s a
great difficulty in making pictorial representations of such nebulae.
Most of them are very faint--so faint, indeed, that they can only be
seen with close attention even in powerful instruments. In making
drawings of these objects, therefore, it is impossible to avoid
intensifying the fainter features if an intelligible picture is to be
made. With this caution, however, we present Plate XVI., which exhibits
several of the more remarkable nebulae as seen through Lord Rosse's great
telescope.

[Illustration: Fig. 99.--The Nebula N.G.C., 1,499.

(_By E.E. Barnard, Lick Observatory, September 21, 1895._)]

The actual nature of the nebulae offers a problem of the greatest
interest, which naturally occupied the mind of the first assiduous
observer of nebulae, William Herschel, for many years. At first he
assumed all nebulae to be nothing but dense aggregations of stars--a very
natural conclusion for one who had so greatly advanced the optical power
of telescopes, and was accustomed to see many objects which in a small
telescope looked nebulous become "resolved" into stars when scrutinised
with a telescope of large aperture. But in 1864, when Sir William
Huggins first directed a telescope armed with a spectroscope to one of
the planetary nebulae, it became evident that at least some nebulae were
really clouds of fiery mist and not star clusters.

We shall in our next chapter deal with the spectra of the fixed stars,
but we may here in anticipation remark that these spectra are
continuous, generally showing the whole length of spectrum, from red to
violet, as in the sun's spectrum, though with many and important
differences as to the presence of dark and bright lines. A star cluster
must, of course, give a similar spectrum, resulting from the
superposition of the spectra of the single stars in the cluster. Many
nebulae give a spectrum of this kind; for instance, the great nebula in
Andromeda. But it does not by any means follow from this that these
objects are only clusters of ordinary stars, as a continuous spectrum
may be produced not only by matter in the liquid or solid state, or by
gases at high pressure, but also by gases at lower pressure but high
temperature under certain conditions. A continuous spectrum in the case
of a nebula, therefore, need not indicate that the nebula is a cluster
of bodies comparable in size and general constitution with our sun. But
if a spectrum of bright lines is given by a nebula, we