150 
POPULAE SCIENCE REVIEW. 
physical conditions wMcli differ from those of the luminiferous 
photosphere. Dark spots on the sun have been constantly 
observed from the time of Gralileo to the present, and for a 
long period a careful record of their appearances and numbers 
has been kept. A dark spot may be described as commencing 
in some minute pores or dots upon the brilliant solar surface. 
These increase ; by some they have been thought to have a 
visible motion. They assume an umhral blackness, and acquire 
at first an irregular and changeable shape. These conditions 
will be understood from the accompanying woqdcut (fig. 1), from 
a drawing by Mr. 
Carrington, of a 
group of spots ob- 
served on the 8th 
August, 1860. * 
When these have 
attained somemea- 
surable size, a 
numhra begins to 
be formed, whichis, 
as Herschel says, 
a circumstance 
strongly favomfing 
the origination of 
the spot in a distur- 
bance from below 
upward. Large 
spots are always 
^ attendedby/acu?ce, 
^ ^ which are vein-like 
lines appearing on 
the photosphere, 
but still more lumi- 
nous thanit. These 
are well shown in 
the woodcut (fig. 2), 
from a photograph taken by Mr. Warren De la Rue ; the asso- 
ciated one (fig. 3) giving — upon the same authority — the condi- 
tion of a well-formed solar dark spot. It is difficult to convey 
an idea of the vast size and extent over which these evidences 
of solar activity spread. Dark spots, covering an area of 
between seven and eight hundred millions of square miles, 
are not uncommon. Herschel measured one which occupied 
no less than three thousand seven hundred and eighty millions 
© s 
m 
(5a' 
Fig. 1. 
* Observations of the Spots on the Sun from Nov. 9, 1853, to March 24, 
1861, made at Eedhill by Kichard Christopher Carrington, F.K.S. 
