Its extent in the parallel is nearly one degree and half, and in the meridional direction about fifty-two minutes." It appears that this diffused nebulosity is very extensive; for, of fifty-two nebula of this description which had never been before observed, Herschel found them to occupy no less than 152 square degrees. A specimen of an extensive diffusive nebula of this description is represented in fig. 59.

Sir W. Herschel has presented us with fourteen specimens related to this class, of what he terms nebulosities joined to nebula, one of which is represented in fig. 60, where a bright nebulous speck is connected with a faint nebulosity, which seems to proceed from it as from a central point, increasing in breadth, in proportion to the distance, till it terminates in a kind of irregular margin. Fig. 61 represents what is called a milky nebula with condensation. It appears to be a roundish nebula, condensed towards the central parts. It is natural to suppose, when we see a gradual increase of light, that there is a condensation of the substance which produces it in the space which appears brightest, or, at least, that the luminous substance is deeper in the brighter space. Some of the nebulosities of this class are not always extensively diffused, but are sometimes met with in detached collections, near to each other, but completely separate, as represented at a, b, c, fig. 62.

A diffused nebulosity of this kind may be seen about six or seven degrees due east from the star Zeta Cygni, near the back or tail of Anser. Its right ascension is 20h 38', and north declination 30° 6'. Another, whose right ascension is 20h 49', and north declination 31° 3', is found about three or four degrees northwest of Zeta Cygni, and within two or three degrees of the preceding.

Figures 63, 64, and 65 are representations of nebula which are brighter in more than one place, which appearance is supposed to be owing to so many predominant seats of attraction, owing to a superior preponderance of the nebulous matter in those places causing a division of it,, from which will arise three or four distinct nebulæ.

Figs. 66 to 71 are representations of nebulæ of various descriptions. The three upper figures, numbered 66, are. nebulæ that are suddenly much brighter in the middle. A nucleus to which these nebulæ seem to approach is considered as indicating consolidation; and that, should we have reason to conclude that a solid body can be formed of condensed nebulous

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matter, the nature of which has been chiefly deduced from its shining quality, we may possibly be able to view it with respect to some other of its properties. The three figures, No. 67, represent extended nebula and round nebula, that show the progress of condensation. These nebulæ appear farther condensed than the preceding, and appear surrounded with the rarest nebulous matter, which, not having as yet been consolidated with the rest, remains expanded about the nucleus in the shape of a very extended atmosphere. The three figures in the third row from the top of the plate, marked No. 68, and the first figure to the left hand of No. 69, represent nebula which are almost of a uniform light, and nebula that draw progressively towards a period of final condensation. "In the course of the gradual condensation of the nebulous matter," says Sir W. Herschel, "it may be expected that a time must come when can no longer be compressed, and the only cause which we may suppose to put an end to the compression is, when the consolidated mass assumes hardness. From the size of the nebulæ, as we see them at present, we cannot form an idea of the original bulk of the nebulous matter they contain; but let us admit, for the sake of computation, that the nebulosity of a certain nebula, when it was in a state of diffusion, took up a space of ten minutes in every cubical direction of its expansion, then, as we now see it collected into a globular compass of less than one minute, it must of course be more than 1900 times denser than it was in its original state. This proportion of density is more than double that of water to air."

The small nebulæ represented in No. 70 are stellar nebulæ, which approach to the appearance of stars, and one or two of doubtful character. The four figures marked No. 71 represent separate views of the gradual condensation of the nebulous substance. In these we may evidently perceive a striking gradation in the light and brilliancy of the central parts. The figure on the left-hand side represents an object nearly in its original state of nebulosity; the next towards the right appears considerably condensed towards the central parts; the third figure represents a condensation still greater; and the one on the right hand exhibits a condensation nearly complete, or a huge luminous body surrounded with a lucid atmosphere. Each of these is the representative of an extensive class of objects of this description.

SECTION III.-On Planetary Nebula.

This designation is given to a class of nebula which bear a very near resemblance to planetary disks when seen through telescopes. But, notwithstanding their planetary aspect, some small remaining haziness, by which they are more or less surrounded, evinces their nebulous origin. They are somewhat extraordinary objects, with round or slightly oval disks, in some instances quite sharply terminated, in others a little hazy at the borders, and of a light exactly equable, or only a very little mottled, which in some of them approaches in vividness to the light of actual planets. The following are some of Sir W. Herschel's remarks on these bodies: If we should suppose them to be single stars with large diameters, we shall find it difficult to account for their not being brighter, unless we should admit that the intrinsic light of some stars may be very much inferior to that of the generality, which, however, can hardly be imagined to extend to such a degree. We might suppose them to be comets about their aphelion, if the brightness as well as magnitude of their diameters did not oppose this idea; so that, after all, we can hardly find any hypothesis so probable as that of their being nebula; but then they must consist of stars that are compressed and accumulated in the highest degree. At a subsequent period, Sir William remarks, "When we reflect on the circumstances connected with these bodies, we may conceive that, perhaps, in progress of time, these nebulæ, which are already in a state of compression, may be still farther compressed, so as actually to become stars. It may be supposed that solid bodies, such as we suppose the stars to be, from the analogy of their light with that of our sun when seen at the distance of the stars, can hardly be formed of a condensation of nebulous matter; but if the immensity of it required to fill a cubical space which will measure ten minutes, when seen at the distance of a star of the eighth or ninth magnitude, is well considered and properly compared with the very small angle our sun would subtend at the same distance, no degree of rarity of the nebulous matter to which we have recourse can be any objection to the solidity required for the construction of a body of equal magnitude with our sun."

The nature of these nebulæ is involved in considerable dark