light. There would probably be, at some parts of the wave, extensions in
the areas of the light-transmitting clouds, resulting in the passage of
more rays from below. Another phenomenon, less common but more striking,
appears also to be in harmony with the hypothesis. I refer to those
bright spots, of a brilliancy greater than that of the photosphere,
which are sometimes observed. In the course of a physical process so
vast and so active as that here supposed to be going on in the Sun, we
may expect that concurrent causes will occasionally produce ascending
currents much hotter than usual, or more voluminous, or both. One of
these, on reaching the stratum of luminous and illuminated cloud forming
the photosphere, will burst through it, dispersing and dissolving it,
and ascending to a greater height before it begins itself to condense:
meanwhile allowing to be seen, through its transparent mass, the
incandescent molten shell of the sun's body.

[The foregoing passages, to most of which I do not commit myself as more
than possibilities, I republish chiefly as introductory to the following
speculation, which, since it was propounded in 1865, has met with some
acceptance.]

"But what of the spots commonly so called?" it will be asked. In the
essay on the Nebular hypothesis, above quoted from, it was suggested
that refraction of the light passing through the depressed centres of
cyclones in this atmosphere of metallic gases, might possibly be the
cause; but this, though defensible as a "true cause," appeared on
further consideration to be an inadequate cause. Keeping the question in
mind, however, and still taking as a postulate the conclusion of Sir
John Herschel, that the spots are in some way produced by cyclones, I
was led, in the course of the year following the publication of the
essay, to an hypothesis which seemed more satisfactory. This, which I
named at the time to Prof. Tyndall, had a point in common with the one
afterward published by Prof. Kirchhoff, in so far as it supposed cloud
to be the cause of darkness; but differed in so far as it assigned the
cause of such cloud. More pressing matters prevented me from developing
the idea for some time; and, afterwards, I was deterred from including
it in the revised edition of the essay, by its inconsistency with the
"willow-leaf" doctrine, at that time dominant. The reasoning was as
follows:--The central region of a cyclone must be a region of
rarefaction, and, cons