A study of the beautiful mezzotint plates, from the drawings of the Earl of Rosse, contained in Professor Nichol's splendid work, "The Architecture of the Heavens," will clearly disclose the forms, as revealed by a powerful telescope, of many of these gaseous nebulæ. Of such nebulæ, Appleton's Cyclopædia says, "nebulæ proper, or those which have not been definitely resolved, are found in nearly every quarter of the firmament, though abounding especially near those regions which have fewest stars. Scarcely any are found near the Milky Way, and the great mass of them lie in the two opposite spaces farthest removed from this circle. Their forms are very various, and often undergo strange and unexpected changes as the power of the telescope with which they are viewed is increased, so as not to be recognizable in some cases as the same objects." An example of this is shown in Plate X. (Figs. 1 and 2) of Professor Nichol's work, which gives a greatly enlarged view of those shown in Figs. 1 and 2 of Plate IX. (For Fig. 2 of Nichol's Plate X., see illustration of nebula with double sun, in previous chapter.) Professor Nichol says, "In every instance examined, save one, the planetary nebulæ are nebulæ with hollow centers." The inference which this writer makes, that such a planetary nebula consists of "a grand annular cluster of stars," has been since disproved by the discoveries of the spectroscope, but the telescopic form remains true, and still awaits further interpretation. While the irresolvable nebulæ seem to seek some retired spot in space for their processes, like certain animals when incubating, this rule is not universal. Of this, Appleton's Cyclopædia says, "The density of nebular distribution increased with the distance from the galactic zone for the irresolvable nebulæ, but diminished with that distance for the clusters.... There is not a gradual condensation of nebulæ towards two opposite regions, near the poles of the galactic zone, but the nebulæ are gathered into streams, nodules, and irregular aggregations such as we find in the grouping of stars.... Between stars and nebulæ their arrangement follows the law of contrast. There are two remarkable exceptions to this law,--the Magellanic Clouds. In these, where stars of all orders, from the ninth magnitude to irresolvable stellar aggregations, are as richly gathered as in the galactic zone, nebulæ of all orders are also gathered richly, even more so than anywhere else over the whole heavens." In the same work, article "Nebula," it is stated of the planetary nebulæ, "There are several which have perfectly the appearance of a ring, and are called annular nebulæ.... Some appear to be physically connected in pairs like double stars. Most of the small nebulæ have the general appearance of a bright central nucleus enveloped in a nebulous veil. This nucleus is sometimes concentrated as a star and sometimes diffused. The enveloping veil is sometimes circular and sometimes