. 
C. 8. Hastings— Constitution of the Sun. 43 
There are, however, certain evident limitations to these con- 
ditions; in other words, we cannot expect to see all the dark 
lines doubled by any increase of dispersive power. For in- 
stance, a line must have a marked tendency to broaden with 
increased pressure, otherwise the duplication cannot be pro- 
nounced. Again, the layer of rare vapor must be thin, or its 
temperature cannot be relatively high throughout, as demanded 
y the theory. This evident condition doubtless gives the rea- 
son why the hydrogen lines, though the broadest in the solar 
spectrum, are not sensibly double. 
he theory of the constitution of the sun above proposed, 
may be briefly recapitulated thus: 
Convection currents, directed generally from the center of 
the sun, start from a lower level where the temperature is prob- 
ably above the vaporizing temperature of every substance. 
As these currents move upward they are cooled, mainly by 
expansion, until a certain element (probably of the carbon 
group), is precipitated. This precipitation, restricted from the 
nature of the action, forms the well-known granules. There is 
nothing which has come under my observation which would 
indicate a columnar form in these granules under ordinary cir- 
cumstances. 
The precipitated material rapidly cools, on account of its 
great radiating power, and forms a fog or smoke, which settles 
slowly through the spaces between the granules till revolatilized 
below. It is this smoke which produces the general absorp- 
tion at the limb and the “rice grain” structure of the photo- 
sphere, 
_ When any disturbance tends to increase a downward convee- 
tion current, there is a rush of vapors at the outer surface o 
the photosphere toward this point. These horizontal currents, 
or winds, carry with them the cooled products of precipitation 
which, accumulating above, dissolve slowly below in sinking. 
is body of ‘smoke’ forms the solar spot. 
The upward convection currents in the region of the spots 
are bent horizontally by the centripetal winds. Yielding their 
heat now by the relatively slow process of radiation, the loci 
of precipitation are much elongated, thus giving the region 
immediately surrounding a spot the characteristic radial struc- 
ture of the penumbra. : 
This conception of the nature of the penumbra implies a 
ready interpretation of a remarkable phenomenon, amply at- 
tested by the most skillful observers, and, as far as my i 
edge goes, wholly unexplained; namely, the brightening of 
the inner edge of the penumbra in every well-developed spot.* 
* Relating to this phenomenon, see important observations by Professor Lang- 
ley, this Journal, vol. ix (1875), p. 194; also Le Soleil, par Le P. A. Secchi, Paris, 
1875, chap. rv, p. 80, and particularly fig. 46, p. 90, with explanatory text. 
