We are talking here quite freely about nebulae because we have met one long ago when we discussed the gradual evolution of our own system, and we know quite well that a nebula is composed of luminous faintly-glowing gas of extreme fineness and thinness. We see in the sky at the present time what we may take to be object-lessons in our own history, for we see nebulae of all sorts and sizes, and in some stars are mixed up, and in others stars are but dimly seen, so that it does not require a great stretch of the imagination to picture these stars as being born, emerging from the swaddling bands of filmy webs that have enwrapped them; and other nebulae seem to be gas only, thin and glowing, with no stars at all to be found in it. We still know very little about these mysterious appearances, but the work of classifying and resolving them is going on apace. Nebulae are divided into several classes, but the easiest distinction to remember is that between white nebulae and green nebulae. This is not to say that we can see some coloured green, but that green appears in the spectrum of some of the nebulae, while the spectrum of a white nebula is more like that of a star. It is fortunate for us that in the sky we can see without a telescope one instance of each of the several objects of interest that we have referred to. We have been able to see one very vivid example of a variable star; we have seen one very beautiful example of a star cluster; and it remains to look for one very good example of a white nebula. Just as in finding Algol you were doing a little bit of practical work, proving something of which you had read, so by seeing this nebula you will remember more about nebulae in general than by reading many chapters on the subject. This particular nebula is in Andromeda, and is not far from Algol; and it is not difficult to find. It is the only one that can be well seen without a telescope, and was known to the ancients; it is believed to have been mentioned in a book of the tenth century! If you take an imaginary line down from the two left-hand stars of Cassiopeia, and follow it carefully, you will come before long to a rather faint star, and close to it is the nebula. When you catch sight of it you will, perhaps, at first be disappointed, for all you will see is a soft blur of white, as if someone had laid a dab of luminous paint on the sky with a finger; but as you gaze at it night after night and realize its unchangeableness, realize also that it is a mass of glowing gas, an island in space, infinitely distant, unsupported and inexplicable, something of the wonder of it will creep over you. [Illustration: _Dr. Max Wolf._ THE GREAT NEBULA IN ANDROMEDA.]