152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
downward, but at the edge of the sun we look obliquely, and there 
the 3,000 or 4,000 miles will all be found in a layer of the sun perhaps 
not more than 100 miles in thickness. In a body 800,000 miles in 
diameter a thickness of 100 miles is practically negligible, certainly 
so for any telescopic observations which can be made from the earth, 
hence, naturally, the boundary of the sun is seen to be sharp. 
The contrast of brightness between the center and the edge of the 
sun follows at once from what has just been said. For at the center 
we look far deeper than at the edge, and, naturally, see thereby gases 
which are much hotter than those which are perceived in the com- 
paratively superficial layer which is seen at the sun’s edge. Attending 
the increase of temperature there must be an increase of brightness, 
and this will be greater for red and infra-red rays than for violet and 
ultra-violet rays, in accordance with laboratory experiments on the 
relations of radiation and temperature. Thus the contrast of bright- 
ness between the center and the edge of the sun will be greater in the 
red and infra-red than in the violet and ultra-violet. 
The sun spots appear to be whirlpools where the gases of the interior 
are pouring out toward the exterior in forms similar to a waterspout. 
They are cooled by expansion as they reach the surface, and the par- 
tial vacuum formed in the center of the whirl sucks in the superincum- 
bent and very light gases, hydrogen and calcium, above the sun’s 
surface. The magnetic field found by Hale in sun spots is due, no 
doubt, to the rotation of the electrically charged material in the spots. 
The solar spectrum, with its numerous dark lines, is due to the pres- 
ence of the gases of the chemical elements which are found upon the 
earth. These gases are cooler at the boundary of the sun than they 
are within, where the principal part of the light comes from, hence, 
as noted above, the effect will be to produce dark lines on a bright 
background. The irregular mottled appearance of the sun’s surface 
is probably due to differences of temperature which exist in so great 
a body, and thereby produce variations in brightness of different 
parts. 
It is impossible to go further and touch upon the very interesting 
questions connected with the sun’s place among the stars, the de- 
pendence of plant growth upon solar radiation, and the relations 
between the temperature of the earth and the radiation of the sun. 
These matters, and many details, which it has been impossible to 
mention in this short account, are discussed by the writer in a book 
entitled ‘‘The Sun,” to which and to the original sources of informa- 
tion and to longer treatises the interested reader is invited to turn. 
