is be so, it is reasonable to suppose that the evolution of some of
the stars may not have progressed so far as has that of the sun, and
thus we may be able actually to witness stars in the earlier phases of
their development. Let us see how far the telescope responds to these
anticipations.

The field of view of a large telescope usually discloses a number of
stars scattered over a black background of sky; but the blackness of
the background is not uniform: the practised eye of the skilled observer
will detect in some parts of the heavens a faint luminosity. This will
sometimes be visible over the whole extent of the field, or it may even
occupy several fields. Years may pass on, and still there is no
perceptible change. There can be no illusion, and the conclusion is
irresistible that the object is a stupendous mass of faintly luminous
glowing gas or vapour. This is the simplest type of nebula; it is
characterised by extreme faintness, and seems composed of matter of the
utmost tenuity. On the other hand we are occasionally presented with the
beautiful and striking phenomenon of a definite and brilliant star
surrounded by a luminous atmosphere. Between these two extreme types of
a faint diffused mass on the one hand, and a bright star with a nebula
surrounding it on the other, a graduated series of various other nebulae
can be arranged. We thus have a series of links passing by imperceptible
gradations from the most faintly diffused nebulae on the one side, into
stars on the other.

The nebulae seemed to Herschel to be vast masses of phosphorescent
vapour. This vapour gradually cools down, and ultimately condenses into
a star, or a cluster of stars. When the varied forms of nebulae were
classified, it almost seemed as if the different links in the process
could be actually witnessed. In the vast faint nebulae the process of
condensation had just begun; in the smaller and brighter nebulae the
condensation had advanced farther; while in others, the star, or stars,
arising from the condensation had already become visible.

But, it may be asked, how did Herschel know this? what is his evidence?
Let us answer this question by an illustration. Go into a forest, and
look at a noble old oak which has weathered the storm for centuries;
have we any doubt that the oak-tree was once a young small plant, and
that it grew stage by stage until it reached maturity? Yet no one has
ever followed an oak-tree through its various stages