ration: PLATE III.

SPOTS AND FACULAE ON THE SUN.

(FROM A PHOTOGRAPH BY MR. WARREN DE LA RUE, 20TH SEPT., 1861.)]

If we examine sun-spots under favourable atmospheric conditions and
with a telescope of fairly large aperture, we perceive a great amount of
interesting detail which is full of information with regard to the
structure of the sun. The penumbra of a spot is often found to be made
up of filaments directed towards the middle of the spot, and generally
brighter at their inner ends, where they adjoin the nucleus. In a
regularly formed spot the outline of the penumbra is of the same general
form as that of the nucleus, but astronomers are frequently deeply
interested by witnessing vast spots of very irregular figure. In such
cases the bright surface-covering of the sun (the photosphere, as it is
called) often encroaches on the nucleus and forms a peninsula stretching
out into, or even bridging across, the gloomy interior. This is well
shown in Professor Langley's fine drawing (Plate II.) of a very
irregular spot which he observed on December 23-24, 1873.

The details of a spot vary continually; changes may often be noticed
even from day to day, sometimes from hour to hour. A similar remark may
be made with respect to the bright streaks or patches which are
frequently to be observed especially in the neighbourhood of spots.
These bright marks are known by the name of _faculae_ (little torches).
They are most distinctly seen near the margin of the sun, where the
light from its surface is not so bright as it is nearer to the centre of
the disc. The reduction of light at the margin is due to the greater
thickness of absorbing atmosphere round the sun, through which the light
emitted from the regions near the margin has to pass in starting on its
way towards us.

None of the markings on the solar disc constitute permanent features on
the sun. Some of these objects may no doubt last for weeks. It has,
indeed, occasionally happened that the same spot has marked the solar
globe for many months; but after an existence of greater or less
duration those on one part of the sun may disappear, while as frequently
fresh marks of the same kind become visible in other places. The
inference from these various facts is irresistible. They tell us that
the visible surface of the sun is not a solid mass, is not even a liquid
mass, but that the globe, so far as we can see it, consists of matter in
the gaseous, or vaporous, conditi