolvable forever, yet it is highly probable that they do not materially differ from the resolvable and partly resolvable clusters, except by their enormous distance from us, which probably reaches the extreme boundary of our visible universe. THE NEBULÆ PLATE XV Besides the foggy, luminous patches which have just been described, a few hazy spots of a different kind are also visible to the naked eye on any clear, moonless night. These objects mainly differ from the former in this particular, that when viewed through the largest telescopes in existence they are not resolved into stars, but still retain the same cloudy appearance which they present to the unassisted eye. On account of the misty and vaporous appearance which they exhibit, these objects have been called _Nebulæ_. Of the 26 nebulous objects visible to the naked eye in the whole heavens, 19 belong to the class of star-clusters, and 7 to the class of nebulæ. Among the most conspicuous nebulæ visible to the unassisted eye, are those in the constellations Argo Navis, Andromeda and Orion.