nulation is not uniform over the surface of the photosphere; in some
parts it is indistinct, and appears to be replaced by interlacing
filamentous bands, which are most apparent in the penumbrae of the spots
and around the spots themselves. The 'granules' are the tops of
ascending masses of intensely luminous vapour; the comparatively dark
'pores' consist of similar descending masses, which, having radiated
their energy, are returning to be again heated underneath the surface of
the photosphere.

In certain regions of the photosphere several dark patches are usually
visible, which are called 'sun-spots.' At occasional times they are
almost entirely absent from the solar disc. It has been observed that
they occupy a zone extending from 10 deg. to 35 deg. north and south of the
solar equator, but are not found in the equatorial and polar regions of
the Sun. A sun-spot is usually described as consisting of an irregular
dark central portion, called the _umbra_; surrounding it is an edging or
fringe less dark, consisting of filaments radiating inwards called the
_penumbra_. Within the umbra there is sometimes seen a still darker
spot, called the _nucleus_. The umbra is generally uniformly dark, but
at times filmy luminous clouds have been observed floating over it. The
nucleus is believed to be the orifice of a tubular depression in the
floor of the umbra, prolonged downwards to an unknown depth. The
penumbra is brightest at its inner edge, where the filaments present a
marked contrast when compared with the dark cavity of the umbra which
they surround and overhang. Sometimes lengthened processes unite with
those of the opposite side and form bands and 'bridges' across the
umbra. The darkest portion of the penumbra is its external edge, which
stands out conspicuously against the adjoining bright surface of the
Sun. One penumbra will sometimes enclose several umbrae whilst the nuclei
may be entirely wanting.

[Illustration: FIG. 6.--A Sun-spot magnified.

(_Janssen._)]

Sun-spots usually appear in groups; large isolated spots are of rare
occurrence, and are generally accompanied by several smaller ones of
less perfect formation. The exact moment of the origin of a sun-spot
cannot be ascertained, because it arises from an imperceptible point; it
grows very rapidly, and often attains its full size in a day.

Prior to its appearance there is an unusual disturbance of the solar
surface over the site of the spot: luminous rid