dary are not seen, and there is a gradual fading away at the edge, the last traces of which appear either as a luminous mist, or cloud-like filament, which become finer till they cease altogether to be seen. The Dumb-Bell cluster, in Vulpecula, and the Crab cluster, in Taurus, both of which have been resolved into Stars, are instances of this,

75. In some of these star-clusters the increase of brightness from the edge to the centre is so rapid that it would appear that the stars are actually nearer together at the centre than they are near the edge of the cluster; in fact, that there is a real condensation towards the centre.

LESSON V.-NEBULÆ.

CLASSIFICATION AND

DESCRIPTION.

76. We now come to the Nebula. "Nebula" is a Latin word signifying a cloud, and for this reason the name has been given to everything which appeared cloudlike to the naked eye or in a telescope. The group in Perseus, for instance, appears like a nebula to the naked eye; in the smallest telescope, however, it is separated into stars.

77. Every time a telescope larger than any formerly used has been made use of, however, numbers of what were till then called nebulæ, and about which as nebulæ nothing was known, have been found to be nothing but star-clusters, some of them of very remarkable forms, so distant that even in telescopes of great power they could not be resolved,-that is to say, could not be separated into distinct stars.

78. Now, this is what has happened ever since the discovery of telescopes. Hence it was thought by some that all the so-called nebulæ were, in reality, nothing but distant star-clusters.

79. One of the most important discoveries of modern times, however, has furnished evidence of a fact long ago conjectured by some astronomers,—namely, that some of the nebulæ are something different from masses of stars, and that the cloud-like appearance is due to something else besides their distance and the still comparatively small optical means one can at present bring to bear upon them.

80. This discovery is so recent that there has not yet been time to sort out the real nebulæ from those which, by reason of their great distance, appear like nebulæ. We are compelled, therefore, in this book to accept as nebulæ all formerly classed as such which up to this time have not been resolved into stars.

81. Nebulæ, then, may be divided into the following classes:

1.-Irregular nebulæ.

2.-Ring nebulæ and Elliptical nebulæ.
3.-Spiral, or Whirlpool nebulæ.

4.-Planetary nebulæ.

5.-Nebulæ surrounding stars.

82. Some of the irregular nebula-those in the constellations Orion and Andromeda, for example—are visible to the naked eye on a dark night.

83. The great nebula of Orion is situated in the part of the constellation occupied by the sword-handle and surrounding the multiple-star Theta (0). The nebulosity near the stars is flocculent, and of a greenish white tinge. There seems no doubt that the shape of this nebula and the position of its brightest portions are changing. One part of it appears, in a powerful telescope, startlingly like

D

the head of a fish. On this account it has been termed the Fish-mouth nebula.

84, Two other fine irregular nebulæ are visible in the Southern hemisphere: one is in the constellation Dorado, the other surrounds Eta (7) Argûs. The latter occupies a space equal to about five times the apparent area of the Moon.

85. We have classed the ring-nebulæ and elliptical nebulæ together because probably the latter are, in several instances, ring-nebulæ looked at sideways. The finest 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 entirely 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 John 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.