If you look at it with an opera-glass it appears to be distinctly elongated.  You can see more of its structure when you view it in larger instruments, but its nature was never made clear until some beautiful photographs were taken by Mr. Roberts (Fig. 9).  Unfortunately, the nebula in Andromeda has not been placed in the best position for its portrait from our point of view.  It seems as if it were a rather flat-shaped object, turned nearly edgewise towards us.  To look at the pattern on a plate, you would naturally hold the plate so as to be able to look at it squarely.  The pattern would not be seen well if the plate were so tilted that its edge was turned towards you.  That seems to be nearly the way in which we are forced to view the nebula in Andromeda.  We can trace in the photograph some divisions extending entirely round the nebula, showing that it seems to be formed of a series of rings; and there are some outlying portions which form part of the same system.  Truly this is a marvellous object.  It is impossible for us to form any conception of the true dimensions of this gigantic nebula; it is so far off that we have never yet been able to determine its distance.  Indeed, I may take this opportunity of remarking that no astronomer has yet succeeded in ascertaining the distance of any nebula.  Everything, however, points to the conclusion that they are at least as far as the stars.

[Illustration:  FIG. 9.  THE GREAT NEBULA IN ANDROMEDA.]

It is almost impossible to apply the methods which we use in finding the distance of a star to the discovery of the distance of the nebulae.  These flimsy bodies are usually too ill-defined to admit of being measured with the precision and delicacy required for the determination of distance.  The measurements necessary for this purpose can only be made from one star-like point to another similar point.  If we could choose a star in the nebula and determine its distance, then of course, we have the distance of the nebula itself; but the difficulty is that we have, in general, no means of knowing whether the star does actually lie in the object.  It may, for anything we can tell, lie billions of miles nearer to us, or billions of miles further off, and by merely happening to lie in the line of sight, appear to glimmer in the nebula itself.

If we have any assurance that the star is surrounded by a mass of this glowing vapor, then it may be possible to measure that nebula’s distance.  It will occasionally happen that grounds can be found for believing that a star which appears to be in the glowing gas does veritably lie therein, and is not merely seen in the same direction.  There are hundreds of stars visible in a good drawing or a good photograph of the famous object in Andromeda, and doubtless large numbers of these are merely stars which happen to lie in the same line of sight.  The peculiar circumstances attending the history of one star seem, however, to warrant us in making the assumption that it was certainly