ring nebula is the 57th in Messier's catalogue (written 57 M. for short). It is in the constellation Lyra. The finest elliptical nebula is the one in Andromeda to which we have before referred. This nebula, the 31st of Messier's catalogue (31 M.) when viewed in large instruments, shows several curious black streaks running in the direction in which the nebula is longest.

86. The spiral or whirlpool nebulæ are represented by that in the constellation of Canes Venatici (51 M.). In an ordinary telescope this presents the appearance of two globular clusters, one of them surrounded by a ring at a considerable distance, the ring varying in brightness, and being divided into two in a part of its length. But in a larger instrument the appearance is cntirely changed. The ring turns into a spiral coil of nebulous matter, and the outlying mass is seen connected with the main mass by a curved band. 33 M. Piscium, and 99 M. Virginis, are other examples of this strange phenomenon, which indicate to us the action of stupendous forces of a kind unknown in our own universe.

87. The fourth class, or planetary nebulæ, were so named by Sir William Herschel, as they shine with a planetary and often bluish light, and are circular or slightly elliptical in form. 97 M. Ursa Majoris and 46 M. Argûs may be taken as specimens.

88. We come lastly to the nebulæ surrounding stars, or nebulous stars. The stars thus surrounded are apparently like all other stars, save in the fact of the presence of the appendage; nor does the nebulosity give any signs of being resolvable with our present telescopes. Iota (1) Orionis, Epsilon (e) Orionis, 8 Canum Venaticorum, and 79 M. Ursa Majoris, belong to this class.

LESSON VI.-Nebula (continued). Their Faintness. Variable Nebula. Distribution in Space. Their Structure. Nebular Hypothesis.

89. Having stated and described the several classes into which nebulæ may be divided, their general features and structure have next to be considered.

90. Like the stars, they are of different brightnesses, but as yet they have not been divided into magnitudes. This, however, has been done in a manner by determining what is termed the space-penetrating power or light-grasping power of the telescope powerful enough to render them visible. Thus supposing nebulæ to consist of masses of stars, it has been estimated that Lord Rosse's great Reflector, the most powerful instrument as yet used in such inquiries, penetrates 500 times further into space than the naked eye can; that is, can detect a nebula or cluster 500 times further off than a star of the sixth magnitude.

91. Now, if we suppose that a sixth magnitude star is 12 times further off than a star of the first magnitudeand this is within the mark-and that, as we have seen in Art. 27, light requires 120 years to reach us from such a star, the telescope we have referred to penetrates so profoundly into space that no star can escape its scrutiny "unless at a remoteness that would occupy light in over spanning it sixty thousand years."

92. An idea of the extreme faintness of the more distant nebulæ may be gathered from the fact, that the light of some of those visible in a moderately-large instrument has been estimated to vary from 100 to 2000 of the light of a single sperm candle consuming 158 grains of material per hour, viewed at the distance of a quarter of a mile; that is, such a candle a quarter of

a mile off is 20,000 times more brilliant than the nebula!1

93. The phenomenon of variable, lost, new, and temporary stars has its equivalent in the case of the nebulæ, the light of which, it has been lately discovered, is in some cases subject to great variations.

94. In 1861 it was found that a small nebula, discovered in 1852 in Taurus, near a star of the tenth magnitude, had disappeared, the star also becoming dimmer. In the next year the nebula increased in brightness again; but was completely invisible 1877-1880. There is, besides, strong evidence of the periodical variability of two nebula-one situated in Cetus, the other in Virgo.

95. In Art. 30 the marked character of the distribution of the stars of our universe, giving rise to the appearance of the Milky Way, was pointed out. The distribution of the nebulæ, however, is very different; in general they lie out of the Milky Way, so that they are either less condensed there, or the visible universe (as distinguished from our own stellar one) is less extended in that direction. They are most nunerous in a zone which crosses the Milky Way at right angles, the constellation Virgo being so rich in them that a portion of it is termed the nebulous region of Virgo. In fact, not only is the Milky Way the poorest in nebulæ, but the parts of the heavens furthest away from it are richest.

The recent application of photography to celestial physics has shown that nebulæ are far more widely distributed than was formerly supposed. Its power of integrating the successive effects of very feeble luminosities is not possessed by the eye, and consequently many nebulæ which will never be visible to the eye of man have been successfully photographed.

96. We now come to the question, What is a Nebula?

1 Huggins.

The answer is-A true nebula consists of a sparse swarm of meteorites, the luminosity of which is due to the heat produced by collisions. The interspaces are partly filled with hydrogen and magnesium and other vapours, which are volatilised out of the meteorites. Amongst true nebulæ, are the great nebula in Orion, that surrounding Eta (ʼn) Argus; the ring nebula in Lyra, and all planetary nebulæ.

97. When, therefore, we see, in what we know to be a true nebula, closely associated points of light, we must not regard the appearance as an indication of resolvability into true stars. These luminous points, in some nebulæ at least, must be looked upon as probably denser aggregations of the meteorites composing the nebulæ.

98. The nebular hypothesis supposed that all the countless bodies which are distributed through space once existed in the condition of gaseous matter, but recent researches have shown that all of them have their origin in meteorites. According to this view, a nebula is to be regarded as a future star, for, by the gradual contraction of the mass which will be produced by gravitation, the collisions between the meteorites will become more numerous and violent, and the nebula will get hotter and brighter, first forming a star of Group II., then of Group III., and, if hot enough, of Group IV. During the subsequent cooling, it will become a star of Group V., then of Group VI., and finally it will become a cold body like a planet. (See Arts. 65 and 504a.) It may take long years to prove, or disprove, this hypothesis; but the tendency of recent observations assuredly is to show its correct

ness.