nebula of Orion is the so-called Trapezium, already described. That these four stars form a system by themselves cannot be doubted. The darkness of the nebula immediately around them suggests that they were formed at the expense of the nebulous mass.

Great interest has recently been excited in the spiral form of certain nebulæ. The great spiral nebula M. 51 in Canes Venatici has long been known. We reproduce a photograph of this object and another. It is found by recent studies at the Lick Observatory that a spiral form can be detected in a great number of these objects by careful examination.

Another striking feature of numerous nebulæ is their varied and fantastic forms, of which we give a number of examples. The "Triphid nebula," figured in our frontispiece, is a noted one in this respect.

The great nebula of Andromeda is second only to that of Orion. It also is plainly visible to the naked eye and can be more readily recognised as a nebula

than can the other. It has frequently been mistaken for a comet. Seen through a telescope of high power, its aspect is singular, as if a concealed light were seen shining through horn or semi-transparent glass.

Another nebula which, though not conspicuous to the naked eye, has attracted much attention from astronomers, is known, from the figure of one of its branches, as the Omega nebula. Sir John Herschel, who first described this object in detail, says of it: "The figure is nearly that of the Greek capital Omega, somewhat distorted and very unequally bright." From one base of the letter extends out to the east a long branch with a hook at the end, which in most of the drawings is more conspicuous than the portion included in the Omega. The drawings, however, vary so much that the question has been raised. whether changes have not taken place in the object. As in other cases, this question is one which it is not yet possible to decide. The appearance of such objects varies so much with the aperture of the telescope and the conditions of vision that it is not easy to decide whether the apparent change may not be due to these causes. It is curious that in a recent photograph, the Omega element of it, if I may use the term, is far less conspicuous than in the older drawings, and is, in fact, scarcely recognisable.

Among the most curious of the nebulæ are the annular ones, which, as the term implies, have the form of a ring. It should be remarked that in such cases the interior of the ring is not generally entirely black, but is filled with nebulous light. We may,

therefore, define these objects as nebula which are brighter round their circumference than in the centre. The most striking of the annular nebula is that of Lyra. It may easily be found from being situated about half-way between the stars Beta and Gamma. Although it is visible in a medium telescope, it requires a powerful one to bring out its peculiar features in a striking way. Recently it has been photographed by Keeler with the Crossley reflector of the Lick Observatory, who found that the best general impression was made with an exposure of only ten

minutes.

The ring, as shown by Keeler's photograph, has a quite complicated structure. It seems to be made up of several narrower bright rings, interlacing somewhat irregularly, the spaces between them being filled with fainter nebulosity. One of these rings forms the outer boundary of the preceding end of the main ring. Sweeping around to the north end of the minor axis, it becomes very bright, perhaps by superposition on the broader main ring of the nebula at this place. It crosses this ring obliquely, forming the brightest part of the whole nebula, and then forms the inner boundary of the main ellipse toward its following end. The remaining part of the ring is not so easily traced, as several other rings interlace on the south end of the ellipse.

The central star of this nebula has excited some interest. Its light seems to have a special actinic power, as the star is more conspicuous on the photographs than to the eye.

There are several other annular nebula which are fainter than that of Lyra. The one best visible in our latitudes is known as H IV. 13, or 4565 of Dreyer's catalogue. It is situated in the constellation Cygnus, which adjoins Lyra. Both Herschel and Lord Rosse have made drawings of it. It was photographed by Keeler with the Crossley reflector on the nights of August 9 and 10, 1899, with exposures of one and two hours, respectively. Keeler states that the nebula, as shown by these photographs, " is an elliptical, nearly circular ring, not quite regular in outline, pretty sharply defined at the outer edge." The outside dimensions are:

Major axis..
Minor axis....

Position angle of major axis

42"-5

40."5

32°.

The nebula has a nucleus with a star exactly in the center. This is very conspicuous on a photograph, but barely if at all visible with a 36-inch reflector.

Another curious class of nebula are designated as planetary, on account of their form. These consist of minute, round disks of light, having somewhat the appearance of a planet. The appellation was suggested by this appearance. These objects are for the most part faint and difficult.

It is impossible to estimate the number of nebulæ in the heavens. New ones have from time to time. been discovered, located, and described by many observers during the last thirty years. Among these