y enabling them to work their
way into the upper regions, where the temperature has so fallen that the
vapour becomes chilled into cloud. A necessary consequence of the rapid
cooling of these clouds, and the consequent radiation of heat on a
large scale, would be the formation of what we may perhaps describe as
smoke, which settles by degrees through the intervals between the clouds
(making these intervals appear darker) until it is again volatilised on
reaching a level of greater heat below the clouds. This same smoke is
probably the cause of the well-known fact that the solar limb is
considerably fainter than the middle of the disc. This seems to arise
from the greater absorption caused by the longer distance which a ray of
light from a point near the limb has to travel through this layer of
smoke before reaching the earth. It is shown that this absorption cannot
be attributed to a gaseous atmosphere, since this would have the effect
of producing more dark absorption lines in the spectrum. There would
thus be a marked difference between the solar spectrum from a part near
the middle of the disc and the spectrum from a part near the limb. This,
however, we do not find to be the case.

With regard to the nature of sun-spots, the idea first suggested by
Secchi and Lockyer, that they represent down rushes of cooler vapours
into the photosphere (or to its surface), seems on the whole to accord
best with the observed phenomena. We have already mentioned that the
spots are generally accompanied by faculae and eruptive prominences in
their immediate neighbourhood, but whether these eruptions are caused by
the downfall of the vapour which makes the photospheric matter "splash
up" in the vicinity, or whether the eruptions come first, and by
diminishing the upward pressure from below form a "sink," into which
overlying cooler vapour descends, are problems as to which opinions are
still much divided.

A remarkable appendage to the sun, which extends to a distance very much
greater than that of the corona, produces the phenomenon of the zodiacal
light. A pearly glow is sometimes seen in the spring to spread over a
part of the sky in the vicinity of the point where the sun has
disappeared after sunset. The same spectacle may also be witnessed
before sunrise in the autumn, and it would seem as if the material
producing the zodiacal light, whatever it may be, had a lens-shaped
form with the sun in the centre. The nature of this obje