hydrogen.  This is an extremely light material, as you see by a little balloon which ascends so prettily when filled with it.  Hydrogen also burns very readily, though the flame is almost invisible.  When I blow a jet of oxygen through the hydrogen, I produce a little flame with a very intense heat.  For instance, I hold a steel pen in the flame, and it glows and sputters, and falls down in white-hot drops.  It is needless to say that, as a constituent of water, hydrogen is one of the most important elements on this earth.  It is, therefore, of interest to learn that hydrogen in some form or other is a constituent of the most distant objects in space that the telescope has revealed.

Photographing the Nebulae.

[Illustration:  FIG. 8.  THE PLEIADES.]

Of late years we have learned a great deal about nebulae, by the help which photography has given to us.  Look at this group of stars which constitutes that beautiful little configuration known as the Pleiades (Fig. 8).  It looks like a miniature representation of the Great Bear; in fact, it would be far more appropriate to call the Pleiades the Little Bear than to apply that title to another quite different constellation, as has unfortunately been done.  The Pleiades form a group containing six or seven stars visible to the ordinary eye, though persons endowed with exceptionally good vision can usually see a few more.  In an opera-glass the Pleiades becomes a beautiful spectacle, though in a large telescope the stars appear too far apart to make a really effective cluster.  When Mr. Roberts took a photograph of the Pleiades he placed a highly sensitive plate in his telescope, and on that plate the Pleiades engraved their picture with their own light.  He left the plate exposed for hours, and on developing it not only were the stars seen, but there were also patches of faint light due to the presence of nebulae.  It could not be said that the objects on the plate were fallacious, for another photograph was taken, when the same appearances were reproduced.

When we look at that pretty group of stars which has attracted admiration during all time, we are to think that some of those stars are merely the bright points in a vast nebula, invisible to our unaided eyes or even to our mighty telescopes, though capable of recording its trace on the photographic plate.  Does not this give us a greatly increased notion of the extent of the universe, when we reflect that by photography we are enabled to see much which the mightiest of telescopes had previously failed to disclose?

Of all the nebulae, numbering some thousands, there is but a single one which can be seen without a telescope.  It is in the constellation of Andromeda, and on a clear dark night can just be seen with the unaided eye as a faint stain of light on the sky.  It has happened before now that persons noticing this nebula for the first time have thought they had discovered a comet.  I would like you to try and find out this object for yourselves.