102 
ASTRONOMY: HALE AND ELLERMAN 
THE MINUTE STRUCTURE OF THE SOLAR ATMOSPHERE 
By George E. Hale and Ferdinand Ellerman 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Received by the Academy, January 10. 1916 
During a total eclipse of the sun, when the light of the disk is com- 
pletely cut off by the moon, the solar atmosphere is momentarily re- 
vealed. The exceedingly faint corona, extending millions of miles into 
space, can be seen only at such times. But the more brilliant chromo- 
sphere, the comparatively shallow atmosphere of luminous gases which 
completely encircles the sun, and the prominences which rise out of it, 
can be observed on any clear day with the aid of a spectroscope. First 
applied to this purpose in 1868, the spectroscope has yielded a large 
store of information regarding the number, distribution, and nature 
of the prominences and the structure of the upper chromosphere, as 
seen in elevation at the sun's 
limb. It has also permitted the 
observation of certain phenomena 
of the solar atmosphere in pro- 
jection against the disk, but on 
account of the brilliant back- 
ground, only their general out- 
lines can be thus detected. In 
FIG. 1. DIRECT PHOTOGRAPH OF SUN-SPOT order to study their details we 
GROUP, 1915, AUG. 7, SCALE: SUN'S DIAMETER 
= 24 CM. (Negative reproduction mUSt haVC reCOUrSC tO thc SpCC- 
troheliograph. 
With this instrument, first applied to the investigation of the solar 
atmosphere in 1892, a large number of new phenomena have been brought 
to light. The spectroheliograph may be briefly described a^ a moving 
spectroscope, driven at a uniform rate across the solar disk, and admitting 
to the photographic plate through a narrow slit the light of a single 
spectral line. Thus monochromatic images of the solar atmosphere, 
showing the otherwise invisible clouds of hydrogen or of calcium, iron, 
or other vapor (the fiocculi) are recorded permanently for study. In a 
region on the solar image, for example, where direct observation shows 
nothing, or perhaps a group of sun-spots, the spectroheliograph may 
disclose extensive phenomena of great interest in the solar atmosphere 
(compare fig. 1, a direct photograph of a group of sun-spots, with fig. 
5, Plate I, showing two spectroheliograms of the hydrogen fiocculi 
above and surrounding the group). The purpose of this paper is to com- 
